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Concept
Aphotic zone
Summary
The aphotic zone (aphotic from Greek prefix ἀ- + φῶς "without light") is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. Above the aphotic zone is the photic zone, which consists of the euphotic zone and the disphotic zone. The euphotic zone is the layer of water in which there is enough light for net photosynthesis to occur. The disphotic zone, also known as the twilight zone, is the layer of water with enough light for predators to see but not enough for the rate of photosynthesis to be greater than the rate of respiration.
The depth at which less than one percent of sunlight reaches begins the aphotic zone. While most of the ocean's biomass lives in the photic zone, the majority of the ocean's water lies in the aphotic zone. Bioluminescence is more abundant than sunlight in this zone. Most food in this zone comes from dead organisms sinking to the bottom of the lake or ocean from overlying waters.
The depth of the aphotic zone can be greatly affected by such things as turbidity and the season of the year. The aphotic zone underlies the photic zone, which is that portion of a lake or ocean directly affected by sunlight.
In the ocean, the aphotic zone is sometimes referred to as the dark ocean. Depending on how it is defined, the aphotic zone of the ocean begins between depths of about to and extends to the ocean floor. The majority of the ocean is aphotic, with the average depth of the sea being deep with the deepest part of the sea, being the Challenger Deep in the Mariana Trench, is about deep. The depth at which the aphotic zone begins in the ocean depends on many factors. In clear, tropical water sunlight can penetrate deeper and so the aphotic zone starts at greater depths. Around the poles, the angle of the sunlight means it does not penetrate as deeply so the aphotic zone is shallower. If the water is turbid, suspended material can block light from penetrating, resulting in a shallower aphotic zone.
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