Some twilight occurs in the mesopelagic zone, but creatures below the mesopelagic must be able to live in complete darkness. The hadal zone refers to the greatest depths, deeper than the abyssal zone. The mesopelagic zone extends from 200 metres (656 ft) to 2,000 metres (6,562 ft). The aphotic zone is further divided into the mesopelagic, bathyal, abyssal, and hadal zones. Temperatures can range from roughly 0 ☌ (32 ☏) to 6 ☌ (43 ☏). If the water is turbid, suspended material can block light from penetrating, resulting in a shallower aphotic zone. Around the poles, the angle of the sunlight means it does not penetrate as deeply so the aphotic zone is shallower. In clear, tropical water sunlight can penetrate deeper and so the aphotic zone starts at greater depths. The depth at which the aphotic zone begins in the ocean depends on many factors. The majority of the ocean is aphotic, with the average depth of the sea being 4,267 m (13,999 ft) deep with the deepest part of the sea, being the Challenger Deep in the Mariana Trench, is about 11,000 m (36,000 ft) deep. Depending on how it is defined, the aphotic zone of the ocean begins between depths of about 200 m (660 ft) to 800 m (2,600 ft) and extends to the ocean floor. In the ocean, the aphotic zone is sometimes referred to as the dark ocean. The mesopelagic is sometimes referred to as the dysphotic zone. Zones of the water column as defined by the amount of light penetration. All but the epipelagic zone make up the aphotic zone. The Dark Ocean The layers of the pelagic zone. The aphotic zone underlies the photic zone, which is that portion of a lake or ocean directly affected by sunlight. The depth of the aphotic zone can be greatly affected by such things as turbidity and the season of the year. Most food in this zone comes from dead organisms sinking to the bottom of the lake or ocean from overlying waters. Bioluminescence is more abundant than sunlight in this 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. The depth at which less than one percent of sunlight reaches begins the aphotic zone. 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 euphotic zone is the layer of water in which there is enough light for net photosynthesis to occur. Above the aphotic zone is the photic zone, which consists of the euphotic zone and the disphotic zone. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. 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.
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