The sea ice around Antarctica forms a huge, seasonally growing and shrinking habitat for numerous species. But not all sea ice is the same. More specifically, sea ice provides many different habitats for different species: In the pack ice ((annual and perennial), in fast ice, between platelet ice, and in channels or pockets in the ice. But climate change is altering these different sea ice habitats for ice algae, krill and other zooplankton, fish, and large animals like penguins and seals. How will these speciesrespond?
To help answer this question, a 27-member international research team conducted the first Marine Ecosystem Assessment for the Southern Ocean (MEASO) to examine what climate-driven changes mean for species associated with sea ice in the lower levels of the food web.
Their study, published in the journal Frontiers in Ecology and Evolution, first reviews existing knowledge on the relationships between sea ice and associated ice algae and the species that depend on it, their status, and what causes changes in sea ice.
In the ice-covered regions of the Southern Ocean, algae that live in the sea ice – especially diatoms – contribute significantly to primary production. This benefits copepods, various krill species and salps, which not only find food in or under the ice, but also shelter from predators. In addition, the sea ice acts as a nursery for many of them. Fish, such as the Antarctic silverfish Pleuragramma antarcticum, also use sea ice as a habitat. They lay their eggs in platelet ice that forms under the fast ice in Terra Nova Bay in the Ross Sea. The ice flakes provide ideal protection from predators for the eggs and larvae.
The research team then used qualitative network models to examine the responses of sea ice flora and fauna to disturbances such as rising air and sea temperatures, increasing storms, and shortening of the annual sea ice duration. They concluded that algae, copepods, krill, and fish respond negatively to warming and a shortened sea ice phase and are likely to decline. Salp populations, on the other hand, would benefit from shorter sea ice duration and an increased number of days with temperatures above 0°C and would likely increase.
The increase in storms appears to lead to a decrease in ice algae and phytoplankton because of the associated decrease in light availability, even though more nutrients would be available due to better mixing. Krill and copepods, on the other hand, would have more habitat available due to a larger ice volume, which in turn could benefit their predators.
With less ice cover, krill, copepods, and fish that rely on the presence of sea ice would most likely fare poorly. In contrast, salps develop better under conditions with less sea ice.
In addition, the team also examined the differences between the various MEASO sectors of the Southern Ocean: Atlantic, Western Pacific, Eastern Pacific, and Indian sectors. They found that upwelling of relatively warm circumpolar deep water can lead to a decrease in platelet ice in the East Pacific and Indian sectors compared to the Atlantic sector. Thus, the number of Antarctic silverfish there would also decrease. Silverfish are important prey for Weddell seals, whales, Adélie penguins, emperor penguins, and other seabirds. On the western Antarctic Peninsula, where they were once very common, their numbers have already declined sharply.
While the model calculations of the current study provided important insights into possible responses of the sea ice biota to climate-driven changes, the team emphasized that data collection to date is patchy and large areas of the Southern Ocean sea ice zone have never been studied or have been studied over too short a time period. As a result, they call for “rigorous scientific data collection drawn from species and ecosystems” across all sectors to improve predictions of sea ice species responses. Ideally, this should involve long-term cross-disciplinary monitoring of the coupled interactive sea ice-ocean–atmosphere-biology-biogeochemistry system in key regions, the authors say.
Julia Hager, PolarJournal
Featured image: Michael Wenger
Link to study: Swadling Kerrie M. et al, Biological responses to change in Antarctic sea ice habitats. Frontiers in Ecology and Evolution 10, 2023. DOI 10.3389/fevo.2022.1073823.
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