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 sup(15)N 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 (sup(15)NO3 sup(-) - sup(15-14) N2 ) at open ocean stations averaged 9.1 plus or minus 1.0 nmol Nl sup(-1) d sup(-1) (n = 15), and coastal rates averaged 33.2 plus or minus 12.4 nmol Nl sup(-1) d sup(-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 TgNa sup(-1) plus or minus 18 TgNa sup(-1), which is within the range (10-44 TgNa sup(-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 Nl sup(-1) vs. 13 mu g at Nl sup(-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