Denitrification processes in the Arabian Sea

Abstract

Recent information on some consequences of the acute mid-water oxygen deficiency in the Arabian Sea, especially on carbon-nitrogen cycling, is reviewed. An evaluation of published estimates of water column denitrification rate suggests an overall rate in the vicinity of 30Tg Ny^-1, but the extent of benthic contribution remains unknown. A decoupling of denitrification from primary production, unique to the Arabian Sea, is revealed by nitrite, electron transport system (ETS) activity and bacterial production data. Results of both enzymatic and microbiological investigations strongly point to a major role of organic carbon other than that sinking from surface layers in supporting denitrification. Although denitrification is associated with an intermediate nepheloid layer, it seems unlikely that the excess carbon comes with particles re-suspended along the continental margins and transported quasi-horizontally into the ocean interior; instead, the particle maximum may directly reflect a higher bacterial abundance. It is proposed that denitrification may be predominantly fuelled by the dissolved organic matter.