Melting ice is an emblem of climate change. For sea ice, the Arctic has long grabbed most headlines for its alarming rate of decline. Recently, however, Antarctica has begun to follow suit.
About a decade ago, everything changed. After decades of relative stability, and within just a few years, an ocean area nearly the size of Greenland suddenly became sea-ice free. At first, scientists suspected it might be a temporary blip. Now it is described as a step change, with large ocean areas remaining ice-free ever since.
This shift has serious consequences for Antarctica’s marine life. The decline in ice was so abrupt that it challenged many existing computer models of the Southern Ocean and its ecosystems. Models do not predict step changes well. Field researchers, too, could not respond quickly enough to document how the sudden loss of sea ice was affecting the plants and animals that depend on it.
Our 2025 study examined the loss of sea ice from a different perspective. We used satellite imagery to identify the wavelengths of light reflected from the upper ocean back into space.
Just as landscapes can be classified, we divided the ocean into distinct “seascapes” based on the light they reflect. This reveals information about phytoplankton — the microscopic drifting algae that support the rest of the marine food web. Changes in light reflections show both how much phytoplankton is present and which types dominate.
Surprisingly, we found that large and remote areas of the Southern Ocean shifted from very low concentrations of phytoplankton to more moderate levels. Nearly 70% of the Southern Ocean now has, on average, more phytoplankton than before the ice decline about ten years ago.
At first glance, an increase in food supply might seem positive. But sea ice supports unique marine ecosystems in several ways. It provides shelter and nursery spaces within its structure. Sea ice also nurtures hotspots of food, supporting large algae called diatoms that are easily consumed and passed up Antarctic food chains.
Diatoms are a key food source for Antarctic krill — shrimp-like crustaceans that also rely on sea ice as a nursery habitat. Krill, in turn, are eaten by penguins, whales and many other marine species. They are also targeted by a major fishery worth hundreds of millions of dollars.
However, krill do not appear to be benefiting from the increase in phytoplankton following the dramatic loss of sea ice. Instead, gelatinous filter feeders known as salps are associated with the expanding ice-free seascapes.
Another recent study sheds further light on what may be happening. It found that the step change in sea ice triggered a sudden shift in phytoplankton composition. Tiny phytoplankton known as cryptophytes began increasing rapidly after the ice loss.
Research is only beginning to map how this “new normal” of reduced sea ice is reshaping Antarctic ecosystems. Any long-term shift in the balance between krill and salps could have significant consequences for Southern Ocean ecosystems and their role in nutrient cycling.
The Conversation