The epipelagic zone is the ocean’s sunlit surface layer, extending from the surface to roughly 200 metres.
Enough light penetrates this zone for photosynthesis, so it contains much of the ocean’s primary production. Phytoplankton use sunlight to convert carbon dioxide and water into organic matter, forming the base of many marine food webs. Zooplankton, fish, turtles, seabirds, and marine mammals feed directly or indirectly on this production.
The exact depth of the photic layer varies with water clarity, season, and latitude. Clear tropical water can transmit useful light deeper than turbid coastal water. The epipelagic zone is therefore a practical ecological boundary rather than a perfectly fixed horizontal line.
Below it lies the mesopelagic, or twilight, zone, where light is too weak for significant photosynthesis. Many animals migrate between these layers each day: they feed near the surface at night and descend during daylight. This vertical movement is called diel vertical migration and transports carbon and nutrients through the water column.