The Photic zone reference article from the English Wikipedia on 24-Jul-2004
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Photic zone

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The photic zone is the depth of the water, whether in a lake or an ocean, that is exposed to sufficient sunlight for photosynthesis to occur. The depth of the photic zone can be greatly affected by seasonal turbidity. 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,

The aphotic zone is that portion of the ocean that is exposed to no direct sunlight.

The transparency of water, which affects the photic zone, is simply measured with a Secchi disk.

See also: Pelagic zone