Photic Zone

The photic zone is the sunlit upper layer of the ocean where enough light penetrates to support net photosynthesis, also called the euphotic zone. It extends to the depth where photosynthetically available radiation falls to about 1 percent of its surface value, roughly 200 m in the clearest open ocean and only a few meters in turbid coastal water. Nearly all marine primary production and the base of the carbon-fixing biological pump occur here. Below it lies the dysphotic (twilight) zone and then the aphotic deep ocean. Acidification now pushes the aragonite saturation horizon up toward the photic zone in upwelling margins.