Photic zone
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The photic zone or euphotic zone (Greek 'well lit') is the depth of the water in a lake or ocean, that is exposed to sufficient sunlight for photosynthesis to occur. The depth of the photic zone can be greatly affected by seasonal turbidity. It extends from the atmosphere-water interface downwards to a depth where light intensity falls to 1% of that at the surface (also called euphotic depth), so its thickness depends on the extent of light attenuation in the water column. Typical euphotic depths vary from only a few centimetres in highly turbid eutrophic lakes, to around 200 metres in the open ocean. Since the photic zone is the only zone of water where primary productivity occurs (an exception being the productivity connected with abyssal hydrothermal vents along mid-oceanic ridges), the depth of the photic zone is generally proportional to the level of primary productivity that occurs in that area of the ocean. About 90% of all marine life lives in this region.
The transparency of the water, which determines the depth of the photic zone, is simply measured with a Secchi disk.
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