Variation of amino acid in the sinking particulates in the northeastern Arabian Sea and the northern Bay of Bengal

Abstract

Sediment traps are widely used to get insight into how particulate matter forms and settles in the water column. Earlier studies in the Northern Indian Ocean have shown that the sinking flux depends mainly on surface primary production and varies seasonally and inter-annually. In the case of the Bay of Bengal, river runoff plays a significant role in the sinking flux. As a part of the SIBER-INDIA program, a mooring consisting of sediment traps was deployed at three depths (495 m, 1205 m, and 2915 m) in the Arabian Sea (Northeastern Arabian Sea sediment trap, NEAST; 16.87°N, 67.77°E) and at 580 m, 1110 m, and 1635 m in the Bay of Bengal (Bay of Bengal sediment trap, BoBST; 18.21°N, 89.63°E). We present here data on total fluxes, concentrations of particulate carbon and nitrogen, biogenic opal, and amino acid composition for the period 2017–2018 in both basins. Particle flux in the Arabian Sea was influenced by monsoon-related processes with higher flux during the northeast monsoon (NEM) and southwest monsoon (SWM) seasons, whereas the flux was higher in BoB during SWM. Particulate nitrogen (PN), particulate organic carbon (POC), and total hydrolysable amino acid (THAA) mostly decreased with depth in all traps due to the decomposition of labile organic constituents in the sinking flux. Changes in the decomposition of organic matter with depth were further reflected in the POC/PN molar ratios, which increased with depth in the Arabian Sea. A low POC/PN molar ratio associated with the higher THAA concentration in most of the shallow traps indicates the presence of fresher organic matter. Among the THAA, Glycine (Gly) was the dominant amino acid in all the samples followed by Aspartic acid (Asp), Glutamic acid (Glu), Serine (Ser), and Alanine (Ala). The amino acid-based degradation index (DI) reveals the degradation of labile organic matter with depth in the NEAST. However, the middle trap in the BoB showed a different trend with higher total and lithogenic fluxes, POC, PN, and DI values compared to the shallow trap indicating lateral advection in the bay. Monsoonal changes and mesoscale eddies make these basins distinct from one another and influence their sinking fluxes and biogeochemical settings.