Figure 1: The 'biological pump' is a collective property of a complex phytoplankton-based food web. | Nature

Figure 1: The 'biological pump' is a collective property of a complex phytoplankton-based food web.

From: Stirring times in the Southern Ocean

Figure 1

Together with the 'solubility pump' (right), which is driven by chemical and physical processes, it maintains a sharp gradient of CO2 between the atmosphere and the deep oceans where 38 × 1018 g of carbon is stored. Using sunlight for energy and dissolved inorganic nutrients, phytoplankton convert CO2 to organic carbon, which forms the base of the marine food web. As the carbon passes through consumers in surface waters, most of it is converted back to CO2 and released to the atmosphere. But some finds its way to the deep ocean where it is remineralized back to CO2 by bacteria. The net result is transport of CO2 from the atmosphere to the deep ocean, where it stays, on average, for roughly 1,000 years. The food web's structure and the relative abundance of species influences how much CO2 will be pumped to the deep ocean. This structure is dictated largely by the availability of inorganic nutrients such as nitrogen, phosphorus, silicon and iron. Iron is the main limiting nutrient in the Southern Ocean, which is why the SOIREE experiment1,2,3 was conducted there. (Figure modified from a graphic by Z. Johnson.)

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