Defined as without light (Dark zone). Of or relating to the region of a body of water that is not reached by sunlight and in which Photosynthesis is unable to occur. The Aphotic Zone of the ocean is the water deeper than about 800 meters (2,625 feet), beyond which no light penetrates. Aphotic zone is the zone in which most photosynthetic algae cannot survive, due to light deficiency. Contrast with Photic Zone.