The open ocean, which extends over nine-tenths of the entire oceanic area, has a total productivity that is more or less equivalent to a plant assimilation of about 15 thousand million tons of carbon per year.
Investigations in the Sargasso Sea have led Riley to estimate that about 75 per cent of such production is consumed by zooplankton and heterotrophic organisms in the uppermost part of the water column.
Twenty per cent of the total production falls to the animals and heterotrophs of mesopelagic levels, the bathypelagic zone and the bottom, Mesopelagic metabolism down to and including the level of the oxygen minimum layer accounts for most of the deep-sea consumption of organic matter, which leaves less than 5 per cent to the bathypelagic and benthic organisms. Riley suggests that total amounts consumed in the bathypelagic and benthic zones are probably about equal.
In the former, consumption is thinly (but unevenly) spread over a water column with a mean depth range of some 3,000 metres, whereas benthic consumption is concentrated into a narrow depth range in, an and just over the deep-sea floor. In temperate and subarctic regions of Pacific, which are something like three times are productive as the waters of the Sargasso Sea, Bogdanov estimated that 10 per cent of the organic matter produced in the euphotic zone reaches depths below 3,000 metres, but in tropical and south temperate latitudes the corresponding figure is less than 5 per cent.
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Clearly, there are certain outstanding questions. How is organic material transported from the euphotic zone to the depths? What are the metabolic requirements of deep-sea animals and how are they met? Concerning the first question, there is the gravitational settling of particulate matter and the movements of animals, whether daily, seasonal or ontogenetic, up and down the water column.