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Monday, 28 October 2013 13:12

Coastal Marine Eutrophication

Coastal Marine Eutrophication

Main Contributors:

Thorsten Blenckner, Johanna Yletyinen

Other Contributors:

Summary

Eutrophication is a complex process that turns low-nutrient, clear water sea to a murky, high-nutrient sea. Marine eutrophication processes differ from lakes due to the open physical structure of the sea, higher diversity of biotic habitats and more complex hydrological structure. Increases in nutrients (both nitrogen and phosphorus) increase primary production, leading to a higher turbidity, and may threaten ecosystem stability and animal as well as human health. Decomposition of the increased biomass results in increased consumption of oxygen in deep water, which may lead to hypoxia and anoxic bottoms with severe consequences for benthic organisms. Light availability can become too low to sustain macroalgae and/or submerged plants.

Scientific knowledge on the eutrophication is considerable and major commitments have been made to reduce eutrophication. These include institutional arrangements, nutrient reduction goals, assessment of progress and second-generation goals. Coastal marine eutrophication has occurred in the Baltic Sea and Chesapeake Bay.

Drivers

Key direct drivers

  • External inputs (eg fertilizers)

Land use

  • Fisheries
  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Marine & coastal

Key Ecosystem Processes

  • Primary production
  • Nutrient cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries

Regulating services

  • Water purification

Cultural services

  • Recreation
  • Aesthetic values

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Typical spatial scale

  • Sub-continental/regional

Typical time scale

  • Decades

Reversibility

  • Irreversible (on 100 year time scale)

Evidence

  • Models
  • Contemporary observations
  • Experiments
  • Other

Confidence: Existence of RS

  • Well established – Wide agreement in the literature that the RS exists

Confidence: Mechanism underlying RS

  • Well established – Wide agreement on the underlying mechanism

Links to other regime shifts

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Key References

  1. Andersen L, Rydberg L. 1988. Trends in nutrient and oxygen conditions within the Kattegat: effects of local nutrient supply. Estuar Coast Shelf Sci 26, 559u2013579.
  2. Boesch DF. 2002. Challenges and opportunities for science in reducing nutrient over-enrichment of coastal ecosystems. Estuaries 25, 886u2013900.
  3. Bonsdorff E et al. 1997. Coastal eutrophicationu202f: causes, consequences and perspectives in the archipelago areas of the northern Baltic Sea. Estuarine, Coastal and Shelf Science 44, 63u201372.
  4. Borysova O et al., 2005. Eutrophication in the Black Sea region. Impact assessment and causal chain analysis. Kalmar.
  5. Boynton WR, Kemp WM & Keefe C. 2009. A comparative analysis of nutrients and other factors influencing estuarine phytoplankton production. In Estuarine Comparisons. Academic Press, Inc. New York.
  6. Caddy JF. 1993. Toward a comparative evaluation of human impacts on fishery ecosystems of enclosed and semiu2010enclosed seas. Reviews in Fisheries Science, 1, 57u201395.
  7. Cloern J. 2001. Our evolving conceptual model of the coastal eutrophication problem. Marine Ecology Progress Series 210, 223u2013253.
  8. Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board, N.R.C. 2000. Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution. National Academy Press.Washington, DC.
  9. Conley DJ et al. 2009. Controlling eutrophication: nitrogen and phosphorus. Science 324, 1014u20131015.
  10. Goldman JC, McCarthy JJ, Peavey DG. 1979. Growth rate influence on the chemical composition of phytoplankton in oceanic waters. Nature 279, 210u2013215.
  11. Hobbie J. 2000. Estuarine Science: A Synthetic Approach to Research and Practice. Hobbie J (ed.) Island Press. Washington, DC.
  12. Howarth RW. 1988. Nutrient limitation of net primary production in marine ecosystems. Annual review of ecology and systematics 19, 89u2013110.
  13. Justic D. et al. 2009. Global change and eutrophication of coastal waters. ICES Journal of Marine Science 70, 1528u20131537.
  14. Kemp WM et al. 2005. Eutrophication of Chesapeake Bayu202f: historical trends and ecological interactions. Marine Ecology Progress Series 303, 1u201329.
  15. Llope M et al. 2011. Overfishing of top predators eroded the resilience of the Black Sea system regardless of the climate and anthropogenic conditions. Global Change Biology 17, 1251u20131265
  16. Mort HP et al. 2007. Phosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2. Geology 35, 483.
  17. Mort HP et al. 2010. Phosphorus recycling and burial in Baltic Sea sediments with contrasting redox conditions. Geochimica et Cosmochimica Acta 74, 1350u20131362.
  18. Nixon, S.W., 1995. Coastal marine eutrophication: a definition, social causes and future concerns. OPHELIA 41, 199u2013219.
  19. Nystru00f6m M et al. 2012. Confronting feedbacks of degraded marine ecosystems. Ecosystems 15, 695u2013710
  20. Paerl HW 1997. Coastal eutrophication and harmful algal bloomsu202f: Importance of atmospheric deposition and groundwater as new nitrogen and other nutrient sources. Limnology and oceanography 42, 1154u20131165.
  21. Smith VH, Joye SB, Howarth RV. 2006. Eutrophication of freshwater and marine ecosystems. Limnology and Oceanography 51, 351u2013355.
  22. Smith VH, Tilman GD, Nekola JC. 1999. Eutrophication: impacts of excess nutrient inputs on freshwater, marine, and terrestrial ecosystems. Environmental pollution 100, 179u201396.
  23. Smith VH. 2003. Eutrophication of freshwater and coastal marine ecosystems a global problem. Environmental Science and Pollution Research 10, 126u2013139.
  24. Vahtera E et al. 2007. Internal Ecosystem Feedbacks Enhance Nitrogen-fixing Cyanobacteria Blooms and Complicate Management in the Baltic Sea. AMBIO 36, 186u2013194.
  25. Walker D et al. 2001. Ecological significance of seagrass: Assessment for management of environmental impact in Western Australia. Ecol Eng 16, 323u2013330.

Citation

Thorsten Blenckner, Johanna Yletyinen. Coastal Marine Eutrophication. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-13 06:25:36 GMT.
Monday, 09 September 2013 13:59

West Antarctic Ice Sheet collapse

West Antarctic Ice Sheet collapse

Main Contributors:

Johanna Yletyinen

Other Contributors:

Garry Peterson

Summary

Indication exists for a possible regime shift of collapsed West Antarctic Ice Sheet (WAIS) due to the warming climate. As the atmosphere and oceans warm as a result of global warming, ice sheets are predicted to shrink in size, resulting in raised sea level. The WAIS is a marine ice sheet, surrounded by floating ice shelves with the main part of the sheet below sea-level (Oppenheimer 1998). It is considered to be capable of past and future collapses bringing about several meters sea level rise (Mercer 1978; Oppenheimer & Alley 2004). The two WAIS regimes consist of the intact ice sheet and disintegrated WAIS. The global warming-induced future WAIS collapse could cause a sea level rise of approximately 3-5 meters with significant societal and economic impacts. Marine fauna that is adapted to sea ice dynamics would be directly impacted through habitat changes, food web interaction alterations and shifts in marine isotherms (Rogers et al. 2012; Clarke et al. 2007). Many uncertainties remain about the mechanisms of the WAIS system, drivers of the observed change and future scenarios. It is suggested that the warming of the oceanic deep water currently causes significant basal melting and thinning of the ice sheet. A basin-scale ice model study, published in 2014, provides strong evidence that the collapse has already begun (Joughin et al. 2014.)

Drivers

Key direct drivers

  • Global climate change

Land use

  • Conservation
  • Tourism
  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Polar
  • Planetary

Key Ecosystem Processes

  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries

Regulating services

  • Climate regulation

Cultural services

  • Recreation
  • Knowledge and educational values

Human Well-being

  • Livelihoods and economic activity
  • Social conflict

Key Attributes

Typical spatial scale

  • Sub-continental/regional

Typical time scale

  • Centuries

Reversibility

  • Irreversible (on 100 year time scale)

Evidence

  • Models
  • Paleo-observation
  • Contemporary observations

Confidence: Existence of RS

  • Contested – Reasonable evidence both for and against the existence of RS

Confidence: Mechanism underlying RS

  • Speculative – Mechanisms have been proposed, but little evidence as yet

Links to other regime shifts

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

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Citation

Johanna Yletyinen, Garry Peterson. West Antarctic Ice Sheet collapse. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-08-28 20:02:35 GMT.
Monday, 19 March 2012 12:04

North Pacific Ocean

North Pacific Ocean

Main Contributors:

Johanna Yletyinen

Other Contributors:

Thorsten Blenckner, Reinette (Oonsie) Biggs

Summary

A climatic regime shift took place in the North Pacific Ocean during the winter 1976-77. It caused significant impacts on the physical and biological conditions leading to severe distribution and abundance changes of plankton and fish species. Physical changes include intensification of the wintertime Aleutian Low pressure system, change in Pacific-North America (PNA) teleconnection pattern, and regional cooling or warming. The 1977 climate shift is associated with an abrupt transition from a negative to positive phase of the Pacific Decadal Oscillation (PDO). In 1989, a new regime shift occurred characterized by declining fish stocks, but the changes were not as remarkable or pervasive as in the 1976-77, and the changes caused not a return of the system back to the pre-1977 conditions. The 1976-77 and 1989 North Pacific Ocean climatic regime shifts were caused by natural shifts in ocean climate. Studies have shown that regime shifts have occurred in the North Pacific for centuries, although their durations seem to have diminished from 50-100 years to even 10 years. 

Type of regime shift

  • Climatic Regime Shift

Ecosystem type

  • Marine & coastal

Land uses

  • Fisheries

Spatial scale of the case study

  • Sub-continental/regional (e.g. southern Africa, Amazon basin)

Continent or Ocean

  • Pacific Ocean

Region

  • North Pacific Ocean

Countries

  • Not relevant

Locate with Google Map

Drivers

Key direct drivers

  • Environmental shocks (eg floods)
  • Global climate change

Impacts

Key Ecosystem Processes

  • Primary production
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries

Cultural services

  • Spiritual and religious

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Spatial scale of RS

  • Sub-continental/regional

Time scale of RS

  • Months

Reversibility

  • Readily reversible

Evidence

  • Models
  • Paleo-observation

Confidence: Existence of RS

  • Well established – Wide agreement in the literature that the RS exists

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Alexander M, Capotondi A, Miller A, Chai F, Brodeur R, Deser C. 2008. Decadal variability in the northeast Pacific in a physical-ecosystem model: Role of mixed layer depth and trophic interactions. Journal of Geophysical Research 113, 1-13.
  2. Alheit J, Bakun A. 2010. Population synchronies within and between ocean basins: Apparent teleconnections and implications as to physical-biological linkage mechanisms. Journal of Marine Systes 79, 267-285.
  3. Anderson PJ, Piatt JF. 1999. Community reorganization in the Gulf of Alaska following ocean climate regime shift. Marine Ecology Progress Series 189, 117-123.
  4. Badjeck M-C, Allison EH, Halls AS, Dulvy NK. 2010. Impacts of climate variability and change on fishery-based livelihoods. Marine Policy 34, 357-383.
  5. Benson AJ, Trites AW. 2002. Ecological effects of regime shifts in the Bering Sea and eastern North Pacific Ocean. Fish and Fisheries 3, 95-113.
  6. Benson AJ, Trites AW. 2002. Ecological effects of regime shifts in the Bering Sea and eastern North Pacific Ocean. Fish and Fisheries 3, 95-113.
  7. Chavez FP, Ryan J, Lluch-Cota SE, Niquen MC. 2003. From anchovies to sardines and back: multidecaldal change in the Pacific Ocean. Science 299, 217-221.
  8. Chiba S, Aita MN, Tadokoro K, Saino T, Sugisaki H, Nakata K. From climate regime shifts to lower-trophic level phenology: Synthesis of recent progess in retrospective studies of the western North Pacific. Progress in Oceanography 77, 112-126.
  9. Drinkwater KF, Beaugrand G, Kaeriyama M, Kim S, Ottersen G, Perry RI, Pörtner HO, Polovina JJ, Takasuka A. 2010. On the processes linking climate to ecosystem changes. Journal of Marine Systems 79, 374-488.
  10. Hare SR, Mantua NJ. 2000. Empirical evidence for North Pacific regime shifts in 1977 and 1989. Progress in Oceanography 47, 103-145.
  11. Hartmann B, Wendler G. 2005. The significance of the 1976 Pacific climate shift in the climatology of Alaska. Journal of Climate 18, 4824-4839.
  12. Jin FF. 1997. A theory of interdecadal climate variability of the North Pacific ocean-atmosphere system. Journal of Climate 10, 1821-1835.
  13. McBeath J. 2004. Management of the commons for biodiversity: lessons from the North Pacific. Marine Policy 28, 523-539.
  14. McGowan JA, Bograd SJ, Lynn RJ, Miller AJ. 2003. The biological response to the 1977 regime shift in the California Current. Deep Sea Research II 50, 2567-2582.
  15. McGowan JA, Cayan DR, Dorman LM. 1998. Climate-ocean variability and ecosystem response in the Northeast Pacific. Science 281, 210-217.
  16. Megrey BA, Rose KA, Shin-ichi I, Hay DE, Werner FE, Yamanaka Y, Aita MN. 2007. North Pacific basin-scale differences in lower and higher trophic level marine ecosystem responses to claimte impacts using a nutrient-phytoplankton-zooplankton model coupled to a fish bioenergetics model. Ecological Modelling 202, 196-210.
  17. Miller AJ, Schneider N. 2000. Interdecadal climate regime dynamics in the North Pacific Ocean: theories, observations and ecosystem impacts. Progress in Oceanography 47, 355-379.
  18. Overland J, Rodionov S, Minobe S, Bond N. 2008. North Pacific regime shifts: Definitions, issues and recent transitions. Progress in Oceanography 77, 92-102.
  19. Wooster WS, Zhang CI. 2004. Regime shifts in the North Pacific: early indications of the 1976-1977 event. Progress in Oceanography 60, 183-200
  20. Wu L, Lee DE, Liu Z. 2005. The 1976/77 North Pacific climate regime shift: the role of subtropical ocean adjustment and coupled ocean-atmosphere feedbacks. Journal of Climate 18, 5125-5140.
  21. Yatsu A, Aydin KY, King JR, McFarlane GA, Chiba S, Tadokoro K, Kaeriyama M, Watanabe Y. 2008. Elucidating dynamic responses of North Pacific fish populations to climatic forcing: Influence of life-history strategy. Progress in Oceanography 77, 252-268.
  22. Yoo S, Batchelder HP, Peterson WT, Sydeman WJ. 2008. Seasonal, interannual and event scale variation in North Pacific ecosystems. Progress in Oceanography 77, 155-181.
  23. Zhang CI, Lee JB, Kim S, Oh J-H. 2000. Climatic regime shifts and their impacts on marine fisheries resources in Korean waters. Progress in Oceanography 41, 171-190.

Citation

Johanna Yletyinen, Thorsten Blenckner, Reinette (Oonsie) Biggs. North Pacific Ocean. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-11-20 12:10:02 GMT.
Thursday, 29 December 2011 19:49

Izmit Bay, Turkey

Izmit Bay, Turkey

Main Contributors:

Johanna Yletyinen

Other Contributors:

Summary

Izmit Bay has been affected by oil spill and fire, and sewage discharge.

Type of regime shift

Ecosystem type

  • Marine & coastal

Land uses

  • Fisheries

Spatial scale of the case study

  • Local/landscape (e.g. lake, catchment, community)

Continent or Ocean

  • Europe

Region

  • Western Asia

Countries

  • Turkey

Locate with Google Map

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Tüfekçi et al. 2010. Phytoplankton composition and environmental conditions of a mucilage event in the Sea of Marmara. Turkish Journal of Biology 34, 199-210.

Citation

Johanna Yletyinen. Izmit Bay, Turkey. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 17:33:15 GMT.
Thursday, 29 December 2011 19:36

Aveiro Lagoon, Portugal

Aveiro Lagoon, Portugal

Main Contributors:

Johanna Yletyinen

Other Contributors:

Summary

Hypoxia recorded in the 1980s.

Type of regime shift

Ecosystem type

  • Marine & coastal
  • Freshwater lakes & rivers

Land uses

  • Fisheries

Spatial scale of the case study

  • Local/landscape (e.g. lake, catchment, community)

Continent or Ocean

  • Europe

Region

  • South Europe

Countries

  • Portugal

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Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Lopes JF, Dias JM, Cardoso AC, Silva CIV. 2005. The water quality of the Ria de Aveiro lagoon, Portugal: From the observations to the implementation of a numerical model. Marine Environmental Research 60, 594-628.

Citation

Johanna Yletyinen. Aveiro Lagoon, Portugal. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 17:33:41 GMT.
Thursday, 29 December 2011 19:30

Mondego River, Portugal

Mondego River, Portugal

Main Contributors:

Johanna Yletyinen

Other Contributors:

Summary

Hypoxia recorded in the 1990s.

Type of regime shift

Ecosystem type

  • Freshwater lakes & rivers

Land uses

  • Fisheries

Spatial scale of the case study

  • Local/landscape (e.g. lake, catchment, community)

Continent or Ocean

  • Europe

Region

  • South Europe

Countries

  • Portugal

Locate with Google Map

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Flindt MR, et al. 1997. Description of the three shallow estuaries: Mondego River (Portugal), Roskilde Fjord (Denmark) and the Lagoon of Venice (Italy). Ecological Modelling 102, 17-31.

Citation

Johanna Yletyinen. Mondego River, Portugal. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 17:33:56 GMT.
Thursday, 29 December 2011 19:23

Tysfjord & Ofotfjord, Norway

Tysfjord & Ofotfjord, Norway

Main Contributors:

Johanna Yletyinen

Other Contributors:

Summary

Hypoxia recorded in the 2000s.

Type of regime shift

Ecosystem type

  • Marine & coastal

Land uses

  • Fisheries

Spatial scale of the case study

  • Local/landscape (e.g. lake, catchment, community)

Continent or Ocean

  • Europe

Region

  • North Europe

Countries

  • Norway

Locate with Google Map

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Diaz R, Rosenberg R. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321, 926-929.
  2. OSPAR Commission. 2003. OSPAR Integrated Report 2003 on the Eutrophication Status of the OSPAR Maritime Area Based Upon the First Application of the Comprehensive Procedure. Eutrophication Series. Available online http://www.eutro.org/documents/p00189_Eutrophication Status Report 2003.pdf (Last accessed 29.12.2011)

Citation

Johanna Yletyinen. Tysfjord & Ofotfjord, Norway. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 17:34:07 GMT.
Thursday, 29 December 2011 19:18

Tonsbergfjord, Norway

Tonsbergfjord, Norway

Main Contributors:

Johanna Yletyinen

Other Contributors:

Summary

Hypoxia recorded in the 2000s.

Type of regime shift

Ecosystem type

  • Marine & coastal

Land uses

  • Fisheries

Spatial scale of the case study

  • Local/landscape (e.g. lake, catchment, community)

Continent or Ocean

  • Europe

Region

  • North Europe

Countries

  • Norway

Locate with Google Map

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Diaz R, Rosenberg R. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321, 926-929.
  2. OSPAR Commission. 2003. OSPAR Integrated Report 2003 on the Eutrophication Status of the OSPAR Maritime Area Based Upon the First Application of the Comprehensive Procedure. Eutrophication Series. Available online http://www.eutro.org/documents/p00189_Eutrophication Status Report 2003.pdf (Last accessed 29.12.2011)

Citation

Johanna Yletyinen. Tonsbergfjord, Norway. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 17:34:13 GMT.
Thursday, 29 December 2011 19:14

Stolefjord, Norway

Stolefjord, Norway

Main Contributors:

Johanna Yletyinen

Other Contributors:

Summary

Hypoxia associated with transport of nutrients and organic matter.

Type of regime shift

Ecosystem type

  • Marine & coastal

Land uses

  • Fisheries

Spatial scale of the case study

  • Local/landscape (e.g. lake, catchment, community)

Continent or Ocean

  • Europe

Region

  • North Europe

Countries

  • Norway

Locate with Google Map

Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Diaz R, Rosenberg R. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321, 926-929.
  2. OSPAR Commission. 2003. OSPAR Integrated Report 2003 on the Eutrophication Status of the OSPAR Maritime Area Based Upon the First Application of the Comprehensive Procedure. Eutrophication Series. Available online http://www.eutro.org/documents/p00189_Eutrophication Status Report 2003.pdf (Last accessed 29.12.2011)

Citation

Johanna Yletyinen. Stolefjord, Norway. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 17:41:39 GMT.
Thursday, 29 December 2011 19:11

Steindalsfjord, Norway

Steindalsfjord, Norway

Main Contributors:

Johanna Yletyinen

Other Contributors:

Summary

Hypoxia recorded in the 2000s.

Type of regime shift

Ecosystem type

  • Marine & coastal

Land uses

  • Fisheries

Spatial scale of the case study

  • Local/landscape (e.g. lake, catchment, community)

Continent or Ocean

  • Europe

Region

  • North Europe

Countries

  • Norway

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Alternate regimes

Oligotrophic regime

Coastal regions serve as an attraction to people, and contribute to marine resources, recreation and transport, which on the other hand cause high human impact on these ecosystems. An oligotrophic coastal system has clear water and often submerged vegetation. Commercially preferred fish species may be abundant, because coastal ecosystems support many fisheries and are highly productive due to the nutrients from the land via runoff, or deep ocean via upwelling (Boesch 2002). Many seas experience some natural hypoxia/anoxia and eutrophication, but in smaller extent and shorter time period than in the eutrophic regime. The limited amount of nutrients keeps the growth of primary production and algal blooms restricted.

Eutrophic regime

Estuaries and coastal marine ecosystem receive anthropogenic pollutants rapidly from rivers and streams from drainage basins. In eutrophic marine ecosystems phytoplankton and epiphytic algae biomass, as well as nuisance blooms of gelatinous zooplankton, have increased. Changes in species composition can take place in all trophic levels. Phytoplankton species may have shifted to taxa that are toxic or inedible. Animal species composition often changes to include less recreationally and commercially desired species. Macroalgal and vascular plants can experience changes in biomass and species composition or disappear completely. (Smith et al. 1999; Smith 2003)

Eutrophic regime causes economic losses due to restoration and damaged goods and health threats to humans exposed to algal toxins. Water clarity is reduced and humans perceive the water overall as less aesthetic.

Drivers and causes of the regime shift

Eutrophication commonly takes place in marine coastal waters (Smith et al. 1999; Cloern 2001). The primary cause of marine eutrophication is excessive increase in nutrient concentration (Nixon 1995) from riverine loads, originating from fertilized agricultural areas, urban sewage and industrial wastewaters (e.g. Bonsdorff et al. 1997). The nutrient loads from land to sea have been successively increased as a result of land clearing, population growth, industrial development, increased use of fossil fuels and increased use of fertilizers in agriculture (Boesch 2002). In addition, oceanic upwelling transports nutrient-rich waters to water surface and atmospheric nitrogen can enter the ocean (Boesch 2002; Paerl 1997). Estuaries may be naturally eutrophic as the nutrient loads from land are concentrated in confined channels, but cultural eutrophication often increases the nutrients significantly. In marine systems nitrogen is usually the key limiting nutrient (Borysova et al. 2005), but the limiting nutrient might differ between locations.

Also other human impacts contribute to the changes observed in many coastal areas (Cloern 2001), such as fishing (removal of top-predators causing food web reorganizations). For instance in the Black Sea the intense eutrophication is suggested to result from the combination of nutrient inputs, low grazing pressure on phytoplankton and favorable climatic influences (Llope et al. 2011). Climate change is projected to intensify eutrophication by potentially increasing the water temperature and thereby strengthening the vertical stratification potentially leading to larger deep anoxic water volumes or by increasing the discharge due to higher rainfalls (Justic et al. 2009). Local characteristics of the sea, such as water residence time, stratification and tides affect the intensity of eutrophication and the ecosystem vulnerability to it (Cloern 2001; Justic et al. 2009).

How the regime shift works

The biological growth and algal blooms in an oligotrophic sea are restricted by the limited availability of nutrients, in particular nitrogen and phosphorus. Diverse processes tend to regulate algal growth in coastal environments, for instance herbivores control algal biomass and elevated oxygen concentrations suppress recycling of nitrogen and phosphorus (Kemp et al. 2005).

In summary, nutrient addition increases algal biomass and after substantial nutrient loading affects bottom-water oxygen and nutrient recycling, water clarity and benthic primary production (Kemp et al. 2005). Biomass increases lead to a high remineralisation of organic matter at the sea bottom leading to a drecrease in oxygen concentration (Borysova et al. 2005). Oxygen within sulphates is used by bacteria, resulting in the release of sulfur that will capture the free oxygen still available in the upper layers. Sulfur also causes the specific smell of sediments in eutrophic water. As the deep water oxygen concentration decrease towards anoxic conditions, benthic orgranisms will decrease and eventually disappear.

Positive feedback loops are found in nutrient recycling and water clarity. In the nutrient feedback loop, increase in nitrogen and phosphorus increase the amount of algae and turbidity, which decreases oxygen concentration in deep water, leading to lower redox and denitrification processes. This causes increased recycling of nutrients (ammonium, phosphate), and increases the nutrient availability causing a higher phytoplankton biomass. In the coastal water clarity feedback the increased turbidity and abundant phytoplankton leading to less light and benthic production, which results in less nitrogen and phosphorus uptake and increased resuspension, which inhibit the growth of seagrass once again means more algae and turbidity. (Kemp et al. 2005, Nyström et al. 2012).

Impacts on ecosystem services and human well-being

In eutrophic systems nutrient availability regulates primary production levels. The increased occurrence of harmful algal blooms are of great concern in marine systems (Smith et al. 1999). Provisioning ecosystem services are lost when toxins cause mortality of both wild and farmed fish as well as shellfish, and other animals. The composition and structure of food webs may be altered (Caddy 1993; Smith et al. 2006), potentially affecting the resilience of the coastal ecosystem. Damage and destruction of habitats (e.g. Kemp et al. 2005; Walker et al. 2001; Andersen & Rydberg 1988) and weakened quality of nursery and spawning grounds (Borysova et al. 2005) cause commercial fish species to migrate away or die.

Over half of the world population lives within coastal areas and most of the world's fisheries are connected to estuaries and near-shore habitats (Smith 2003 ref. Hobbie 2000), making the marine eutrophication a serious problem. Humans experience illness or even mortality due to contaminated fish and seafood and harmful algal blooms. Economic losses are caused by water quality problems, expenditures undertaken to reduce or avoid the damaging effects, damage on market goods and decreased tourism (Committee on the Causes and Management of Eutrophication, Ocean Studies Board, Water Science and Technology Board 2000; Cloern 2001).

Management options

Options for preventing the regime shift

Returning to the pristine, oligotrophic state is seldom a realistic option especially with continuation of anthropogenic nutrient inputs (Boesch 2002) and the human population growth. Eutrophication can be avoided, or restored, by defining a good ecological status for example in the European coasts through the implemented Water Framework Directive (WFD), for the system and the standards (nutrient reductions) needed to achieve it (Boesch 2002). The effect of current regulations to meet these standards is estimated and additional means to satisfy the established ecological goals identified, as was also done by the first assessment of the success of the WFD. Overall, the management strategy should be integrated to include for instance the catchment basin and atmospheric impact. The use of nutrients can be reduced already in the point of release and pollution should be removed before they end off at the sea. Ecosystem based fisheries management could improve the resilience of food-web against perturbations (Llope et al. 2011)

Options for restoration of desirable regimes

The key action to decrease eutrophication is limiting (Smith 2003; Smith et al. 2006; Conley et al. 2009) and most measures needs to be taken in the surrounding catchments. Commitments and organized efforts have been made in several regions to reverse eutrophication. Examples of institutional arrangements are regulations to reduce emissions, multinational directives for the national level such as the WFD, coastal management programs and educational efforts (Boesch 2002).

Different ecosystems need separate, specific management frameworks including several sectors. Local to regional nutrient reduction programs are formed in combination of scientific knowledge and politics. Reductions can be done through wastewater treatment, emission regulations, increased efficiency of nutrient use in agriculture, reduced fertilizer use, manure management, enhancement of nutrient sinks and reduction of urban runoff (Boesch 2002).

Estimation of progress requires observations, continuous monitoring of the nutrient sources and tracking the changes in the sea back to the particular action. A time lag of years may occur before the management effects can be seen in the sea, in particular as internal phosphorus loading from sediments continues.  Modeling enables forecasting the future consequences for nutrient reduction for, for instance, water clarity, algal blooms and oxygen levels. (Boesch 2002)

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

Alternate regimes

Cooling North Pacific regime (since 1976-77)

The 1977 climate shift was a transition from the permanent warming since the 1960s conditions towards a cooling regime. The central North Pacific Ocean and northwest Pacific Ocean now began to cool, and the northeast and subarctic Pacific Ocean to warm.

Such changes clearly influenced the primary and secondary production in the North Pacific, although there are regional variations. Generally speaking, the trophic level changes include increases in phytoplankton and zooplankton, changes in fish species composition and declines in subarctic top predator populations. The response of salmon to such changes is very species-specific and can therefore not be generalized. However, several studies suggest that the North Pacific sardine and anchovy stocks vary naturally in decadal cycles due to cold and warm periods: the warm sardine regime (intensification of Aleutian Low pressure) switches to a cold, anchovy dominated regime (relaxation of the Aleutian Low) every ca 25 years. An anchovy regime ended in 1975 and a sardine regime occurred from 1975 to mid-1990s.

Declining fish production regime (1989 – (possibly) 1998)

The 1977 regime shift had an almost equal balance in fish abundance, as the stocks both increased and decreased, whereas in 1989 the ecological changes consisted largely of widespread declines in productivity. Physical changes include intensification of the winter and summer Arctic vortex, weakened winter Aleutian low and subarctic circulation, and summer warming throughout most of the central North Pacific and coastal northeast Pacific Ocean. The sea surface temperature (SST) change in the Northern Pacific occurred concurrently with the SST change in the tropical Pacific during the 1976-77 climate transition period, but in the 1989 climate transition, the SST change was limited to the North Pacific.

Warming occurred in the Central North Pacific Ocean, in the Kuroshio-Oyashio system and in the California Current. However a, winter cooling of the coastal waters took place in the northern Gulf of Alaska and Bering Sea. The Kuroshio Current slowed down and wind stress and vertical mixing decreased, leading to earlier spring phytoplankton blooms. In the Gulf of Alaska and Bering Sea, the 1989 regime shift was associated with some of the lowest salmon catches in the history of the Canadian fishery. The nutrient concentrations and biological production decreased in all four regions. Based on the climate and ocean indices, a "new" ocean climate regime began in 1998. However, the overall biological consequences are still unknown, since not all ecosystems have responded to the 1998 regime shift. 

Drivers and causes of the regime shift

The North Pacific regime shifts are probably caused by climatic variations. The Pacific Decadal Oscillation (PDO), an interdecadal climate variability, refers to the cyclical variations of SSTs in the whole North Pacific Ocean. Both the strong 1976-77 and the weaker 1989 regime shifts were most probably caused by the change in the PDO patterns.

During warmer periods, abundant zooplankton support strong recruitment of both forage and predatory fishes, which in turn control forage fish. At the onset of a new cold regime, the biomass of predators remain high and predation continues to control the biomass of forage fish, but bottom up processes begin to limit fish recruitment. Some fish species in the Gulf of Alaska may have experienced a shift between bottom-up control in the 1980s (high production) and top down control. 

How the regime shift worked

Changes in the PDO patterns most probably caused the two regime shifts. During the 1976-77 regime shift, deepening and eastward shift of the Aleutian Low caused the warm, moist air to move over Alaska and cold air over the North Pacific Ocean. It caused large changes in the patterns of surface-heat flux, ocean current advection, turbulent mixing and horizontal transport. Strong mid-ocean upwelling is believed to increase productivity, and the associated horizontal divergence transports nutrients and plankton into coastal areas. These phenomena may have been responsible for the improved overall productivity in the North Pacific in the 1980s. Plankton production is positively correlated with fish production and there was a general increase in plankton during the 1980s.

Variations in salinity and SST affected zooplankton and fish abundance and their recruitment. The responses of the marine mammals and sea birds to regime shifts are difficult to estimate because of the human influence, complex natural responses to natural phenomena and delayed or muted response. Although the 1976-77 and 1989 regime shifts were caused by natural climate variability, the response of the fish may have been influenced by humans. Overfishing may have caused changes in community structure, fish age structure and energy cycling, and this may have alter the response of the fish to the otherwise natural regime shift. The disentangle of the climate effect and the overfishing is difficult. 

Impacts on ecosystem services and human well-being

The 1976-77 and 1989 regime shifts affected humans mainly through changes in the provisioning ecosystem services. The 1976-77 regime shift fisheries response was nearly balanced with variations in species biomasses but in the post-1989 regime there were widespread declines in fish stocks (for regional variations, see Ecosystem services at level 3). Around the time of the 1976-77 regime shift there were regional variations in the phytoplankton and zooplankton biomasses. The bottom-up regulation of overall productivity in the North Pacific Ocean appears to be closely related to the upper ocean changes characteristic to the positive PDO.

Changes in fish stocks affected human societies directly through seafood availability and economy, and indirectly by decreasing the ecosystem stability through declines in species richness, genetic diversity and productivity. Fish, marine mammals, sea birds and shellfish have been essential resources for the North Pacific native people. Especially declines in the salmon stocks affected negatively the livelihoods of the indigenous human societies for which salmon forms a cultural core of traditions, economy, food, health and even religious beliefs 

Management options

As the 1976-77 and 1989 regime shifts were caused by natural climatic changes, the changes in food web can chiefly be influenced through maintaining the natural resilience against the future climate changes. It seems very likely that the PDO will continue to change polarity every few decades as it has done over the past century, and with it the abundance of salmon and other species sensitive to environmental conditions will change in the North Pacific. It is believed that the buffering impact of species diversity on the resilience of an ecosystem generates security in economy and ecosystem management.

Marine reserves and fisheries closures may increase species diversity and consequently fish production. The resilience of fish populations to regime shifts caused by natural climate variability can probably be maintained by managing fish stocks in a way that doesn't alter the sensitivity of the marine systems to climate variability. Eliminating locally adapted species by overfishing may decrease the stability of a marine ecosystem and its ability to recover in a changing environment. Various studies have looked at what constitutes an optimum management strategy for fisheries that undergo regime shifts. The disadvantage of regime-specific harvest rate strategy (see: Leverage points at Level 3) is that scaling down from the high fishing capacity after the high biomass regime would most probably cause difficult economic and social problems. Also, if the low biomass is overestimated, the stock might be overharvested. The North Pacific fisheries are not managed as one unit. Regional, cooperative resource management is important in international marine ecosystems with migratory fish species. Some studies propose that a new management institution should be created for the North Pacific Ocean to do research on ecological interactions, to create a framework for decision-making and to ensure equal benefits. 

Key References

  1. Diaz R, Rosenberg R. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321, 926-929.
  2. OSPAR Commission. 2003. OSPAR Integrated Report 2003 on the Eutrophication Status of the OSPAR Maritime Area Based Upon the First Application of the Comprehensive Procedure. Eutrophication Series. Available online http://www.eutro.org/documents/p00189_Eutrophication Status Report 2003.pdf (Last accessed 29.12.2011)

Citation

Johanna Yletyinen. Steindalsfjord, Norway. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 18:16:04 GMT.
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