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

摘要

Rates of canonical, i.e. heterotrophic, water-column denitrification were measured by ~(15)N 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 (~(15)NO_3~-→~(15-14)N_2) at open ocean stations averaged 9.1±1.0nmol 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 TgNa~(-1) ± 18TgNa~(-1), which is within the range (10-44TgNa~(-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 Nfutation, 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.