Implications of the recent loss of Antarctic sea ice for phytoplankton and summer feeding habitats of salps and krill

Antarctic sea ice showed a profound, step-wise reduction around 2016-2017, but the scarcity of modern time series means that we know very little about how Southern Ocean biota have responded. Given the paucity of direct field data, we combined satellite data with KRILLBASE, a large historical salp and krill database, to examine how the new, low-ice era has changed the feeding habitat of these key plankton species. Most (69%) of the Southern Ocean area has experienced an increase in mean summer chlorophyll a (chl a) values since the step-change in sea ice. We went beyond bulk chl a indices to classify the Southern Ocean into 14 optical water types based on their spectral reflectance. At this finer resolution, the 2 species showed more habitat differentiation than discernible from chl a alone, with salps being strongly correlated with those optical water types that increased in area after the 2017 reduction in sea ice. These water types have moderate phytoplankton concentrations (~0.4 mg chl a m-3), and large expanses of the Indian-Pacific sector improved from being too oligotrophic into better feeding habitats for salps. We also show fundamental differences in habitat requirements of both species between the Atlantic and the Indian-Pacific sectors, but overall, our feeding habitat indices suggested that the modern low-ice era has become more favourable for salps. Antarctic sea ice is a crucial part of the climate system, and the recent era of extreme variability and record lows has major ramifications for food webs and biogeochemical cycles.

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