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

Silvia E. Bulow, Jeremy J. Rich, Hema S. Naik, Anil K. Pratihary, Bettie Ward

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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.

Original languageEnglish (US)
Pages (from-to)384-393
Number of pages10
JournalDeep-Sea Research Part I: Oceanographic Research Papers
Issue number3
StatePublished - Mar 2010

All Science Journal Classification (ASJC) codes

  • Oceanography
  • Aquatic Science


  • Anammox
  • Arabian Sea
  • Denitrification
  • Oxygen minimum zone


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