Deep Sea
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The deep sea begins below about 200 m, where sunlight becomes inadequate for photosynthesis. From there to about 1,000 m, the mesopelagic or "twilight" zone, sunlight continues to decrease until it is gone altogether. Considering the volume of water above the deepest parts of the ocean, it's no wonder that hydrostatic pressure is one of the most important environmental factors affecting deep sea life. Pressure increases 1 atmosphere (atm) for each 10 m in depth. The deep sea varies in depth from 200 m to about 11,000 m, therefore pressure ranges from 20 atm to more than 1,100 atm. With the exception of hydrothermal vent communities where hot water is emitted into the cold waters, the deep sea temperature remains between about -1 to about +4°C. However, water never freezes in the deep sea.
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