Retention and loss of fixed nitrogen in a Pacific oxygen minimum zone

Abstract

Marine oxygen depleted zones (ODZs) are regions of the ocean where oxygen concentrations are low enough to induce anaerobic microbial processes. These processes include denitrification and anammox which lead to loss of fixed nitrogen that would fuel primary production. Nitrite oxidation, which has recently been detected in ODZs, converts nitrite, a substrate for the two nitrogen loss processes, back into nitrate, a key nutrient. Understanding the kinetics of these processes causing the retention or loss of fixed nitrogen is vital for understanding their effects on the marine nitrogen budget. In this study, the effects of varying nitrite and nitrate concentration on denitrification, anammox, and nitrite oxidation rates were examined using \(^{15}\)NO\(_{2}\)\(^{-}\) stable isotope tracer experiments in seawater samples from the Eastern Tropical North Pacific ODZ. Nitrite oxidation rates were found to increase with increased nitrite concentration and to fall below detection limit with increased nitrate concentration. Denitrification rates were below detection limit, and anammox rates were independent of nitrite and nitrate concentrations. Additionally, nitrite dismutation, a possible mechanism for anaerobic nitrite oxidation was investigated by comparing the stoichiometry of nitrate and N\(_{2}\) production with measured N\(_{2}\) production rates. The very low measured N\(_{2}\) production was not consistent with nitrite dismutation as a sink for fixed nitrogen.