The Role of Water Masses, Biological Processes, Remineralization and Reversible Scavenging in Controlling the Distribution of Dissolved Nickel in the Arabian Sea

Abstract The present study describes the distribution of dissolved nickel (dNi) by sampling and analyzing seawater in the Arabian Sea. It aims to understand how different biogeochemical processes impact the distribution of dNi. The concentration of dNi ranges from 1.4–4.6 nM in surface waters to 6.8–12 nM in the deeper waters (>1,000 m) of the Arabian Sea. The distribution of dNi in the Arabian Sea is modulated by sources such as dust, riverine and submarine groundwater discharge, sinks such as biological uptake and adsorption processes, as well as internal cycling such as remineralization, reversible scavenging, and consumption in intense oxygen deficient zone in the water column influence the distribution of dNi in the Arabian Sea. Dissolved nickel has a higher concentration in the surface waters of the Arabian Sea, unlike other micronutrients such as Fe, Cd, Zn, Co, etc., potentially due to its unavailability for biological uptake. A significant portion of dNi (30%–50%) in the intermediate and deeper waters of the Arabian Sea is derived from the remineralization of organic matter, and the reversible scavenging process, similar to that in the Pacific Ocean. This is confirmed by the presence of additional dNi in excess of the preformed dNi supplied by water mass mixing in the Arabian Sea. The results of this study indicate significant depletion of dNi over Phosphate in the intense oxygen minimum zone of the Arabian Sea, implying a complex interplay of factors such as its loss as sulphides or Fe oxides or POC and due to varying ecosystem composition.