Plankton are drifting organisms that live in the surface layers of the ocean. They live in the top layer of the ocean, called the epipelagic zone. They are not strong enough to swim against ocean currents. The term is in contrast to nekton, who can control their movements. There are three groups:
Phytoplankton: which live at the surface of the ocean and photosynthesise (use light to make sugars and other molecules).
Eukaryote algae: diatoms, coccolithophores, some dinoflagellates.
Bacteria: cyanobacteria.
Zooplankton: small protozoans or metazoans: Ctenophores; jellyfish; rotifers; foraminifera; tiny crustacea and other animals. Some eggs and larvae of larger animals such as fish, crustaceans, and annelids. Apart from the eggs, they all feed on other plankton. Some groups fall into both categories. Dinoflagellates can be either photosynthetic producers or heterotroph consumers; many species are mixotrophic depending upon their circumstances. It is also hard to fit viruses into this scheme; yet they are present in great numbers.

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