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MARINE ECOSYSTEMS (Chapter 14)
Lead Author: Christine Michel
Contributing Authors: Bodil Bluhm, Violet Ford, Vincent Gallucci, Anthony J. Gaston, Francisco J. L. Gordillo, Rolf Gradinger, Russ Hopcroft, Nina Jensen, Kaisu Mustonen, Tero Mustonen, Andrea Niemi, Torkel G. Nielsen and Hein Rune Skjoldal
Harbour seal Photo: Stefan Schejok, Shutterstock.com
Arctic marine ecosystems host a vast array of over 2,000 species of algae, tens of thousands of microbes and over 5,000 animal species, including unique apex species such as the polar bear Ursus maritimus and narwhal Monodon monoceros, commercially valuable fish species, large populations of migratory birds and marine mammals, and some of the largest colonies of seabirds on the planet. Current estimates also suggest that many species are yet to be discovered.
The marine Arctic is characterized by a wide range of and large variability in environmental conditions. The Arctic Ocean has the most extensive shelves of all oceans, covering about 50% of its total area. It comprises diverse ecosystems such as unique millennia-old ice shelves, multi-year sea ice, cold seeps and hot vents, and their associated communities.
The Arctic is undergoing major and rapid environmental changes including accelerated warming, decrease in sea ice cover, increase in river runoff and precipitation, and permafrost and glacier melt. These changes together with new opportunities for economic development create multiple stressors and pressures on Arctic marine ecosystems.
All Eskimos (Siberian Yupik) emphasize their connection with the sea – boys have dreams of becoming hunters. The sea gives birth to our whole life … Tatyana Achirgina in Novikova (2008).
Throughout the Arctic, ecosystem changes are already being observed. Changes in the distribution and abundance of key species, range extensions and cascading effects on species interactions are taking place, influencing Arctic marine food web architecture. Unique habitats such as ice shelves and multi-year ice are rapidly shrinking.
With continued warming and sea ice decline, measures should be put in place to monitor areas of particular biological significance and uniqueness in support of preservation and protection measures. Moreover, the complexity and regional character of Arctic ecosystem responses to environmental changes calls for the establishment of long-term marine ecosystem observatories across the Arctic, in support of sustainable management and conservation actions.
Arctic marine ecosystems are important constituents of global biodiversity. Arctic marine ecosystems are habitats to a vast array of over 5,000 animal species and over 2,000 species of algae and tens of thousands of microbes (see Josefson & Mokievsky, Chapter 8, Daniëls et al., Chapter 9 and Lovejoy, Chapter 11). The marine Arctic also provides habitat for large populations of marine mammals and birds (see Reid et al., Chapter 3 and Ganter & Gaston, Chapter 4), some of which form colonies that are among the largest seabird colonies on the planet. The unique characteristics of Arctic marine ecosystems also contribute directly to global diversity. For example, Arctic sea ice ecosystems support biodiversity at various scales ranging from unique microbial communities to apex predator species such as the polar bear Ursus maritimus and walrus Odobaenus rosmarus whose ecology is closely associated with the sea ice environment.
Indirectly, the Arctic Ocean plays a key role in shaping the global biodiversity of marine and terrestrial ecosystems as it plays an essential role in the Earth climate system. The Arctic Ocean also influences marine ecosystems of the Atlantic Ocean directly, as waters and sea ice exiting the Arctic Ocean affect the physical, chemical and biological characteristics of the North Atlantic. Conversely, the Arctic Ocean receives waters from the Pacific and Atlantic Oceans, and therefore Arctic marine ecosystems are influenced by global changes that influence biodiversity in these oceans.
The Arctic is subject to rapid environmental changes. The current increase in global temperature is most rapid in the Arctic, with a predicted summer temperature increase of up to 5 °C over this century (IPCC 2007), and surface water temperature anomalies as high as 5 °C recorded in 2007 (Steele et al. 2008). Arctic sea ice, a key defining characteristic of the Arctic Ocean, is declining faster than forecasted by model simulations (in Meltofte et al., Chapter 1), with the potential for a summer ice-free Arctic within the next few decades (Stroeve et al. 2007, Wang & Overland 2009). The effects of these and other environmental changes (e.g. changes in freshwater input, shoreline erosion) on Arctic marine ecosystems are already documented (e.g. Wassmann et al. 2010, Weslawski et al. 2011). These changes, together with increased economic interest and development in the Arctic, put pressure on the biodiversity of Arctic marine ecosystems and on the species that inhabit them.
Vulnerabilities, adaptation and looking forward
As primary production fuels marine food webs through its transfer to pelagic and benthic organisms, regional increases in primary production may be expected to augment the production of fish and shellfish species, some of which have commercial value. Recent increases in primary production associated with changes in sea ice cover on two geographically opposed shelves, the Beaufort and Laptev shelves, have been linked to observed/ modeled increases in the sedimentation of organic material (Lalande et al. 2009, Lavoie et al. 2009). In addition, studies from Arctic areas (Svalbard) suggest that benthic biota respond to fluctuations in regional climate patterns (Beuchel et al. 2006). Enhanced environmental forcing leading to warmer winters with less sea ice, earlier onset of melting and increased precipitation in Kongsfjorden during the decade 1993-2004 (Svendsen et al. 2002) may have benefited the brown algae Desmarestia sp. due to the increased availability of light and nutrients (Beuchel & Gulliksen 2008). These results point to changes in marine ecosystem architecture and biodiversity on Arctic shelves, where sea ice cover is in a state of transition.
At the same time, recent studies indicate that the increased freshwater content in the Arctic Ocean, through the effect of stratification on plankton community structure (Li et al. 2009), decreases the efficiency of transfer of organic material in Arctic marine food webs (Kirchman et al. 2009, Cai et al. 2010). Therefore, an increase in overall production in the Arctic Ocean may not necessarily lead to more abundant harvestable species, as the composition of communities largely determines the fate of material in marine systems. Recent modelling also highlights the regional character of ecosystem responses to climatic forcing (Slagstad et al. 2011).
The response of Arctic marine ecosystems to on-going changes depends on complex interactions between community structure, trophic interactions, species-specific adaptation and fitness in regard to environmental conditions, superimposed upon anthropogenic stressors that often have a strong local influence. The cumulative effects of the thinning of the ice pack, its enhanced export in relation to atmospheric circulation patterns, and warmer ocean temperatures may continue to alter Arctic sea ice and associated ecosystems dramatically. How these and other emergent environmental and anthropogenic forcings will affect ecosystem biodiversity in the marine Arctic, and in downstream marine systems, is unknown.
Patterns of changing diversity will likely depend on regional characteristics and habitat types, but also on the connectivity of ocean areas with boreal/southern regions. In areas connected to boreal waters, increases in advection can result in the transport of more sub-Arctic species northward. In regions isolated from advection of boreal waters, such as the Canadian Arctic Archipelago, changes in biodiversity may be slower and mainly influenced by local changes. Trans-Arctic migrations from the Pacific to the Atlantic Ocean are likely to occur increasingly, as Arctic sea ice continues to melt and could cause restructuring of marine food webs. The presence of the Pacific diatom Neodenticula seminae in the North Atlantic Ocean in the late 1990s after > 800,000 years of absence, was attributed to increased transport of Pacific waters through the Canadian Archipelago (Reid et al. 2007). Such trans-Arctic expansions are likely to continue, reflecting the influence of the Arctic on global marine biodiversity.
Some unique habitats, species and elements of Arctic marine ecosystems are particularly vulnerable to ongoing changes. The unique habitats associated with Arctic ice shelves that have evolved over thousands of years are eroding and may be irrevocably lost in the current and predicted future climate. Multi-year ice and its associated habitats are at risk of vanishing, with major but largely unknown direct and indirect effects on Arctic marine ecosystem architecture. Ice-associated biodiversity is at risk, with species such as the polar bear exemplifying climate-related impacts on Arctic marine biodiversity.
As changes are occurring in the Arctic, marine species and Arctic residents need to adapt. Hence, much local human transport that hitherto has taken place over ice may now use ships and boats for most of the year, and hunting techniques developed for hunting on ice may be replaced by open water hunting methods. Traditional ways may have to evolve, as expressed by this Inuit hunter:
A buddy of mine is into making little sleds out of aluminum, which you can use as a little kayak or boat. If you're out on the ice and you have to cross an open lead you can use that. It's one of the things that can help. I'm going to get one of those. It's combined as a little sleigh and, if you have to, you can use it as a boat. That's one way I can adapt.
Species with more plasticity are likely to better adapt to a variable and changing environment than species with narrow tolerances and strict physiology or life history. For example, copepods and krill in the Barents Sea MIZ show marked trophic plasticity, shifting from herbivory during the bloom to omnivory when fresh material is less abundant. Predator fishes such as Atlantic cod also show high feeding plasticity, shifting their prey from fishes to zooplankton in response to changes in abundance. Such flexibility in feeding strategies may provide an advantage in highly variable environments such as the MIZ (Tamelander et al. 2008). Phenotypic plasticity is also expected to dominated responses of marine mammals to climate change in the short term (Gilg et al. 2012). Accordingly, biodiversity can offer functional redundancy and increase the resilience of marine systems to multiple stressors. However, this resilience ultimately depends on the response of each species to individual and combined stressors and the resulting trophic interactions.
Since the Arctic is at the northern limit of distribution of many species, northward range extensions due to a warming climate are likely to shift the balance of species as the sub-Arctic biome takes over the present Arctic and true Arctic species are pushed northwards or go extinct. Such changes, as examplified by shifts in top predator species in Hudson Bay (i.e. killer whales versus polar bears, see Section 14.5.4), will affect ecosystem functioning and transfer pathways. In addition, extensive alterations in the physical and biogeochemical structure of Arctic marine ecosystems are currently taking place, with unknown consequences for these ecosystems and the species that inhabit them. We cannot predict the tradeoffs between the potential loss of unique ecosystems such as ice shelves and the introduction of new species via northwards range extensions and modifications in habitats.
Knowledge gaps and challenges
One of the greatest impediments to understanding the ongoing changes in the biodiversity of Arctic marine ecosystems is the fragmented nature of much of the existing knowledge and the lack of consistent and regular long-term monitoring programs in most Arctic marine regions, including unique or vulnerable ecosystems. A commitment to long-term studies is essential in this regard, and the establishment of the Arctic Marine Biodiversity Monitoring Plan supported by CAFF (Gill et al. 2011) is an important step towards this goal.
The effects of disturbances and stressors on Arctic marine biodiversity are not well understood. The lack of baseline information in many areas, the wide range of ecosystems and the impact of cumulative effects make it difficult to predict the direction of changes. The multiple stressors currently affecting Arctic marine ecosystems operate simultaneously at various temporal and spatial scales, emphasizing the need for local and concerted biodiversity assessment and monitoring. There is also a need to develop indicators that properly reflect the unique characteristics of Arctic marine ecosystems. For example, habitat fragmentation, used as a global biodiversity indicator, could be characterized in the marine Arctic using a variety or combination of indicators including sea ice extent and water mass distribution indices. These physical/chemical indicators could then serve as structuring elements upon which to monitor associated ecosystem biodiversity trends. Shifts in ecosystem structure, species interactions and trophic pathways need to be understood in the context of shortand long-term trends, in order to develop management strategies to maintain the diversity and sustainability of Arctic marine ecosystems. To this effect, it is essential to include biological elements in monitoring programs for the marine Arctic.
To gain new knowledge and make sensible projections about climate impacts on carbon dynamics and sequestering in Arctic marine ecosystems, key organisms from the base of marine food webs need to be considered, parameterized and included in research and modeling efforts We also need to better understand the ecophysiology of key species to be able to better parameterize bulk processes and rates.
We still have a limited inventory and understanding of the current status of Arctic marine diversity, and particularly so for the small microbial communities and benthic invertebrates. There is still much to learn about the biodiversity of extreme habitats and organisms in the Arctic. For example, there is recent evidence of the widespread occurrence of cold seeps in the marine Arctic, but the organisms inhabiting these unique habitats are poorly described. Similarly, unique habitats associated with sea ice and ice shelves are poorly understood and their biodiversity is largely unknown. This special biodiversity in the Arctic presents opportunities for advancements in biotechnology, medical research and even the search for life on other planets. Deep basins of the Arctic Ocean, which were largely inaccessible, are becoming ice-free in summer, bringing new opportunities for research and exploration. As one of the last frontiers on Earth, the marine Arctic still holds many discoveries with respect to the biodiversity of its ecosystems and the species that inhabit them
Key points and recommended actions
The marine Arctic spans a wide range of environmental conditions including extremes in temperature, salinity, light conditions and the presence (or absence) of sea ice, leading to diverse Arctic marine ecosystems. These ecosystems are experiencing rapid changes in their chemical, physical and biological characteristics together with unprecedented socio-economic pressures. Changes in the distribution and abundance of key species and cascading effects on species interactions and the structure and functionality of marine food webs are already observed.
Range extensions are taking place throughout the Arctic, with a northward expansion of sub-Arctic species and a narrowing of Arctic habitats that have existed over millions of years such as multi-year ice and ice shelves. Under current climate scenarios, the loss of these unique ecosystems could be irreversible.
Arctic marine ecosystems are influenced by large-scale processes and their connectivity to the Pacific and Atlantic Oceans. However, the strong regionality in physicochemical conditions and in observed trends and their drivers precludes generalization of ecosystem responses to current and predicted environmental changes.
With continued warming and sea ice decline, measures should be put in place to monitor areas of particular biological significance and uniqueness in support of preservation and protection measures. One such area is N Greenland and the northeastern Canadian Archipelago, predicted to be the last refuge where multiyear ice and its associated species will persist.
Establishing a network of long-term biological observatories of marine ecosystems across the Arctic is highly recommended. It is essential that biological communities and ecosystem processes are characterized in conjunction with physico-chemical observations as part of monitoring activities in the Arctic.
Pan-Arctic coordination of research and monitoring activities, using standardized methods in Arctic oceanography and taking advantage of new technologies, is encouraged in order to document and forecast trends in Arctic marine ecosystem biodiversity.
Key species at all trophic levels and ecological processes that best allow characterization of marine food webs should be identified and included in future monitoring programs across the Arctic.
Concerted international efforts and associated national funding programs should be dedicated to better understanding changes in the functioning of Arctic marine ecosystems, including process studies to relate