euphotic zone The upper, illuminated zone of aquatic ecosystems: it is above the compensation level and therefore the zone of effective photosynthesis. In marine ecosystems it is much thinner than the deeper aphotic zone, typically reaching 30 m in coastal waters but extending to 100–200 m in open ocean waters. In freshwater ecosystems it is subdivided into littoral (shallow edge) and limnetic zones. Compare aphotic zone; dysphotic zone; photic zone; and profundal zone.

euphotic zone The upper, illuminated zone of acquatic ecosystems: it is above the compensation level and therefore the zone of effective photosynthesis. In marine ecosystems it is much thinner than the deeper aphotic zone, typically reaching 30 m in coastal waters but extending to 100–200 m in open ocean waters. In freshwater ecosystems it is subdivided into littoral (shallow edge) and limnetic zones. Compare APHOTIC ZONE; DYSPHOTIC ZONE; PHOTIC ZONE; and PROFUNDAL ZONE.

euphotic zone (epipelagic zone; photic zone) The topmost layer of a lake or sea in which there is sufficient light for net primary production, i.e. where the energy fixed by photosynthesis exceeds that lost by respiration. The depth varies, depending on such factors as turbidity, supply of nutrients in the water, tidal turbulence, and temperature. For example, high nutrient levels will encourage a greater biomass of phytoplankton near the surface, which causes shading and consequent reduction in depth of the euphotic zone. It typically ranges from 1 m to about 30 m in lakes and coastal waters, and rarely reaches depths of more than 200 m in the open ocean. At depths between 200 and 1000 m blue light may still penetrate sufficiently to allow limited photosynthesis. This is sometimes referred to as the dysphotic zone or mesopelagic zone. Below this is the aphotic zone, where no light penetrates.

euphotic zone The upper, illuminated zone of an aquatic ecosystem.

euphotic zone See PRIMARY PRODUCTIVITY.