Dissolved selenium in the Northeastern Indian Ocean

Selenium is considered as one of essential micronutrients in marine ecosystems, and is involved in a number of enzymatic reactions, and hence plays an important role in various metabolic pathways. However, the biogeochemistry of selenium in the ocean is still poorly described in the literature because of lack of observational data. In this study, we reported the concentration and chemical speciation of dissolved selenium in the Eastern Indian Ocean (EIO) based on observations and sample analyses made in 2017 and 2018. In surface waters, concentration of Se(IV), Se(VI) are all low relative to dissolved organic selenium (DOSe), being related to the biological uptake in the euphotic zone. The regeneration of Se in the mesopelagic waters induces that concentration increases with depth for inorganic fraction of dissolved pool, with a systematic change in species ratio, indicating active transformations there. In the deep part of vertical profiles, spatial structure of Se shows a similar trend with dissolved oxygen (DO) and is consistent to the pattern of deep water circulation.