Yet, due to their low abundance relative to plankton, marine vertebrates are not included in most models of marine biogeochemistry. Humans have had a substantial impact on the abundance, biomass, and size structure of populations of fish and whales.
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In the open ocean, the carbon cycle is assumed to be free of direct human influences. On land, humans directly influence the carbon stored in terrestrial ecosystems through logging and the burning of forests and grasslands. Dead phytoplankton cells are the dominant component of the flux, but zooplankton feces and vertical migrations can contribute significantly. Carbon is removed from the euphotic zone by the sinking of organic matter. On annual or longer scales, the availability of these nutrients is controlled by physical processes such as vertical mixing, upwelling, or atmospheric deposition. Primary production in the ocean is limited by the availability of macronutrients (principally, nitrogen) and micronutrients, notably iron. Research on the organic carbon cycle in the ocean has focused on the processes that limit primary productivity and the removal of carbon from the euphotic zone through the biological pump. Whether an ecosystem acts as a source or sink for atmospheric carbon dioxide depends on the relative rates of photosynthesis and respiration. Ecosystems play a central role in regulating the concentration of atmospheric carbon dioxide.
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