Vulnerability of Southern Ocean biota to climate change

Marine algae contribute to the production of oxygen and sequestration of atmospheric CO2 through photosynthesis, while producing organic matter that supports higher levels in the food web. Whilst phytoplankton growth in open water is predicted to increase in areas of sea-ice reduction (and has locally been observed) (4, 5), it also decreases when nutrients are reduced due to consumption and increased water stratification. Shifts in species composition from larger to smaller diatoms and phytoflagellates are driven by changes in sea-ice and ocean dynamics as well as by glacial melt-induced salinity variability (Figure 2) (5, 6). Since phytoplankton is the foundation of the marine food web, such climate sensitive changes can affect the entire marine ecosystem. The complexity and high biological diversity of other Antarctic microbial groups (bacteria, archaea and unicellular animals) pose additional challenges to assessing ecosystem vulnerability to climate change. These organisms are inextricably linked to phytoplankton and play critical roles in ecosystem functioning and interactions (e.g. biogeochemical, symbiosis, predation; 7).

Microalgae in the lower layers of sea-ice are typically 1000-times denser than in seawater. They are a rich food source for zooplankton as well as for sea-floor communities (benthos) when ice melts and the microalgae sink towards the sea-bed. In scenarios of sea-ice reduction, many species will see their distribution diminish. Abundant copepod species like Drescheriella glacialis need sea-ice as a breeding or nursery ground (8). Antarctic krill (Euphausia superba) and silverfish (Pleuragramma antarctica) also depend on sea-ice during part of their life cycle. They are dominant species and represent energy-rich trophic links between plankton and top predators. Apparently, recruitment failure in krill and silverfish is a consequence of warming and sea-ice retreat (9, 10). Their local disappearance and partial replacement by energy-poor gelatinous salps at the Antarctic Peninsula provides evidence for their vulnerability and first responses to climate change. The decline of these key species and other sea-ice biota will have cascading effects on higher trophic levels and consequences for long-term ecosystem health.

Zooplankton are a crucial link in the food web because they feed on phytoplankton and microbes and provide food for higher organisms. Increases in water temperature will probably see species adapted to extreme cold decrease in number, (e.g. the ice krill Euphausia cystallorophias) in favour of warm-water tolerant species such as the small copepod Oithona similis. It is predicted that warm-water species will also move southward and may displace cold-water species near the continent, which will be unable to escape any further south. On the other hand, foraminiferans are increasing in number in the short term and in some regions, for complex ecological reasons (11). Ocean acidification poses an additional threat to Antarctic ecosystems. It can affect organism development and physiology not only for species with calcium carbonate shells, e.g. pteropods (sea butterfly snails) but also for krill, resulting in long-term declines in populations. This will affect the entire marine ecosystem and may alter the uptake of atmospheric CO2 by the ocean.

The benthos contributes significantly to the cycling of carbon and nutrients and to the overall marine biodiversity. In laboratory studies, Antarctic benthic invertebrates, especially larger species, are less resistant to warming than temperate species even though they have been observed to survive temperature increases in the range predicted, which are already occurring West of the Antarctic Peninsula (Figure 3) (3, 12, 13). While some species do have a high thermal tolerance, a comprehensive knowledge of response thresholds to environmental change is generally poor.

Not only single species, but whole communities can be affected by climate change (17). This happens with increases in iceberg scouring, glacier retreat, and changes in sea-ice cover (18, 19). The entire marine ecosystem is also changed by ice shelf disintegration when an extremely nutrient-poor system shifts to more “normal” Antarctic conditions with intensive growth of microalgae serving as food for consumers in a short summer period (20). As a consequence, benthic organisms like sponges, sea-squirts, and brittle stars, can exhibit unexpectedly fast population growth and high mortality (Figure 5) (20, 21).Generally the vulnerabilities of such systems can give rise to top down cascading effects through the food-web, to competitive or bottom-up forces depending on which types of organisms are affected and how (17). The self-repair capacity (resilience) is not known for many species or communities, though evolutionary processes may result in timely genetic adaptation, especially in microorganisms.

Do we already know enough about the impact of climate change on Southern Ocean biodiversity, ecological processes, and the ecosystem services they provide? No – we need more interdisciplinary and internationally coordinated surveys to monitor key species, their adaptability and the environment that shapes them. Long-term in situ experiments, studies of ecosystem linkages and development of future simulations that include effects of multiple stressors and non-linear biological interactions are needed (19, 22). Finally, such information should provide the basis for a comprehensive assessment of Southern Ocean biota vulnerability, sound conservation measures and effective management actions.