<P> Owing in part to their vast size, abyssal planes are believed to be major reservoirs of biodiversity . They also exert significant influence upon ocean carbon cycling, dissolution of calcium carbonate, and atmospheric CO concentrations over time scales of a hundred to a thousand years . The structure of abyssal ecosystems are strongly influenced by the rate of flux of food to the seafloor and the composition of the material that settles . Factors such as climate change, fishing practices, and ocean fertilization have a substantial effect on patterns of primary production in the euphotic zone . </P> <P> Abyssal planes were not recognized as distinct physiographic features of the sea floor until the late 1940s and, until very recently, none had been studied on a systematic basis . They are poorly preserved in the sedimentary record, because they tend to be consumed by the subduction process . </P> <P> The ocean can be conceptualized as being divided into various zones, depending on depth, and presence or absence of sunlight . 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 . Photosynthesis in turn requires energy from sunlight to drive the chemical reactions that produce organic carbon . </P> <P> The stratum of the water column nearest the surface of the ocean (sea level) is referred to as the photic zone . The photic zone can be subdivided into two different vertical regions . 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 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 dysphotic zone is also referred to as the mesopelagic zone, or the twilight zone . Its lowermost boundary is at a thermocline of 12 ° C (54 ° F), which, in the tropics generally lies between 200 and 1000 metres . </P>

Referring to the topographic profile above what description below best describes the abyssal plain