Any water in a sea or lake that is neither close to the bottom nor near the shore can be said to be in the pelagic zone. The word pelagic comes from the Ancient Greek: πέλαγος (pélagos) “open sea”. The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes from the surface of the sea almost to the bottom. Conditions change deeper down the water column; the pressure increases, the temperature drops and there is less light. Depending on the depth, the water column, rather like the Earth's atmosphere can be divided into different layers.
The pelagic zone occupies 1,330 million cubic kilometres (320 million cubic miles) with a mean depth of 3.68 kilometres (2.29 mi) and maximum depth of 11 kilometres (6.8 mi). Fish that live in the pelagic zone are called pelagic fish. Pelagic life decreases with increasing depth. It is affected by light intensity, pressure, temperature, salinity, the supply of dissolved oxygen and nutrients, and the submarine topography. In deep water, the pelagic zone is sometimes called the open-ocean zone and can be contrasted with water that is near the coast or on the continental shelf. However in other contexts, coastal water that is not near the bottom is still said to be in the pelagic zone.
The pelagic zone can be contrasted with the benthic and demersal zones at the bottom of the sea. The benthic zone is the ecological region at the very bottom of the sea. It includes the sediment surface and some sub-surface layers. Marine organisms living in this zone, such as clams and crabs, are called benthos. The demersal zone is just above the benthic zone. It can be significantly affected by the seabed and the life that lives there. Fish that live in the demersal zone are called demersal fish. Demersal fish can be divided into benthic fish, which are denser than water so they can rest on the bottom, and benthopelagic fish, which swim in the water column just above the bottom. Demersal fish are also known as bottom feeders and groundfish.
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