Denitrification exceeds anammox as a nitrogen loss pathway in the Arabian Sea oxygen minimum zone

The oxygen minimum zones (OMZs) of the world oceans are sites of enhanced anaerobic nitrogen transformations, such that the largest OMZ, the Arabian Sea, is responsible for 30-50% of the oceanic water column fixed N loss. Measurements of excess N₂ and nitrate deficits have been interpreted to indicate a large net loss of fixed nitrogen through N₂ production in waters with very low oxygen concentrations. Rates of N₂ production both by denitrification and anaerobic ammonium oxidation (anammox) were investigated by measuring ¹⁵N-N₂ production from ¹⁵ NH₄⁺, ¹⁵ NO₂^-, and ⁴⁶N₂O in incubations at 3-4 depths at each of the three stations in the central region of the Arabian Sea OMZ. In addition to conventional measurements of anammox and denitrification rates from incubations in 12 ml exetainers purged with He, we also investigated the effect of ammonium and organic carbon additions on N₂ production rate. Denitrification was detected at every depth at every station; the maximum rate (25.3±9.1 nmol l^-1 d^-1 from the ¹⁵NO₂ tracer and 21.2±5.6 nmol l^-1 d^-1 from the ⁴⁶N₂O tracer) occurred at the depth of the secondary nitrite maximum in the core of the OMZ. The ⁴⁶N₂O tracer was the most direct method to measure the denitrification rate. Rates of denitrification correlated strongly with water column nitrite concentration. Addition of organic carbon or ammonium did not consistently stimulate denitrification rates. Distribution of anammox was patchy; low rates of anammox production of ²⁹N₂ in ¹⁵ NH₄⁺ incubations were detected at only four of eleven depths. The highest rate of anammox production (4.23±0.35 nmol l^-1 d^-1) occurred near the upper boundary of the OMZ at one station. Overall, denitrification dominated N₂ production at this time in the Arabian Sea OMZ.