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

Abstract

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.