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euphotic zone

Definition

For the purpose of biological investigations, it is convenient to consider the sea as divided vertically into three zones with respect to the amount of light that is present. These are: (a) the euphotic zone, which is abundantly supplied with light sufficient for the photosynthetic processes of plants (in oceans down to 80 m); (b) the disphotic zone, which is only dimly lighted and extends in the open ocean from about 80 m to 200 or more metres. No effective plant production can take place here; (c) the aphotic zone, the lightless region below the disphotic zone. The depth of the euphotic zone varies with the water's extinction coefficient, the angle of incidence of the sunlight, the length of day and cloudiness.

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