photic zone

noun Biology.
the upper layer of a body of water delineated by the depth to which enough sunlight can penetrate to permit photosynthesis.

1970–75 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
Cite This Source Link To photic zone's 21st Century Lexicon
Main Entry:  photic zone
Part of Speech:  n
Definition:  the upper part of a body of water that is exposed to sunlight and permits photosynthesis; also called euphotic zone
Example:  The photic zone is usually less than 100 meters in depth.'s 21st Century Lexicon
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Encyclopedia Britannica

photic zone

surface layer of the ocean that receives sunlight. The uppermost 80 m (260 feet) or more of the ocean, which is sufficiently illuminated to permit photosynthesis by phytoplankton and plants, is called the euphotic zone. Sunlight insufficient for photosynthesis illuminates the disphotic zone, which extends from the base of the euphotic zone to about 200 m. The thicknesses of the photic and euphotic zones vary with the intensity of sunlight as a function of season and latitude and with the degree of water turbidity. The bottommost, or aphotic, zone is the region of perpetual darkness that lies beneath the photic zone and includes most of the ocean waters

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
It's a beautiful thing and also a little daunting, saying goodbye to the sun as
  you sink deep down past the photic zone.
At fall overturn, the nutrients and other compounds may reenter the photic zone
  and stimulate algal productivity.
The depth of the photic zone in a given image is a factor of the sediment load,
  sea conditions, and image quality.
Virtually all marine phytoplankton are buoyant and live in the upper part of
  the water column called the photic zone.
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