Satellite images show how Antarctica’s vanishing sea ice is changing the food chain
Melting ice is an emblem of climate change. For sea ice, the Arctic has been grabbing most of the headlines for its truly alarming rate of decline. But recently Antarctica has followed suit.
Around ten years ago everything changed. After decades of stability and within just a few years, an ocean area nearly the size of Greenland suddenly became sea-ice free. At first, scientists thought this could be a blip, but now it is described as a step change, with large ocean areas remaining ice free ever since.
This has dramatic consequences for the marine life of Antarctica. The ice decline was so sudden it challenged most existing computer models of the Southern Ocean and its ecosystems. Models tend not to predict step changes very well. Likewise, due to the sheer seismic suddenness of ice loss, the boots-on-the-ground fieldworkers could not scramble fast enough to document how the loss of sea ice was affecting the plants and animals living here.
Our 2025 study looked at ice loss from a different perspective. We used satellite imagery to pinpoint the exact wavelengths of light that are reflected from the upper ocean back into space.
Just like landscapes can be classified, we divided the ocean into distinct “seascapes”, based on the wavelengths of light that they reflect. This tells us about the phytoplankton – the tiny drifting planktonic algae that support the rest of the food web. Changes in light reflections indicate how much phytoplankton is present and also which types of species are present.
Surprisingly, we found that large and remote expanses of the Southern Ocean actually increased from very low concentrations of phytoplankton to more moderate levels. Nearly 70% of the Southern Ocean now has, on average, more phytoplankton since the ice declined around ten years ago.
This increase in food supply might sound good. But sea ice supports unique marine ecosystems, and in many ways. For example, it provides nooks and crannies for shelter and nursery. Sea ice also nurtures hotspots of food, supporting large algae called diatoms that are easily eaten and passed up Antarctic food chains.
Diatoms are a key food source for Antarctic krill, shrimp-like crustaceans which also need sea ice as a nursery habitat. Krill in turn are the food source for penguins, whales and other marine species, as well as being the target species for an important fishery valued in hundreds of millions of dollars.
Winners and losers
Krill do not seem to be benefiting from the increases in phytoplankton after the dramatic loss of sea ice. Instead, gelatinous filter feeders known as “salps” associate with the ice-free seascapes that have increased in size.
Salps are a colonial, barrel-shaped group of species that pump water through their transparent bodies, filtering out even the smallest phytoplankton. They are more nutritious than most jellyfish, but much less carbon rich than crustaceans such as krill, who help in the storage of carbon at depth.
A study by another team sheds more light on what was happening. They showed that the step-change in sea ice marked a sudden shift in phytoplankton composition. Suddenly, a group of tiny phytoplankton called cryptophytes started increasing.
Salps act like marine vacuum cleaners that can rapidly and efficiently remove even these small cryptophytes from the water. It looks like the recent low ice era has changed large expanses of ocean from having too little food even for salps into that sweet spot – not super-rich but just good enough for these vacuum cleaners to thrive.
These studies are just starting to map how the “new-normal” low-ice era is reshaping Antarctic ecosystems. Salps are not fished commercially, do not appear so important in storing carbon, and support different types of food chain. Any long-term shift in the relative dominance of krill and salps will have far-reaching ramifications for Southern Ocean ecosystems and their role in nutrient cycling.
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Angus Atkinson receives funding from the European Space Agency (ESA). His contribution to this work was supported by ESA Cluster OCEAN HEALTH THEME 3: Open Ocean Biodiversity, grant number 4000137125/22/I-DT project Biodiversity in the Open Ocean: Mapping, Monitoring and Modelling (BOOMS). He was also supported by the World Wide Fund for Nature and the Natural Environment Research Council (NERC) National Capability (NC) International FOCUS Programme (grant no. NE/X006271/1)
Bob Brewin receives funding from a UK Research and Innovation Future Leader Fellowship (MR/V022792/1).
Victor Martinez Vicente receives funding from the European Space Agency (ESA). His contribution to this work was supported by ESA Cluster OCEAN HEALTH THEME 3: Open Ocean Biodiversity, grant number 4000137125/22/I-DT project Biodiversity in the Open Ocean: Mapping, Monitoring and Modelling (BOOMS)