Assessment of excess nitrate development in the subtropical North Atlantic

Geochemical estimates of N-2 fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 +/- 1.7 x 10(11) mol N yr (-1). Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 +/- 0.9 x 10(11) mol N yr (-1) for similar to 3 8% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 +/- 1.1 x 10 11 mol N yr (-1) for similar to 28% of the total), and, calculated by mass balance of the excess N field, N-2 fixation (adding excess N at 2.6 +/- 2.2 x 10(11) mol N yr (-1) for similar to 33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N-2 fixation in the North Atlantic subtropical gyre is estimated at similar to 4 x 10(11) mol N yr(-1). This relatively low rate of N-2 fixation suggests that i) the rate of N-2 fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N-2 fixation. (c) 2007 Elsevier B.V. All rights reserved.