The euphotic zone (from Greek εὖ eu , "good", and φῶς phōs , Gen. φωτός phōtos " light "), also called euphotic region , photic zone or photic layer , is defined as the upper, light-flooded layer of water in one Sea or inland still waters, in which effective photosynthesis is possible and thus single-cell algae and plants can grow and produce oxygen .
With depth, the light decreases so much (through scattering and absorption ) that more energy is consumed by breathing than can be produced by photosynthesis. Therefore, algae and water plants can no longer live at a relatively shallow depth, although there is still residual light ( dysphotic zone ).
Light of different wavelengths is not attenuated to the same extent: red light (approx. 650 nm) is only represented with 1% at a depth of 10 m, while blue light (approx. 450 nm) penetrates the furthest (in clear water up to 150 m Depth). The aphotic zone begins where the last daylight has disappeared .