The term euphotic (or photic) zone refers to the zone of a given aquatic ecosystem in which there is an optimal level of incoming sunlight, sufficient to allow photosynthesis by plants and photosynthetic bacteria.
The term photic comes from the Greek phòs photós “light”.
In the euphotic zone, as a consequence, the primary production of organic matter deriving from photosynthesis exceeds the consumption of organic matter deriving from respiration.
The photic zone ends with the so-called light compensation level, in which photosynthesis equals respiration. In water masses below this level, respiration is greater than production.