Plankton are the aggregate community of weakly swimming or drifting microorganisms that inhabit all depths of the ocean and bodies of freshwater, and whose study is termed planktology. The name comes from the Greek language - term for "wanderer" or "drifter". While some forms of plankton can move several hundreds of meters vertically in a single day (a behavior called diel vertical migration), their horizontal position is controlled mainly by the currents in the body of water they inhabit. Larger organisms such as squid, fish and marine mammals that can control their horizontal movement and swim against the mean flow of their environment are called nekton. Most planktons will be consumed by many aquatic species, such as fish, marine mammals, bryozoans, and so on.

Plankton are divided into two classeses:

Plankton concentration and distribution is very sensitive to chemical and physical changes in the sea water.

Photomontage. See also larger images.

Hyperiid amphipod, Hyperia macrocephala, ca. 20mm long

Copepod (Calanoida), ca. 1-2mm long

See also: algal bloom, domoic acid

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