The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur. The photic zone is the only zone of water where primary productivity occurs.
The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. Consequently, bioluminescence is essentially the only light found in this zone. Most food comes from dead organisms sinking to the bottom of the lake or ocean from overlying waters.