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Arctic Benthos Borealisation

Feedback mechanisms

Red calcareous algae and sea anemones dominated state

 

Competition for space - Balancing feedback-loops (B1, B2, B3) (local, well-established)

For structuring of the rocky bottom community the most important limiting resource for sessile organisms is space (Dayton 1971). In this case, for example, an increased amount of sea anemones will reduce the remaining space for other sea anemones to colonize (see B2 in CLD), hence the negative relationship between space and sea anemones. The same negative relationship with space also concerns macroalgae and red calcareous algae due to the same mechanism.

Competitive exclusion - Reinforcing feedback-loop (R1) (local, well-established)

Competitive exclusion is caused by direct competition for space between sessile organisms, causing the exclusion of species that are less successful (Beuchel et al. 2006). For example, if red calcareous algae is attached to the substrate it excludes macroalgae from using the same space (see e.g. B1 and B3). Defence mechanisms such as the red calcareous algae’s (Lithothamnion sp.) ability to inhibit the settlement of macroalgae on top of them by sloughing of its epithelial cells affect their competitive strength for space (Johnson and Mann 1986). A natural experiment (manipulated transect) in Smeerenburgfjord concluded that the competition for space was one of the dominating feedback-loops that maintain the Arctic regime, due to macroalgae’s inability to settle in areas where red calcareous algae had not been removed (Kortsch et al. 2012).

Grazing - Balancing feedback-loop (B4) (local, well-established)

Grazers, in this case mainly sea urchins, have a negative relationship with macroalgae, since macroalgae decreases in biomass with increasing grazing. By acting as a food-resource, this in turn also leads to an increase in grazers if macroalgae were to increase, which explains their positive relationship. Combining these interactions creates a balancing loop (see B4).

Chemical defense - Reinforcing feedback-loop (R2) (local, well-established)

The red calcareous algae (e.g. Lithothamnion sp.) has the ability to chemically attract grazers (Kortsch et al. 2012), which explains the positive relationship between red calcareous algae and grazers. As previously mentioned, grazers have a negative relationship on macroalgae (B4) due to feeding on them. By decreasing macroalgae, more space is available for red calcareous algae which spurs their dominance (R1). By including grazers, a new reinforcing feedback-loops is created (R2).

 

Subarctic boreal state

Competition for space - Balancing feedback-loops (B1, B2, B3) (local, well-established)

As previously mentioned, the main feedback-loop is based on the competition for space by sessile organisms (Dayton 1971). The mechanisms also work to balance macroalgae in the new regime, where an increase in algae demands more space. If space is not available, this will restrict the growth of the algae. If a macroalgae is attached it excludes others from attaching to the same substrate thus affecting the amount of available space.

Competitive exclusion - Reinforcing feedback-loop (F1) (local, well-established):

Macroalgae increases their competitive strength by mechanically interfering with the feeding apparatus of several invertebrates (Whitman and Dayton 2001) thus excluding them from the substrate. Also, macroalgae has been shown to increase drag in dense areas (Johnson 2001), which could decrease the food availability for organisms that rely on water currents. As previously mentioned, red calcareous algae also has a negative impact on macroalgae that altogether causes the reinforcing feedback-loop (F1). The maintenance of the macroalgae domination is partly due to favorable environmental conditions (Bischoff and Wiencke 1993), but also due to the negative relationship between the competing species (Kortsch et al. 2012).

Drivers

The main external direct drivers that contribute to the shift include:

Sea ice cover (local/regional, well-established):

From 1980 to 2010, the length of the ice-free season for the West Svalbard region increased at a rate of 3.3 days per year. (Kortsch et al. 2012) Reduced sea ice cover enhances the light conditions in the water column, which promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with more light (Bischoff and Wiencke 1993). The calcareous algae that dominates the Arctic boreal regime however, thrives under low light conditions (Johansen 1981) and was disfavoured by this change. Research suggests that there is a certain time lag between the change in sea ice cover and change in benthos (Beuchel et al. 2006). Further, the loss of sea ice in the region decreases the albedo, which leads to more heat absorption by the sea water (IPCC 2013). Increased sea water temperature induces ice melting, which makes this a reinforcing feedback loop (R3).

Sea surface temperature (local/regional, well-established):

From 1980 to 2010, annual average sea surface temperature (SST) in the West Svalbard region increased by 0.5°C (Kortsch et al. 2012). Just as for sea ice cover, this change disfavoured the calcareous algae, that thrives under low water temperature regimes (Johansen 1981), and favoured the macroalgae that has higher temperature requirements (Bischoff and Wiencke 1993). In other words, the combined changes in SST and sea ice cover influence the feedback loop R1 and shifts the balance in favour of macroalgae. For SST, there is also likely to be a time lag before it changes the abundance of different species (Beuchel et al. 2006).

Nutrient input (local/regional, proposed):

Some research suggests that increased nutrient input could be another driver causing the observed regime shift in the Arctic benthos. With increased precipitation due to global warming, melt-water runoff increases, which could lead to increased nutrient input into the fjord. Higher nutrient levels could spur primary production and hence food supply to benthic populations (Josefsson 1990), and is put forward as a potential mechanism influencing the community structure (Beuchel et al. 2006). The impact of nutrient input remains contested, and has been found not to be a contributing factor to the regime shift in Kongsfjord and Smeerenburgfjord (Kortsch et al. 2012). Nutrient input is therefore not included in the CLD for this regime shift

The main external indirect drivers that contribute to the shift include:

Global warming (global, well-established):

Global warming has accelerated over the last 30 years and has done so at an even faster rate in the Arctic (Hansen et al. 2006). Research indicates that the warming in the Arctic takes place at twice the global average rate (IPCC 2007). The global warming triggers the melting of sea ice and increases the sea surface temperature, and is therefore the main indirect driver of the regime shift observed in Arctic benthos. Further, global warming is influencing the patterns of the NAO (Gillett et al. 2003). With global warming, the NAO is expected to more often be in a positive phase, which is indicated by a positive relationship in the CLD. However, it is contested exactly how the NAO responds to climate change, and different regions are expected to be affected in different ways (AICA 2005). This is why the relationship between global warming and NAO is indicated by a dashed line in the CLD.

The loss of sea ice cover reduces the albedo (i.e. sunlight reflectivity), which leads to further global warming, thereby creating a reinforcing feedback loop (R4). However, at the local scale, the albedo effect is negligible, which is indicated with a dotted line in the CLD.

North Atlantic Oscillation (local/regional, well-established)

The NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic (Beuchel et al. 2006). Over recent decades, the NAO has gradually changed pattern (Gillett et al. 2003), which has been found to correlate with changes in the benthic community structure in the Arctic (Beuchel et al. 2006). The NAO impacts both sea ice cover (Serreze et al 2007) and sea surface temperature (Beuchel et al 2006), and the change in the NAO is therefore considered an indirect driver of the observed regime shift in the Arctic benthos. However, as changing NAO patterns are likely to affect the climatic conditions differently in different regions (AICA 2005), the relation between NAO and benthic community structure remains unclear on a regional scale

In Kongsfjord and Smeerenburgfjord, the regime shift is well-established. Due to lack of data, it remains unclear whether the regime shift has occurred and/or will occur on a broader scale.

Summary of Drivers

# Driver (Name) Type (Direct, Indirect, Internal, Shock) Scale (local, regional, global) Uncertainty (speculative, proposed, well-established)
1 Sea ice cover Direct Local/regional Well-established
2 Sea surface temperature Direct Local/regional Well-established
3 Nutrient input Direct Local/regional Proposed
4 Global warming Indirect Global Well-established
5 NAO Indirect Local/regional Well-established

Key thresholds

Shift from Arctic to subarctic regime

  • SST and light inflow – A tipping point in the system is when there is a shift in dominance from B1 and B2 to B3, i.e. a shift in the competition for space. The external drivers gradually strengthens B3 and weakens B1, which - at the threshold level - reverts the R1 loop from reinforcing red calcareous algae and sea anemones dominance to reinforcing macroalgae dominance in the system. At what combinations of increased light and temperature this shift occurs is still uncertain. Other factors that play a role are initial allocation of species, and (possibly) nutrient levels. All of these factors are highly contextual and site specific, which is indicated by the fact that the regime shift occurred five years later in Smeerenburgfjord than in Kongsfjord, despite facing the same levels of SST and sea ice cover. 

 

Shift from subarctic to Arctic regime

  • SST and light inflow - There are indications that the system could flip back to the Arctic state if environmental conditions are reversed. The thresholds at which this would occur are however unknown.

Leverage points

  • Climate warming (global, well-established):  The increase in atmospheric temperature is ensuring the continuing rise of SST and inflow of light in the Arctic. This is likely eroding the resilience of the Arctic regime, thereby increasing the likelihood that perturbations could push it into a new regime dominated by macroalgae (Kortsch et al. 2012). It is therefore vital to reach global agreement and reduce the emission of greenhouse gases to lessen and eventually halt global warming.

    Geoengineering strategies on a local and regional scale to prevent the rapid warming of the Arctic has been suggested, e.g. strengthening arctic ice sheets by pouring freshwater on them, to increase isolations and in that way lower the arctic temperature (Watts 1997). However these strategies are very controversial and heavily debated (See e.g. (Corner and Pidgeon 2010)).

Ecosystem service impacts

The benthos plays a role in the present and future provision of food for human consumption. Some of the organisms living in the benthos are suitable for human consumption and thereby has a potential to provide nutrition for humans directly. More importantly, the benthos provides habitats and dietary components for commercially fished species in the region (Snelgrove 1999). The main targeted species are cod, haddock, and shrimp, and the total catch of 2010 was just under 100 000 tonnes (Fiskeridirektoratet 2011). The Svalbard region is a managed fisheries protected zone, where fishing is limited by quotas (Ministry of Fisheries and Coastal Affairs 2013a, 2013b). It is highly uncertain how the fisheries will be affected by a general regime shift in the benthic communities. The shift, combined with changes in other environmental factors, could lead to effects on other trophic levels, possibly favouring fish abundance (Grebmeier et al. 2006). It is likely that juvenile cod prefers fleshy algae, because of the habitats it provides (Keats and Steele 1987), and there is support that a general borealisation of the Arctic also means a northward shift in the habitats of Atlantic cod and other species, an invasion of fish that could also potentially impact stocks of Arctic cod negatively (Renaud et al. 2011). The total effects of the changed benthic community structure on fishery and food web structures are, though, very unclear and there is little data to be found on the potential long-term impacts on fishing and human well-being. This uncertainty is problematic, since fishery provides a livelihood for fishermen, and nutrition for consumers. The potential impacts could be on far larger scales than the Svalbard area.

There is support that the regime shift in benthos locally led to increased biodiversity (Beuchel et al. 2006). It is, however, difficult to assess if biodiversity increases also when other environmental factors, such as ocean acidification, are taken into consideration. A more general Arctic benthos regime shift, with consequent loss of ice-specific benthos structures (Duarte et al. 2012), could potentially also lead to general loss of biodiversity and a homogenization of habitats (Weslawski et al. 2011).

Svalbard is a research intensive region, with a growing tourism industry. The benthos provides knowledge and educational ecosystem services and the local shift can be studied to enhance understanding regime shifts mechanisms in the Arctic. The change in the benthos could also generate trophic cascades in the region, changing some of the recreational and aesthetic services, but how, if, and when this happens is still very uncertain. 

Summary of Ecosystem Service impacts on different User Groups

References (if available)
Provisioning Services
Freshwater 0
Food Crops 0
Feed, Fuel and Fibre Crops 0
Livestock 0
Fisheries ? (consumers) Grebmeijer et al. (2006), Duarte et al (2012), Renaud et al (2011)
Wild Food & Products ?
Timber 0
Woodfuel 0
Hydropower 0
Regulating Services
Air Quality Regulation 0
Climate Regulation ?
Water Purification 0
Soil Erosion Regulation 0
Pest & Disease Regulation 0
Pollination 0
Protection against Natural Hazards 0
Cultural Services
Recreation ?
Aesthetic Values ?
Cognitive & Educational +/- (researchers) (Beuchel et al. 2006; Kortsch et al. 2012)
Spiritual & Inspirational 0

Uncertainties and unresolved issues

The regime shift analysis is conducted with relatively few data points, which leads to some uncertainties and unresolved issues.

Certainty of regional benthic regime shift: With few data points and complex mechanisms interacting to drive climate change and benthic regime shifts in the Arctic, it is reasonably certain that a regime shift has happened in the two studied Svalbard fjords, but it is uncertain whether or not a shift will occur on a regional scale.

Reversibility: There is some uncertainty as to how readily reversible the system is. A previously observed shift from subarctic to Arctic benthos indicates that the regime shift is reversible (Drinkwater 2006). However, a reversal might be less likely today due to the temperature increases from climate change.

Other relevant variables and drivers: The data and articles used for this analysis indicate that light and temperature are the key drivers of change in the system. Some research suggests that nutrient levels could explain some of the processes in the regime shift (Beuchel et al. 2006). This variable was found not to be an important explaining factor in Kongsfjord and Smeerenburgfjord (Kortsch et al. 2012). However, it remains uncertain what role nutrient levels play in other contexts.

Threshold level: There is no data suggesting a specific threshold level for the regime shift to happen. There might be several combinations of light and temperature that could cause a shift, depending on the resilience of the Arctic benthos regime, the local conditions, and the initial composition of the benthic cover. Further, the shift in the Smeerenburgfjord occurred with a five year delay compared to that in Kongsfjord, although they likely face very similar levels of SST and light inflow. In other words, the thresholds appear to be highly context-dependent.

Implications for well-being in long run: The observed regime shift is relatively recent and occurred on a local scale. It is therefore difficult to say how a shift in the benthic structure of the Arctic will impact human well-being on larger scales and in the long run.