Zooplankton are a type of plankton that obtains its carbon from other organic compounds. They are organisms drifting in the water of oceans, seas, and bodies of fresh water. The name of zooplankton is derived from the Greek zoon, meaning animal, and planktos, meaning wanderer or drifter. Most zooplankton are too small to be seen individually with the naked eye.

They are a broad categorisation spanning a range of organism sizes that includes both small protozoans and large metazoans. It includes holoplanktonic organisms whose complete life cycle lies within the plankton, and meroplanktonic organisms that spend part of their life cycle in the plankton before graduating to either the nekton or a sessile, benthic existence. Although zooplankton are primarily transported by ambient water currents, many have some power of locomotion and use this to avoid predators or to increase prey encounter rate.

Ecologically important protozoan zooplankton groups include the foraminiferans, radiolarians and dinoflagellates. Important metazoan zooplankton include cnidarians such as jellyfish and the Portuguese Man o’ War, crustaceans such as copepods and krill, chaetognaths or arrow worms, mollusks such as pteropods, and chordates such as salps and juvenile fish. This wide phylogenetic range includes a similarly wide range in feeding behavior such as filter feeding, predation and symbiosis with autotrophic phytoplankton as seen in corals. Zooplankton feed on bacterioplankton, phytoplankton, other zooplankton, detritus and nektonic organisms. As a result, zooplankton are primarily found in surface waters where food resources are most abundant.

Through their consumption and processing of phytoplankton and other food sources, zooplankton play an important role in aquatic food webs, both as a resource for consumers on higher trophic levels including fish, and as a conduit for packaging the organic material in the ecology. Since they are typically of small size, zooplankton can respond relatively rapidly to increases in phytoplankton abundance, for instance, during the spring bloom.

Crude oil and natural gas are the preserved remains of prehistoric zooplankton and algae which had settled to a sea or lake bottom in large quantities under conditions of depleted oxygen. Over geological time the organic matter mixed with mud and was buried under heavy layers of sediment resulting in high levels of heat and pressure. This caused the organic matter to chemically change, first into a waxy material known as kerogen which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis.

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