Coastal versus open-ocean denitrification in the Arabian Sea
1National Institute of Oceanography, Dona Paula, Goa 403 004, India
2School of Oceanography, University of Washington, Seattle, WA 98195, USA
3Wadsworth Center, New York State Department of Health, P.O. Box 509, Albany, New York 12201, USA
4Hydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan
*now at: National Centre for Antarctic and Ocean Research, Headland Sada, Vasco da Gama, Goa 403 804, India
**now at: Department of Geosciences, Princeton University, Goyet Hall, Princeton, NJ 98544, USA
Abstract. The Arabian Sea contains one of the three major open-ocean denitrification zones in the world. In addition, pelagic denitrification also occurs over the inner and mid-shelf off the west coast of India. The major differences between the two environments are highlighted using the available data. The perennial open-ocean system occupies two orders of magnitude larger volume than the seasonal coastal system, however, the latter offers more extreme conditions (greater nitrate consumption leading to complete anoxia). Unlike the open-ocean system, the coastal system seems to have undergone a change (i.e., it has intensified) over the past few decades presumably due to enhanced nutrient loading from land. The two systems also differ from each other with regard to the modes of nitrous oxide (N2O) production: In the open-ocean suboxic zone, an accumulation of secondary nitrite (NO2−) is invariably accompanied by depletion of N2O whereas in the coastal suboxic zone high NO2− and very high N2O concentrations frequently co-occur, indicating, respectively, net consumption and net production of N2O by denitrifiers. The extents of heavier isotope enrichment in the combined nitrate and nitrite (NO3−+NO2−) pool and in N2O in reducing waters appear to be considerably smaller in the coastal region, reflecting more varied sources/sinks and/or different isotopic fractionation factors.
Citation: Naqvi, S. W. A., Naik, H., Pratihary, A., D'Souza, W., Narvekar, P. V., Jayakumar, D. A., Devol, A. H., Yoshinari, T., and Saino, T.: Coastal versus open-ocean denitrification in the Arabian Sea, Biogeosciences, 3, 621-633, doi:10.5194/bg-3-621-2006, 2006.