Plankton are any of the very small floating or drifting life forms of the ocean, especially those near the surface. There are two main kinds of plankton: phytoplankton, which comprises floating plants such as algae, diatoms, and dinoflagellates; and zooplankton, which are floating animals such as radiolarians, plus the larvae and eggs of larger marine animals. Phytoplankton may be so numerous as to color the water and cause it to have a "bloom;" they are eaten by zooplankton, which in turn are an important food for large animals such as whales and countless fishes such as herring.
Phytoplankton are confined to the upper layers of the sea where light can reach, but zooplankton have been found at great depths. Plankton drift under the influence of ocean currents and are vitally important links in the marine food chain.