Marine and freshwater organisms that, because they are unable to move or are too small or too weak to swim against water currents, exist in a drifting, floating state. Plankton is the productive base of both marine and freshwater ecosystems, providing food for larger animals and indirectly for humans, whose fisheries depend on plankton. As a human resource, plankton has only begun to be developed and used. The plantlike community of plankton is called phytoplankton, and the animal-like community is called zooplankton, but many planktonic organisms are better described as protists. Most phytoplankton serves as food for zooplankton, but some of it is carried below the light zone. Zooplankton is used directly as food by fish (including herring) or mammals (including whales), but several links on the food chain usually have been passed before plankton is available for human consumption.

This entry comes from Encyclopædia Britannica Concise.
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