Naik, Hema S., and S.W.A. Naqvi

National Institute of Oceanography, Dona-Paula, Goa, India, Tel: +91-832-2456700 (X4398), Fax: +91-832-2456702/03, E-mail: hema@darya.nio.org

Benthic nitrogen cycling in the eastern Arabian Sea

The western continental margin of India experiences moderate upwelling during summer and fall (May-November), but the existence of a 5-10 m thick low-salinity lens generally prevents the upwelled water from surfacing. High nutrient concentrations (nitrate ~20 mM) beneath the shallow mixed layer greatly enhance primary productivity (up to 7 gC/sq. m/d) which, in turn, raises the oxygen demand in the deeper layer. The ensuing oxygen-depletion triggers large-scale denitrification in near-bottom waters (around 5 million tonnes N/season). Near surface accumulation of nitrous oxide occurs in record high concentrations presumably due to production through denitrification, sustaining a high emission rate to the atmosphere (up to 0.4 million tones/season). Over the inner- and mid-shelf regions, sulphate reduction often follows complete denitrification, affecting marine life and cycling of redox-sensitive metals. The supply of large amounts of organic carbon to sediments also fuels vigorous denitrification in sediments: 0.27-1.45 pico mole nitrate/sq. m/s), corresponding to an annual shelf-sedimentary denitrification rate of 0.4-3.5 million tonnes N for the entire Arabian Sea. Porewater profiles of nitrate also suggest its net loss to the sediments but those of nitrous oxide indicate net production close to the sediment surface (and therefore a positive flux to the overlying waters) and net consumption at deeper levels. The results show that water column denitrification rate is higher than the sedimentary denitrification rate and denitrification process in sediments is an important but not the dominant sink for fixed nitrogen in all areas.