Geographical extent of denitrification in the Arabian Sea in relation to some physical processes

Abstract

Utilizing all the available data, the distribution of nitrite at the secondary nitrite maximum is studied to delineate the boundaries of the denitrification zone and to understand some of the processes that control the extent of dentrification in the Arabian Sea. Nitrite distribution shows little correspondence with primary productivity, and the three major sites of seasonal upwelling appear to be located outside the zone of intense denitrification. This distinguishes the Arabian Sea from the other two major oceanic denitrification sites, located in the Pacific Ocean. The relationship between nitrite and nitrate deficit suggests that the distribution of the former effectively reflects the extent of denitrification. The locus of the most intense denitrification appears to extend southwest from the shelf break off Gujarat. In the northernmost and western parts of the Arabian Sea, denitrification seems to be inhibitEd. by the supply of oxygen from the surface to subsurface layers due to the convective overturning of surface waters during the winter. The northward flow of warm, low-salinity waters from the equatorial region, however, maintains strong stratification in the eastern and central Arabian Sea. It is suggested that the development of reducing conditions in the Arabian Sea many be closely related to this feature. Intense cooling close to the coast of Gujarat, especially within the Gulf of Kutch, may lead to the formation of a lower-salinity water which may advect to depths of approx 6200 m. This provides a mechanism for the injection of appreciable quantities of labile dissolved organic carbon (DOC) into the oxygen-minimum layer and, in conjunction with the high horizontal diffusivity in the Arabian Sea, could account for the observed lack of correspondence between denitrification intensity and primary productivity. The results are examined in the light of available information on the paleoceanography of the region. It is postulated that the absence of low-salinity waters off India, more intense evaporation, and lower oxygen demand (due to the lower overall fertility and decreased input of DOC associated with the advection of shelf water) might all combine in producing a better glacial oxygen balance at mid-depth.