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Page count: 18

Nitrogen and phosphorus distributions, the relative rates of oceanic nitrogen fixation and denitrification, and the physiology and biochemistry of phytoplankton growth suggest that nitrate is the major nutrient limiting export and new production in today's ocean. This condition exists largely because oceanic denitrification can respond rapidly to low oxygen levels and high fluxes of organic matter while a shortage of combined nitrogen is not sufficient for stimulating nitrogen fixation. Over several million year periods, nitrogen fixation has probably kept up with the demand, but imbalances in which denitrification exceeds nitrogen fixation for periods of several thousand years could cause changes of 20 to 30% in oceanic export (new) production. Such differences might contribute to the variation in the atmospheric carbon dioxide record. Phosphate may have been the major limiting nutrient in past oceans, during epochs of giant phosphorite deposit formation and also during warmer epochs, since temperatures >20°C favor nitrogen fixation. An examination of the literature suggests that the Redfield ratio of change for nitrogen and phosphorus uptake by marine phytoplankton (16N:1P, by atoms) may vary appreciably from this value only in atypical marine ecosystems.