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· 2001
New data force us to raise previous estimates of oceanic denitrification. Our revised estimate of ~ 450 Tg N / yr (Tg = 1012 g) produces an oceanic fixed N budget with a large deficit (~ 200 Tg N / yr) that can be explained only by positing an ocean that has deviated far from a steady-state, the need for a major upwards revision of fixed N inputs, particularly nitrogen fixation, or both. Oceanic denitrification can be significantly altered by small re-distributions of carbon and dissolved oxygen. Since fixed N is a limiting nutrient, uncompensated changes in denitrification affect the ocean's ability to sequester atmospheric CO2 via the "biological pump". We have also had to modify our concepts of the oceanic N2O regime to take better account of the extremely high N2O saturations that can arise in productive, low oxygen waters. Recent results from the western Indian Shelf during a period when hypoxic, suboxic and anoxic waters were present produced a maximum surface N2O saturation of > 8000%, a likely consequence of "stop and go" denitrification. The sensitivity of N2O production and consumption to small changes in the oceanic dissolved oxygen distribution and to the "spin-up" phase of denitrification suggests that the oceanic source term for N2O could change rapidly.
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· 1989
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.
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· 1986
During February and March 1985, nitrite levels along the northern (approximately 7° to 10°S) Peruvian coast were unusually high. These accumulations occurred in oxygen-deficient waters, suggesting intensified denitrification. In a shallow offshore nitrite maximum, concentrations were as high as 23 micromoles per liter (a record high). Causes for the unusual conditions may include a cold anomaly that followed the 1982-83 El Niño. The removal of combined nitrogen (approximately 3 to 10 trillion grams of nitrogen per year) within zones of new or enhanced denitrification observed between 7° to 16°S suggests a significant increase in oceanic denitrification.
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