Quick Definition
The sunlit upper layer of water where there is sufficient light for photosynthesis, typically the top 200 m in clear ocean waters.
📖 Full Definition
The euphotic zone (also called the photic zone) extends from the surface to the depth at which light intensity falls to 1% of surface irradiance. In clear, tropical waters this may be 200 m; in turbid coastal or estuarine water it may be less than 1 m. All primary production by phytoplankton occurs within this zone. It is the most productive zone for fish because it sustains the food web from the base up. The lower boundary marks the compensation depth where phytoplankton photosynthesis equals respiration.