The term benthos indicates the collective set or assemblage of organisms at or near the bottom of seas, lakes, and rivers. Many of these organisms are relatively or completely sedentary, moving very slowly or being fixed to the bottom. These assemblages can be further divided into those that live either on the bottom itself or on other organisms—the epifauna (animals) and epiflora (plants)—and animal assemblages that live just below the surface—the infauna. Fishes and other more motile organisms that are habitually part of the benthic system but that maintain the capacity to swim from place to place are described as being demersal.

The benthos contrasts with pelagic organisms in marine and freshwater environments (also called limnetic in freshwater systems). Many organisms spend parts of their life cycles inhabiting both the benthos and the pelagic environments. For example, many types of organisms have a larval stage that disperses as part of the drifting plankton in the pelagic zone before settling to the bottom and developing into a benthic juvenile stage. Other species may reproduce in part within the benthos but have adults that inhabit the pelagic zone as swimming (that is, nektonic) individuals.

Many benthic organisms, especially certain algae, sponges, corals (see Cnidarians and Coral Reefs), and worms (see Annelids) that create substantial biogenic structures as they grow, are important "ecosystem engineers" within the benthos. These organisms contribute structural complexity to various marine habitats, and therefore provide new microenvironments for other benthic organisms.

—Daniel R. Brumbaugh See also: Ecology; Ecosystems; Lakes; Oceans Bibliography

Dodson, Stanley I., et al. 1998. Ecology. Oxford: Oxford University Press; McNaughton, Samuel J., and

Larry L. Wolf. 1979. General Ecology, 2d ed. New York: Holt, Rinehart and Winston; Putman, Rory J. 1994. Community Ecology. London: Chapman and Hall.

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