Arctic Benthos Borealisation
A regime shift occurred on the west coast of Svalbard in 1996 and 2000; the former Arctic benthos was mainly constituted by red calcareous algae and filter feeders whereas the present subarctic benthos is dominated by macroalgae. The main drivers of this shift are increases in sea surface temperature and inflow of light that are both due to global warming and changes in the North Atlantic Oscillation. Changes in benthos could impact other trophic levels, potentially affecting commercial fisheries as well as tourism. The implications for ecosystem services and human well-being are highly uncertain. Management options are mainly to reduce greenhouse gases to combat global warming and an adaptive management approach is also proposed on a local scale.
Key direct drivers
- Global climate change
Key Ecosystem Processes
- Primary production
- Wild animal and plant products
- Aesthetic values
- Knowledge and educational values
- Food and nutrition
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
Typical spatial scale
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Well established – Wide agreement in the literature that the RS exists
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Links to other regime shifts
A regime shift in the rocky bottom communities in two fjords (Smeerenburgfjord and Kongsfjord) on the west coast of Svalbard occurred in 2000 and 1996, respectively. A phenomenon that could occur in areas with similar conditions throughout the Arctic. Regional, but also a local regime shifts will have social-ecological effects due to trophic cascades through the food-web (Grebmeier et al. 2006) with potential consequences for the local low-density human population of Svalbard and for commercial fisheries operating in the area.
With increasing light availability and an increase in sea surface temperature (SST) caused by climate change, the Arctic climate zone will become more similar to subarctic conditions and thus promote species that thrive in these regions. It has been proposed that erect macroalgae will benefit from these novel conditions (Bischoff and Wiencke 1993) thus causing a borealisation of the Arctic benthos.
Arctic benthos (regime 1)
With local variation in species composition, the substrate in Kongsfjord was dominated by sea anemones and red calcareous algae, whereas Smeerenburgfjord consisted of red calcareous algae and several different filter feeders such as sea anemones, sea squirts and barnacles (Kortsch et al. 2012). According to Johansen (1981), the dominating species was the red calcareous algae (Lithothamnion sp.) that thrives in low light and low SST.
Subarctic benthos (regime 2)
After the regime shift had occurred, the species composition was more characteristic to subarctic regions (Kortsch et al. 2012). In Kongsfjord, brown algae cover increased from 8% to 80% in 1996 and thereafter fluctuated around 40%, whereas in 2000 the benthos in Smeerenburgfjord consisted of several brown and red macroalgae species (Kortsch et al. 2012). The species found in this new borealised state (e.g. Desmarestia sp.) generally have high light and temperature requirements (Bischoff and Wiencke 1993).
Drivers and causes of the regime shift
The two key drivers regulating the regime shift in the Arctic benthos are sea ice cover and sea surface temperature (Kortsch et al. 2012). Over the last few decades, there has been a simultaneous increase in sea surface temperature and in the length of the ice-free season in the region (Kortsch et al. 2012), which Beuchel et al. (2006) explain as effects of global warming and changing patterns of the North Atlantic Oscillation (NAO). NAO is a large-scale climatic phenomenon that regulates atmospheric circulation across the North Atlantic, and its changing patterns have led to larger inflows of warm currents in the studied region (Beuchel, Gulliksen, and Carroll 2006). Changing NAO patterns are likely to affect the climatic conditions in different ways in different regions of the Arctic (AICA 2005), which makes it hard to generalize the impacts in benthic structure on a regional scale.
The reduced sea ice cover in the fjords enhances the light conditions in the water column, which, coupled with higher SST, promotes reproduction and growth of erect, boreal macroalgae that thrive under conditions with higher temperature and more light (Bischoff and Wiencke 1993). Correspondingly, the red calcareous algae that dominates in the Arctic regime is disfavoured by the changed conditions as it thrives under low light and low temperature conditions (Kortsch et al. 2012).
How the regime shift works
In the Arctic regime with low light and low temperature conditions, red calcareous algae and filter feeders such as sea anemones dominate the substrate cover. Although macroalgae exist in this regime, they are largely outrivaled in the competition for space (Kortsch et al. 2012). The red calcareous algae control macroalgae settling by an antifouling mechanism of sloughing outer layers, which inhibits overgrowth (Kortsch et al. 2012). Further, the red calcareous algae excrete chemicals that attract grazers feeding on macroalgae. In other words, the dominating processes in the Arctic benthos regime are grazing and competition for space (Kortsch et al. 2012)
Increasing SST and increasing light availability in the water column - conditions that favor macroalgae while negatively impacting the red calcareous algae (Bischoff and Wiencke 1993; Johansen 1981) - promotes a change in benthic community structure. Higher growth rates among macroalgae reduce the effectiveness of the above-mentioned control mechanisms (Kortsch et al. 2012). Further, dense carpets of erect macroalgae can limit the food availability for the sea anemones, and might mechanically interfere with feeding (Beuchel et al. 2006). If macroalgae begins to cover the sea anemones, an energetic cost will result from cleaning off algae, which may reduce its competitive strength (Beuchel et al. 2006). At a certain point, the macroalgae are able to outcompete the previously dominating species, and the system shifts into a new state.
Once the macroalgae has established, the new regime is maintained by the processes of competition for space, including interference with feeding, limiting food availability and overgrowth. Consequently, the same feedbacks operating in the Arctic regime are also active in the subarctic regime, but with a shifted balance of more macroalgae as they are more competitive under the new environmental conditions.
Historical data shows that a somewhat similar benthic regime shift occurred after a warming period in the 1920s and 1930s, leading to a change in benthic community structure with higher abundance of macroalgae. The subarctic regime lasted for several decades, before returning to the Arctic state (Drinkwater 2006). This indicates that the regime shift seen in the Svalbard fjords might be reversible, given that SST and light availability declines. It is, however, not likely that the process of climate change in the Arctic will be reversed in the near future (IPCC 2013), which might lessen the likelihood of a reversal.
Impacts on ecosystem services and human well-being
Benthos plays an important role in marine ecosystems, as food and habitat provider for marine organisms such as commercial fish species (Snelgrove 1999). The shift from Arctic to subarctic benthos could lead to large ecosystem changes that affect provisioning (e.g. food and wild animal products), recreational, and aesthetic ecosystem services. On a local scale, the regime shift led to higher local biodiversity (Kortsch et al. 2012), and will potentially lead to larger fish stocks and more primary production, e.g. more carbon cycling (Grebmeier et al. 2006; Snelgrove 1999). Regionally, biodiversity could decrease due to homogenisation of different ecosystems (Weslawski et al. 2011). If, and how, recreational and aesthetic service provision will change as a result of the regime shift is uncertain.
The potential increase in fish stocks in the subarctic regime would be beneficial for the commercial fishing industry that operates near Svalbard, and the consumers that enjoy their products. Carbon cycling is beneficial on a global scale, but the local contribution to global carbon cycling is negligible. Trophic cascades could affect local livelihoods and tourists if recreational ecosystem services are changed, and effects can be both increase and decrease in human well-being.
Since the main drivers of the regime shift are caused by global warming, decreasing the greenhouse gas emissions to reduce atmospheric temperatures is arguable the most powerful point of intervention. This is, however, a management option associated with several difficulties: a) any serious attempt to deal with climate change would have to be on a global scale, involving a multitude of nations and stakeholders, which so far has proven a difficult task, b) due to time lags in the climate system the borealisation of the benthos could occur in more places across the Arctic than what has been observed, even with successful greenhouse gas reduction, and c) the new regime dominated by macroalgae could be stable, and just reversing or slowing down the effects of global warming might not be enough to push the system back into the first regime.
Any local scale management options aimed to successfully inhibit the borealisation are hard to find due to the global and regional processes that are driving the regime shift. Realising the fact that there are going to be continued changes, taking an adaptive management approach in handling this ecosystem might be the best local management option. Adaptive management is often suggested when facing uncertainty, and its goal is to reduce these uncertainties over time, but also to continuously learn more about the system (Holling 1978). In Svalbard this could mean ecosystem monitoring and research with the goal of increasing knowledge about the Arctic benthos, which would enable decision-making based on up-to-date information about the state of the system. Svalbard can be considered unique in its management conditions, since there is a symbiotic relationship between the tourism industry, research activities and governing institutions (Viken 2010), something that could provide good opportunities for an adaptive management approach.
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