The euphotic zone, or photic zone, is the area of the ocean surface in which enough light penetrates the water for the photosynthesis of algae and other photosynthetic plants to occur.
The lower limit of this zone is determined by the fraction of residual light present in the environment. Below 1% of the surface incident light, the photic zone ends and the aphotic zone begins.
The depth of the euphotic zone therefore depends on the penetration of sunlight, in other words, the water's turbidity.
An area of primary production, this ocean zone is one of the most biologically rich (the other being areas around hydrothermal sources).
The euphotic zone is equivalent to the epipelagic zone. © Finlay McWalter, Wikimedia public domain