THE DYNAMICS OF THE MARINE NITROGEN CYCLE AND ITS INFLUENCE ON ATMOSPHERIC CO2

Reference

Gruber, N., The dynamics of the marine nitrogen cycle and its influence on atmospheric CO2, In: Carbon-Climate Interactions, Eds, M. Follows and T. Oguz, NATO ASI Series, in press, 2004.

Abstract

The bioavailability of nutrients represents one of the most important factors controlling the strength of the biological carbon pump and ultimately the impact of ocean biology on atmospheric \COtw. Among those nutrients, the macro-nutrients nitrate and phosphate play a particularly important role in limiting biological productivity as evidenced by their often near complete exhaustion in surface waters. As near surface nitrate concentrations are generally somewhat lower than those of phosphate relative to the demand by phytoplankton, biological oceanographers have argued historically that nitrate rather than phosphate is the primary macro-nutrient controlling phytoplankton productivity. Geologists, in contrast, regarded phosphate as the primary controlling macronutrient. They argued that while nitrate may indeed be the limiting factor at any given location and time, phosphate is truly the limiting factor on geological time-scales, because the biologically mediated fixation of the much more abundant dinitrogen gas (N2) into organic nitrogen is alleviating the scarcity of bioavailable nitrogen. Phosphate on the other hand, does not have such a biologically mediated source. It is therefore the geologically controlled balance between the riverine (and atmospheric) input of phosphate and its burial on the sea-floor that ultimately controls marine biological productivity. Tyrell (1999) provided a synthesis of these two views by identifying nitrate as the proximate nutrient, while giving phosphate the role of being the ultimate nutrient. Over the last two decades, the clear distinction between the two views started to disappear.

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