LAUSR

Abstract The western continental shelf of India (eastern Arabian Sea) houses the world's largest seasonally-occurring coastal low-oxygen zone. The natural abundance of nitrogen and oxygen isotopes in dissolved nitrate were measured along four transects of the shelf (off Mumbai, Goa, Mangalore and Calicut) to gain insights into nitrogen cycling in this system. Carbon and nitrogen isotope abundances in particulate organic matter were also measured to understand sources and cycling of organic matter. Three of the four transects (with the exception of Mumbai transect) experienced fully anoxic (sulphidic) conditions over the inner shelf. Denitrification signals, as indicated by a linear increase of δ15N and δ18O in nitrate in oxygen-deficient waters, were observed off Mumbai, Goa and Mangalore. The occurrence of nitrite oxidation was indicated by a decoupling of stable N and O isotopes of nitrate. The fractionation factor computed was found to be quite low (6.1‰), as compared to open ocean oxygen minimum zones, presumably due to a large contribution (>49%) of benthic denitrification to N-loss. Autochthonous production and phytoplankton composition are proposed to exert major controls on the C isotopic composition of particulate organic matter while nitrate and ammonium seem to control the N-isotope variability. Overall, this study attempts to fill the knowledge gap in terms of N and C stable isotope studies in a system that is otherwise well documented from the biogeochemical point of view.