Message

Case Studies
Case Studies

Case Studies (325)

Monday, 19 March 2012 12:04

North Pacific Ocean

Written by Johanna

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

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, 15 March 2012 17:02

Yellow River delta, China

Written by Reinette (Oonsie) Biggs

Yellow River delta, China

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

In the history of the formation of the Yellow River delta complex, several shifts in the lower river channel and in the tributary channels have occurred over the last 6000 years. Shifts in the tributary channels caused by silting up in the river mouth because of deposition of sediment which heightened the channel floor. Headward deposition as well as the formation of superlobes was also a driver for channel shifts in the tributaries. Superlobes can either be a result of lower channel shifts through movement of the river mouth or a result of formation of several delta lobes which in turn are caused tributary channel shifts (reinforcing feedback). The shifts in the lower river channel and in the distributaries led to complicated imbrication. Moreover, changes in the coastlines, and sea water depth took place because of these river channel shifts. Lower river channel shifts were brought under artificial control by dykes, or under natural conditions by formation of large crevasses due to deposition of sediment. A large crevasse was for example formed in 1855 and in 1128 a dyke was destroyed to check the advance of the Jin army.

Type of regime shift

Ecosystem type

  • Freshwater lakes & rivers

Land uses

  • Urban
  • Small-scale subsistence crop cultivation
  • Fisheries

Spatial scale of the case study

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

Continent or Ocean

  • Asia

Region

  • Shandong Province, China

Countries

  • China, People's Republic of

Locate with Google Map

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. Xue C. 1993. Historical Changes in the Yellow River delta, China. Marine Geology 113, 321-329.

Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. Yellow River delta, China. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 13:36:45 GMT.
Thursday, 15 March 2012 16:55

River Bollin, UK

Written by Reinette (Oonsie) Biggs

River Bollin, UK

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

Several meander cutoffs occurred within a short time period between 1999 and 2002 on the River Bollin in North West England. Although strong flood events were the direct cause of the cut-offs, several hypotheses exist to explain the underlying reasons why the River Bollin became susceptible to such flooding impacts. One explanation is a change in discharge because of changes in rainfall characteristics, population growth, and land use change. Another hypothesis focuses on the occurrence of exceptionally strong flood events. Natural evolution of meanders without chaotic behaviour might also explain the cutoffs. Furthermore, there were some artificial cutoffs in 1990 when changes in the river course threatened a public footpath. 

Type of regime shift

Ecosystem type

  • Freshwater lakes & rivers

Land uses

  • Urban
  • Fisheries

Spatial scale of the case study

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

Continent or Ocean

  • Europe

Region

  • North West England, United Kingdom

Countries

  • United Kingdom

Locate with Google Map

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. Hooke JM 2003. River meander behaviour and instability: a framework for analysis, Transactions of the Institute of British Geographers 28, Issue 2, 238–253.
  2. Hooke JM. 2004. Cutoffs galore! Occurrence and causes of multiple cutoffs on a meandering river. Geomorphology 61, 225-238.

Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. River Bollin, UK. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-19 08:07:10 GMT.
Thursday, 15 March 2012 16:25

Ucayali River, Peru

Written by Reinette (Oonsie) Biggs

Ucayali River, Peru

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

In 1997, a river channel position shift, the so-called Masisea cutoff, occurred at the Ucayali River in Peru. A 71 km meander loop was cut-off and created a 33-47 km2 large oxbow lake. Besides flood events, especially those in 1997 when the cutoff occurred, the shift was mainly driven by human actions from the late 1980s onward. Residents from nearby upstream villages removed debris and cut vegetation along a small flood channel across the meander neck (where the cutoff took place) in order to improve transit and to establish a toll system. From the 1900s the channel and its connection to a floodplain lake have been used as a shortcut for canoes. A third reason for the cut off of vegetation was to facilitate bank erosion. Later, the channel was systematically maintained and widened with machetes, axes and shovels. Shallow, circular pits were excavated that acted as scour holes and in 1997 even a tractor was used to widen the entrance to the channel. The Masisea cut-off had significant impacts on the ecology and the economy upstream and downstream of the location at which it occurred. Upstream impacts consisted of a decrease in flood events, flood levels, travel time and transportation costs, which led to new economic opportunities such as changes in subsistence crops (e.g. plantain and maize) and cash crops that generated a higher income for people living upstream (e.g. papaya). In addition, the new economic opportunities upstream supported in-migration. However, downstream impacts consisted of an increase in flood events, flood levels, riverbed aggradation, bank erosion, lateral channel shifts and stranded communities. These impacts in turn led to a heightened vulnerability in floodplain agriculture, so that people increased their reliance on fishing and shifted their land-use from perennial to annual crops. Furthermore, the negative impacts led to upstream migration. 

Type of regime shift

Ecosystem type

  • Freshwater lakes & rivers

Land uses

  • Small-scale subsistence crop cultivation
  • Fisheries

Spatial scale of the case study

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

Continent or Ocean

  • South America

Region

  • Ucayali, Peru

Countries

  • Peru

Locate with Google Map

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. Coomes OT, Abizaid C, Lapointe M. 2009. Human Modification of a Large Meandering Amazonian River: Genesis, Ecological and Economic Consequences of The Masisea Cutoff on the Central Ucayali, Peru. Ambio 38, No.3, 130-134.

Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. Ucayali River, Peru. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-19 07:31:20 GMT.

Lake Veluwemeer & Lake Wolderwijd, Netherlands

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

Lake Veluwemeer and Lake Wolderwijd are located in the Netherlands between the provinces Gelderland and Flevoland. They were created in 1957 (Veluwemeer) and 1969 (Wolderwijd) for the purpose of serving as flood polders. Both lakes are shallow and have an average water depth of 1.5 meters. They have been eutrophic since 1957 and 1969. This case study deals with the regime shift from a submerged plant dominated system of Potamogeton pectinatus and Potamogeton perfoliatus to 'meadows' of Charophytes, (especially Chara spp.) in Lake Veluwemeer and Lake Wolderwijd that happened between 1987 and 1993. Potamogeton pectinatus, better known as fennel pondweed, is a submerged plant that has 2 mm wide, dense, long and linear leaves. Potamogeton perfoliatus, also known as clasping-leaf pondweed, is a submerged plant with oval, translucent leaves with no stalk that can be up to 8 cm long. Charophytes are a division of green freshwater algaes that have large thalli (which means they have no organized and distinct parts such as leafs, roots or stems) which can grow up to 120 cm. Chara spp., also known as Stonewort, is a rapidly growing submerged plant, which grows on the bottom of a lake with its roots attached to the sediment. Water depth and turbidity, which affect the availability of in situ light, seem to be the main drivers for the shift from the regime dominated by Potamogeton pectinatus and Potamogeton perfoliatus to the Chara spp. dominated regime. However, this is speculative because it is not clear whether luxuriant air plant growth, high turbulence, nutrients and grazing by herbivorous birds had an impact on the shift, too. 

Type of regime shift

Ecosystem type

  • Freshwater lakes & rivers

Land uses

  • Large-scale commercial crop cultivation
  • Intensive livestock production (eg feedlots, dairies)

Spatial scale of the case study

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

Continent or Ocean

  • Europe

Region

  • Province Flevoland, Province Gelderland

Countries

  • Netherlands

Locate with Google Map

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. Coops H, Doef RW. 1996. Submerged vegetation development in two shallow, eutrophic lakes. Hydrobiologia 340, 115-120.
  2. Van den Berg MS, Coops H, Noordhuis R, Van Schie J, Simons J. 1997. Macroinvertebrate communities in relation to submerged vegetation in two Chara-dominated lakes. Hydrobiologia 342, 143-150.

Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. Lake Veluwemeer & Lake Wolderwijd, Netherlands. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 19:19:37 GMT.
Thursday, 15 March 2012 15:49

Dutch ditches, Netherlands

Written by Reinette (Oonsie) Biggs

Dutch ditches, Netherlands

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

In this case study, a dataset of vegetation censuses and water quality from 641 Dutch ditches were analyzed. Vegetation was non-selectively removed from such ditches once or twice a year to see which regime is dominant, a submerged or a floating plant dominated regime. The dataset was divided in a sparsely vegetated subset (total cover of all taxa 50 %) and a very densely vegetated subset (total cover > 80 %).  

 

For high vegetation densities the results of the case study showed that

- Cover by floating plants was negatively correlated to submerged plant abundance.

- Floating plants showed a positive correlation to nutrient levels of the water column.

- Submerged plants were negatively related to nutrient levels.  

 

For low vegetation densities the results showed that

- Correlations between growth forms and nutrient concentrations are less pronounced.

- Abundances of floating and submerged plants are positively correlated in a phase of regrowth after removal of vegetation.  

 

The key driver that is responsible for a shift to the floating plant dominated regime is a drastic harvest of floating plants. 

Type of regime shift

Ecosystem type

  • Freshwater lakes & rivers

Land uses

  • Large-scale commercial crop cultivation
  • Intensive livestock production (eg feedlots, dairies)

Spatial scale of the case study

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

Continent or Ocean

  • Europe

Region

  • Province Flevoland, Province Gelderland

Countries

  • Netherlands

Locate with Google Map

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. Scheffer M, Szabó S, Gragnani A, van Nes EH, Rinaldi S, Kautsky N, Norberg J, Roijackers RMM, Franken RJM. 2003. Floating plant dominance as a stable state. PNAS 100, Issue 7, 4040-4045.

Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. Dutch ditches, Netherlands. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 19:14:45 GMT.
Thursday, 15 March 2012 15:08

Lake Kariba, Zimbabwe & Zambia

Written by Reinette (Oonsie) Biggs

Lake Kariba, Zimbabwe & Zambia

Main Contributors:

Henning Nolzen

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson

Summary

Lake Kariba is the largest man-made lake in Africa. It is located at the border between the northern part of Zimbabwe and the southern part of Zambia. In the case of Lake Kariba, the shift from a submerged plant dominated regime to a floating plant dominated regime occurred through Salvinia molesta in 1958 during the filling of the lake and Eichhornia crassipes from 1980 onward, was a result of strong water-level fluctuations which enhanced nutrient input from flooded land. 

Type of regime shift

Ecosystem type

  • Freshwater lakes & rivers

Land uses

  • Small-scale subsistence crop cultivation
  • Extensive livestock production (natural rangelands)
  • Fisheries
  • Tourism

Spatial scale of the case study

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

Continent or Ocean

  • Africa

Region

  • Mashonaland West (Zimbabwe), Matabeleland North (Zimbabwe), Southern, (Zambia)

Countries

  • Zambia
  • Zimbabwe

Locate with Google Map

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. Machena C. 1989. Ecology of the Hydrolittoral Macrophyte Communities in Lake Kariba, Zimbabwe. Acta Universitatis Upsaliensis 196, 1-357.
  2. Marshall BE, Junor FJR. 1981. The decline of Salvia molesta on Lake Kariba, Hydrobiologica 83, 477-484.
  3. Mitchell DS. 1969. The Ecology of Vascular Hydrophytes on Lake Kariba, Hydrobiologica 34, 448-460.
  4. Oliver JD. 1993. A review of the biology of Giant Salvinia (Salvinia molesta Mitchell). Journal of Aquatic Plant Management 31, 227-231.
  5. Scheffer M, Szabó S, Gragnani A, van Nes EH, Rinaldi S, Kautsky N, Norberg J, Roijackers RMM, Franken RJM. 2003. Floating plant dominance as a stable state. PNAS 100, Issue 7, 4040-4045.

Citation

Henning Nolzen, Reinette (Oonsie) Biggs, Garry Peterson. Lake Kariba, Zimbabwe & Zambia. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2012-03-17 19:19:44 GMT.
Thursday, 29 December 2011 19:49

Izmit Bay, Turkey

Written by Johanna

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

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

Written by Johanna

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

Locate with Google Map

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

Written by Johanna

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

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.