<P> Plankton (singular plankter) are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current . They provide a crucial source of food to many large aquatic organisms, such as fish and whales . </P> <P> These organisms include bacteria, archaea, algae, protozoa and drifting or floating animals that inhabit--for example--the pelagic zone of oceans, seas, or bodies of fresh water . Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification . </P> <P> Though many planktonic species are microscopic in size, plankton includes organisms over a wide range of sizes, including large organisms such as jellyfish . Technically the term does not include organisms on the surface of the water, which are called pleuston--or those that swim actively in the water, which are called nekton . </P> <P> The name plankton is derived from the Greek adjective πλαγκτός (planktos), meaning errant, and by extension, wanderer or drifter, and was coined by Victor Hensen in 1887 . While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by the surrounding water movement, and plankton typically flow with ocean currents . This is in contrast to nekton organisms, such as fish, squid and marine mammals, which can swim against the ambient flow and control their position in the environment . </P>

What are the largest members of the plankton