Krill form the base of much of the Antarctic marine food web. These small, shrimp-like crustaceans feed mainly on microscopic algae and phytoplankton, then convert that production into food for fish, penguins, seals, whales, and seabirds.
The most important species in this system is Antarctic krill, Euphausia superba. They gather in enormous swarms that can extend for kilometers and may contain vast numbers of individuals. During the Antarctic summer, krill feed intensively beneath sea ice and in open water, taking advantage of seasonal algal growth.
Krill are especially important because so many larger animals depend on them. Baleen whales filter them from the water, while Adélie and emperor penguins, crabeater seals, and many seabirds eat them directly. When krill numbers or distribution change, the effects can spread through the entire ecosystem.
“Plankton” is a broader term for drifting organisms, not a single animal species. Amphipods and copepods are also important Antarctic prey, but krill are the classic and most widely accepted answer to this question.