The Antarctic marine ecosystem in the year 2100

This page is part of the topic Biological responses to 21st climate climate change

“The complexity of the Southern Ocean food web and the non-linear nature of many interactions mean that predictions based on short-term studies of a small number of species are likely to be misleading” (Clarke et al., 2007). The most critical points for biological predictions are (i) the enormous complexity of living communities, including hundreds to thousands of parameters (species and their life traits; for their general sensitivity see Barnes and Peck, 2008), (ii) our limited knowledge of these, and (iii) the lack of fine-scale detail in predictions of change in the physical environment, e.g. of the likelihood and extent of extreme events, and of fine scale spatial resolution. Nevertheless, physical predictions have improved considerably compared to a few years ago, because they now include some measurements of regional as well as global climate change.

Polar ecosystems are experiencing significant environmental changes. In the Antarctic the retreat of glaciers and the disintegration of ice shelves, with waters underneath being the least explored marine systems on Earth, provide the most obvious impacts on coastal marine areas. In offshore systems, a shift of pelagic communities towards the south is interpreted as a consequence of regional changes in sea ice dynamics, especially West of the Antarctic Peninsula, although the average sea ice extent has changed little. There are first signs of increased water temperature also along the coast. Because of an assumed high sensitivity of such biological systems to changes, inshore and offshore waters around the Antarctic Peninsula and at the sea ice margin as well as along the Antarctic Convergence (Polar Front) are the main foci of ecological climate research.

The attempt made here to develop a scenario of how the Antarctic ecosystem might look in 2100 should be considered as a tool for identifying future research needs and recommendations to decision makers rather than as a true prediction. The most important constraint that militates against numerical forecasts is the reliability of a number of assumptions about the biological system and its environment. To make this “pre-stage” of a prediction as robust as possible, the concept is based only on coarse patterns. Air and ocean temperature are assumed to be the main climate-driven environmental parameters for the Southern Ocean ecosystem. In addition CO₂-triggered acidification and increased UV-radiation are likely to alter the ecosystem.

Substantially damage of the sea-floor ecosystem by grounding icebergs has been observed locally, following the climate-induced disintegration of the Larsen B ice shelf, and in very shallow water with a naturally high intensity of disturbance. By 2100 the Larsen C ice shelf and much smaller ice shelves west of the Antarctic Peninsula might also have collapsed, at least partly (Morris and Vaughan, 2003), and the northeast coast of the Antarctic Peninsula will have experienced a significantly elevated disturbance regime preventing the benthos from reaching a climax stage. After 2100, icebergs from the Filchner-Ronne ice shelf will continue to scour their way along the east coast of the Antarctic Peninsula, contributing to maintaining a patchwork of recolonization stages and, consequently, high benthic biodiversity. Iceberg calving events along the west coast of the Peninsula and around East Antarctica are also likely to increase as ice shelves collapse, so increasing benthic biodiversity, though not in so dramatic a manner as in the western Weddell Sea. Between now and 2100 elevated air and water temperature will not have risen to levels at which the large Filchner-Ronne and Ross Ice Shelves might disintegrate and cause large-scale circumpolar damage. A regional lessening or cessation of production of icebergs might cause local development of a benthic community in which competitive displacement leads to increased productivity but reduced biodiversity. An irreversible negative effect will be the loss of the unique marine biological system below the former ice shelves. These areas will serve as habitats of retreat for some vertebrate and invertebrate species that escape from warmed areas in the north and west of the Antarctic Peninsula, but these newly available areas will not completely compensate for habitat-loss elsewhere (J. Gutt, unpublished results).

The development of the sea ice plays a key role in the development of the pelagic as well as the open ocean ecosystem, and in intertidal communities; it has indirect effects on the sub-tidal benthos especially on the shelf but also in the deep-sea. Between now and 2100 all components of the ecosystem closely related to the sea ice will show a significant change in their ecological performance in response to the predicted 33% reduction of sea ice extent. A decline of krill by 38-80% in the Atlantic sector was already noticeable by 2004, having begun in the late 1970s (for most recent results and further references see Atkinson et al., 2008). Our ‘forecast’ is that this decline will not be compensated by increasing near-shore populations in the less affected “refuges” in East Antarctica between approximately 90°W and 105°W. In areas with an originally high krill population size the population is likely to stabilize at a low level. The implication is that over the long term all main krill consumers will experience a serious food limitation. The Minke Whale will lose 5-30% of its ice-associated habitat, while for Blue, Humpback, Fin and Sperm Whales a compressed foraging habitat along Southern Ocean fronts is suggested. This negative effect will be superimposed on the recently observed 9.6%/yr rate of increase in Humpback Whales and similar developments in other large species in recent decades, which runs counter to the observed decrease in krill. Due to the trophic complexity of this ecosystem it is impossible to foresee which animal group will suffer most, but some may suffer less than others while other may even become extinct, at least regionally. Colonies of Emperor and Adélie penguins, which are most closely adapted to a complex sea ice regime, but which are also affected by precipitation, will become extinct locally where their pack ice dominated habitat shifts to an open ocean system. They will be locally displaced by king, gentoo and chinstrap penguins. Long-term field data from an Emperor penguin colony in Terre Adélie (East Antarctica) showed very sensitive response to natural climate changes in both directions, when sea ice increased and decreased (Barbraud and Weimerskirch, 2001). A projection using predictions of warm events from IPCC climate scenarios and combined with data on the population dynamics of this East Antarctic colony forecast a 36% probability of a quasi-extinction by 2100, in an area with rather minor climate change impact (Jenouvrier et al., 2009). Since it will be difficult for this species to find sites for new colonies, the total net population will decrease significantly and the zoographic range will be compacted southward. Adélies will find new colonies where pack-ice becomes more divergent and at newly exposed coastlines when ice shelves disintegrate.

As in the case of penguins, the ice-bound Crabeater, Weddell, Leopard and Ross Seals will regionally become extinct due to changes in both, habitat and food web dynamics. The ice-tolerant Fur and Southern Elephant Seal will shift their geographical extent further south and regionally increase their population size – unless their food resources decline. The Antarctic toothfish also has an ice-related behaviour. A prediction of the effect of a 1.3°C temperature increase that could take place between the next 10-90 years shows a circumpolar decrease of population size. This species would only likely become extinct with an extreme warming coupled with a sea ice retreat of 2 km per year (Cheung et al., 2009). In essence most Antarctic species are adapted to changes in sea ice concentration and its extent. Species of any systematic group with a sufficient initial population size and circumpolar distribution are expected to survive at least in the Pacific sector south of Australia and New Zealand, where according to the predictions the sea ice is likely to remain relatively stable.

The effect of the future reductions of sea ice on primary production is likely to be more complex. Southernmost near-shore waters, which, under ‘normal’ conditions, have a low yearly primary production compared to areas like the Antarctic Peninsula, will become more productive as important components of the open ocean system move farther south (Whitehouse et al., 2008). This shift will coincide with a shift among unicellular algae from larger to smaller diatoms and generally from diatoms to Phaeocystis aggregations, both being less favourable food for krill. One side effect will be more emissions of precursors of dimethyl sufide (DMS) causing extra cloud cover. Using results from the “Climate System Model 1.4-carbon” and experiments on the ecology of phytoplankton, Boyd et al. (2007a) assume that “the rate of secular climate change will not exceed background variability, on seasonal to interannual time-scales, for at least several decades…”. These results might underline the high relevance of the sea ice and its biota to the already observed and expected changes in the Antarctic ecosystem. At the sea floor, increased primary production may present a problem for filter feeders that are principally adapted to low food supply rather than decreased production. However, a few opportunists, e.g. among mobile deposit feeders, are likely to benefit and to extend their distribution range. This process will change the functioning of the benthic system, with significant consequences for cycles of organic and inorganic carbon as well as silicate remineralization. The increase in production will also act to reduce diversity in the deep-sea and on the continental slope, where a few species will benefit above the average, coming to dominate the entire assemblage. The extent of extinction of species between now and 2100 will depend very much on the number of at present largely undiscovered “cryptic” species, those that are visually not discernible from very close relatives, if they have only a limited range of occurrence. Populations of pelagic invertebrates and fish may collapse at the regional level, but due to their good dispersal capacity will survive in refuge areas. In contrast to the above scenario, in which phytoplankton growth increases in coastal waters, we may see in offshore waters especially close to the Antarctic Convergence (Polar Front) a spatially and temporarily hardly predictable decrease of primary productivity resulting from decreased stratification, vertical mixing, and increased microbial oxygen consumption. Such a development may negatively affect higher trophic levels of the food limited pelagic ecosystem.

Between now and 2100 water temperatures will not rise by much more than 1ºC at the sea surface and down to 200 m depth in most areas. This is likely to have less of an effect on major components of the entire ecosystem than will the reduction in sea ice extent. It seems likely that only few species will become extinct as a direct effect of temperature increase, either because they proved unable to cope ecologically and physiologically with such an increase, or because they were restricted in their occurrence to an area with an above average temperature increase. Such biological changes are most likely at water depths down to 200 m, where a relatively high proportion of species is naturally used to environmental variability. Adaptation could result from a mobile life style, from effective dispersal of pelagic eggs and, or through ecological and physiological tolerance. The area with the highest benthic biodiversity and ecosystem functioning, at 150 – 300 m depth will remain much less affected since the predicted warming there is very low.

Most of the above projections do not consider the flexibility of organisms to “compensate” e.g. physiologically and ecologically for climate change impact. For example, two Emperor penguin colonies are known to live and reproduce even on land in contrast to their usual habitat, the sea ice. It can also not be excluded that adaptation through microevolution will become effective and have similar positive effects for a variety of species. While the potential for both mechanisms - flexibility and adaptation - might not be very high in long-lived and specialized species, it has to be recognized that the overwhelming majority of the species survived the last interglacial, when there was more warming than today. Nevertheless, some models of the future of the physical environment predict, at least regionally, temperatures that exceed those of the past interglacial. In this report we have chosen to use the average temperatures thrown up by IPCC 19 models, rather than any of the extremes.

The projected temperature changes are supposed to be insufficient to disrupt the thermal barriers that create the marine biological isolation of Antarctica, except locally where temperatures reach values significantly above average. As a consequence the invasion of species will likely remain restricted to areas where invaders can survive at their physiological limits. Under these circumstances it seems unlikely that invaders will have the potential to outcompete or diminish Antarctic species by predation pressure on a broad scale. In effect the Antarctic marine ecosystem will remain buffered from the direct effects of a global temperature-increase by the continued existence of the large and high ice sheet, which keeps the Antarctic cool and quite different from the Arctic. This assumption is confirmed by a newly developed bioclimate envelope model (Cheung et al., 2009), which predicts for 2050 an extremely sharp but non-continuous gradient between highly affected areas north of and close to the ACC and relatively stable conditions in high latitude Antarctic waters in terms of species invasion, local extinction, and turnover of metazoan species between. If, however, ocean temperatures increase by more than 2ºC from today’s, and ecologically important species are less temperature tolerant in nature than in ecophysiological laboratory experiments, assemblages that cannot escape to colder regions are likely to become extinct by 2100 (Barnes and Peck, 2008). These may comprise thousands of invertebrate and vertebrate, pelagic and benthic species.

Our knowledge of the response of Antarctic calcifying organisms to acidification is extremely poor. Such species (coccolithorphorids and pteropods) can reach locally or regionally dense concentrations offshore, with pteropods also reaching high concentrations inshore at high latitudes. At the seabed, calcifying echinoderms, hydrocorals, bryozoans, and molluscs can shape local biodiversity or abundance hot-spots. Undersaturated conditions might become the most severe climate-induced negative impact on Antarctic benthic communities if key species cannot cope with increase pH, as in the case of the common Antarctic sea-urchin Sterechinus neumayeri; however, some species may even be winners in a more acid ocean, e.g. tunicates (Dupont and Thorndyke, 2009).

One of the most unpredictable parameters that shapes the Antarctic ecosystem is human behaviour. It has ranged from the enormous exploitation of natural resources in the past to the later protection not only of most of these but also of mineral resources. It still ranges on the one hand from more or less continuous global emissions of CO₂ to the atmosphere to proposed geoengineering projects for the Southern Ocean, both of which will or may cause irreversible ecological damage (Smith et al., 2008), to on the other hand the Madrid Protocol, one of the most strict international laws for protecting our global environment and to plans to identify, propose and declare High Seas Marine Protected Areas (HSMPAs) and Vulnerable Marine Ecosystems (VMEs).