Its lowermost boundary is at a thermocline of 12 ☌ (54 ☏), which, in the tropics generally lies between 200 and 1,000 metres. The dysphotic zone is also referred to as the mesopelagic zone, or the twilight zone. The lower portion of the photic zone, where the light intensity is insufficient for photosynthesis, is called the dysphotic zone (dysphotic means "poorly lit" in Greek). The uppermost portion of the photic zone, where there is adequate light to support photosynthesis by phytoplankton and plants, is referred to as the euphotic zone (also referred to as the epipelagic zone, or surface zone). The photic zone can be subdivided into two different vertical regions. The stratum of the water column nearest the surface of the ocean ( sea level) is referred to as the photic zone. Photosynthesis in turn requires energy from sunlight to drive the chemical reactions that produce organic carbon. Nearly all life forms in the ocean depend on the photosynthetic activities of phytoplankton and other marine plants to convert carbon dioxide into organic carbon, which is the basic building block of organic matter. The ocean can be conceptualized as zones, depending on depth, and presence or absence of sunlight. Due to darkness and a water pressure that can reach about 750 times atmospheric pressure (76 megapascal), abyssal plains are not well explored. They are poorly preserved in the sedimentary record, because they tend to be consumed by the subduction process. Deep sea coral reefs are mainly found in depths of 3,000 meters and deeper in the abyssal and hadal zones.Ībyssal plains were not recognized as distinct physiographic features of the sea floor until the late 1940s and, until recently, none had been studied on a systematic basis. Due to scarcity of oxygen, abyssal plains are inhospitable for organisms that would flourish in the oxygen-enriched waters above. Much dissolved oxygen in abyssal plains came from polar regions that had melted long ago. Animals absorb dissolved oxygen from the oxygen-poor waters. Factors such as climate change, fishing practices, and ocean fertilization have a substantial effect on patterns of primary production in the euphotic zone. The structure of abyssal ecosystems is strongly influenced by the rate of flux of food to the seafloor and the composition of the material that settles. They also exert significant influence upon ocean carbon cycling, dissolution of calcium carbonate, and atmospheric CO 2 concentrations over time scales of a hundred to a thousand years. Owing in part to their vast size, abyssal plains are believed to be major reservoirs of biodiversity. There are also amounts of carbon, nitrogen, phosphorus and silicon, due to material that comes down and decomposes. Metallic nodules are common in some areas of the plains, with varying concentrations of metals, including manganese, iron, nickel, cobalt, and copper. The rest is composed chiefly of pelagic sediments. Much of this sediment is deposited by turbidity currents that have been channelled from the continental margins along submarine canyons into deeper water. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Magma rises from above the asthenosphere (a layer of the upper mantle), and as this basaltic material reaches the surface at mid-ocean ridges, it forms new oceanic crust, which is constantly pulled sideways by spreading of the seafloor. The creation of the abyssal plain is the result of the spreading of the seafloor (plate tectonics) and the melting of the lower oceanic crust. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills). They are among the flattest, smoothest, and least explored regions on Earth. Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3,000 and 6,000 metres (9,800 and 19,700 ft).
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