Plankton is made up of organisms which live in suspension in the water of lakes and oceans. Plankton is distinguished from nekton by its inability to swim and direct its movement, as do fishes or cephalopods. The last major environmental group is benton, which live on the sea floor or on the hulls of ships. In the initial stages of their life cycle, many nektonic and benthic organisms are part of the plankton, in the form of eggs or larvae.
Plankton, therefore, include a great variety of organisms, from cyanobacteria that populated the Earth's early oceans, up to huge jellyfish banks, which proliferate in present day seas.
Plankton in fact comprise almost all marine biomass and, being composed mostly of microscopic creatures with a short life cycle, are a key element in the great balance of the terrestrial biosphere.
Plankton biological activity is crucial to oxygen production, carbon sink and cloud formation.