228-233Data collected during cruises 51 and 52 of R.V. Gaveshani (Jan-Feb 1979) indicate that watermasses on the eastern and western margins of the Andaman and Nicobar Islands have similar hydrographic features to a depth of about 1500m; deeper watermasses in the central Andaman basin are separated from the rest of the region by shallow sills and are relatively stagnant. Distribution of nutrients and nutrients-oxygen relationships indicate silicate concentrations and stoicchometric relations which are different from those reported for the Arabian Sea. Rates of denitrification (which terminates at the stage of formation of ammonia), however, appear to be uniform in the northern Indian Ocean
