Denitrification rates and excess nitrogen gas concentrations in the Arabian Sea oxygen deficient zone

Devol, A. H. ; Uhlenhopp, A. G. ; Naqvi, S. W. A. ; Brandes, J. A. ; Jayakumar, D. A. ; Naik, H. ; Gaurin, S. ; Codispoti, L. A. ; Yoshinari, T. (2006) Denitrification rates and excess nitrogen gas concentrations in the Arabian Sea oxygen deficient zone Deep Sea Research Part I: Oceanographic Research Papers, 53 (9). pp. 1533-1547. ISSN 0967-0637

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Rates of canonical, i.e. heterotrophic, water-column denitrification were measured by 15N incubation techniques at a number of coastal and open ocean stations in the Arabian Sea. Measurements of N2:Ar gas ratios were also made to obtain independent estimates of N2 excess resulting from denitrification. Measured denitrification rates (15NO3-15-14N2) at open ocean stations averaged 9.1±1.0 nmol N l-1 d-1 (n=15), and coastal rates averaged 33.2±12.4 nmol N l-1 d-1 (n=18). When extrapolated to the entire Arabian Sea, deep measurements within the offshore perennial suboxic zone indicate an overall denitrification rate of 41 Tg N a-1±18 Tg N a-1, which is within the range (10-44 Tg N a-1) of previous estimates for canonical denitrification in the region based on stoichiometric calculations and electron transport system activity. Nitrogen excess gas measurements predict a larger nitrogen anomaly than estimated by classical stoichiometric methods (maximum anomaly=23 μg at N l-1 vs. 13 μg at N l-1, respectively). This mismatch may result from incorrect assumptions of Redfield stoichiometry inherent in the nitrate deficit calculation, inputs of new nitrogen through N-fixation, N2 contributions from sedimentary denitrification along continental margins, the anammox reaction, and metal catalyzed denitrification reactions. Nevertheless, if denitrification is defined as the conversion of combined nitrogen to a gaseous end product, then the data suggest that denitrification in the Arabian Sea may have been underestimated so far.

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Source:Copyright of this article belongs to Elsevier Science.
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Deposited On:22 Nov 2010 11:08
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