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

Rates of canonical, i.e. heterotrophic, water-column denitrification were measured by N-15 incubation techniques at a number of coastal and open ocean stations in the Arabian Sea. Measurements of N-2:Ar gas ratios were also made to obtain independent estimates of N-2 excess resulting from denitrification. Measured denitrification rates ((NO3-)-N-15 -> N-15-14(2)) 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 Na-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 mu g at N l(-1) vs. 13 mu g 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. N-2 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. (c) 2006 Elsevier Ltd. All rights reserved.