Plankton

Plankton are the organisms that live suspended in water and are swept along by tides and currents because they are too small or too weak to swim against them. Most are microscopic, but the definition depends on how an organism moves rather than its size, so some jellyfish count as plankton. Plankton live in both seawater and fresh water.

Phytoplankton are photosynthetic: mostly single-celled algae such as diatoms and dinoflagellates, along with cyanobacteria. They live in the sunlit surface layer and carry out about half of all photosynthesis on Earth, releasing much of the oxygen in the atmosphere and taking up carbon dioxide. Zooplankton are animals and animal-like protists, such as copepods, krill and other small crustaceans, along with the larvae of fish, crabs and many other animals that drift only while young. Drifting bacteria and viruses are sometimes called bacterioplankton and virioplankton.

Phytoplankton form the base of most aquatic food webs: zooplankton graze on them and are in turn eaten by fish, seabirds and even the largest whales. Many zooplankton rise toward the surface at night to feed and sink deeper by day. Sudden overgrowths of phytoplankton, called blooms, can cause harmful events such as red tides.