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The photic zone, euphotic zone (Greek for "well lit": εὖ "well" + φῶς "light"), or sunlight (or sunlit) zone is the uppermost layer of water in a lake or ocean that is exposed to intense sunlight. It corresponds roughly to the layer above the compensation point, i.e. depth where the rate of photosynthesis equals the rate of respiration over a day or more. Within the photic zone carbon dioxide uptake and oxygen production are therefore positive. This is an important definition for biology but very hard to measure.
The euphotic depth is also often calculated, largely through ease of measurement, to be the depth where light intensity falls to one percent of that at the surface. Accordingly, its thickness depends on the extent of light attenuation in the water column but as incoming light at the surface can vary widely, this says little about the net growth of phytoplankton. Typical euphotic depths vary from only a few centimetres in highly turbid eutrophic lakes, to around 200 meters in the open ocean. It also varies with seasonal changes in turbidity, which can be strongly driven by phytoplankton concentrations such that the depth of the photic zone often decreases as primary production increases.
About 90% of all marine life lives in the photic zone. A small amount of primary production is generated deep in the abyssal zone around the hydrothermal vents which exist along some mid-oceanic ridges.
The zone which extends from the base of the euphotic zone to about 200 metres is sometimes called the disphotic zone. While there is some light, it is insufficient for photosynthesis, or at least insufficient for photosynthesis at a rate greater than respiration. The euphotic zone together with the disphotic zone coincides with the epipelagic zone. The zone below the euphotic zone, is called the aphotic zone. Most deep ocean waters belong to this zone but there are still notable variations of light level within it.