Distribution of organic matter and diatom frustules (diversity, flux) along the western Indian continental shelf related to contrasting physicochemical settings.

Abstract

The western Indian continental shelf (eastern Arabian Sea) exhibits contrasting biogeochemical features. This area becomes highly productive due to summer monsoon-driven coastal upwelling in the south and winter monsoon-induced convective mixing in the north. Additionally, in the northern self, the eastern boundary of the Oxygen Minimum Zone (OMZ) persists but is absent in the south. Phytoplankton blooms are dominated by diatoms that contribute to sedimentary phytodetritus flux supplying major elements (C, N, Si) and food for benthic biota and hence important to address. Here we present the data on organic matter content, diatom frustule flux, abundance, and diversity using surface sediments (core tops collected using a multicorer) from 6 locations (11-21° N) along the shelf in a 2° interval at 200 m isobaths. The organic matter retrieved from the core top was relatively fresh (nearly ∼4.5 years old) as evident from ²¹⁰Pb profiles. Frustule abundance and diversity (the maximum at 15° N and minimum at 19° N) varied from 0.10-18.46 × 10⁴ valves g^-1 and 0.79-2.32, respectively. A total of 36 diatom genera were found with two centric (Thalassiosira and Coscinodiscus), and one pennate (Nitzschia) diatoms as major contributors. The higher contribution of Thalassiosira was observed throughout the shelf dominating the south (11, 13, 15° N), whereas, in the north (17, 19, 21° N) Coscinodiscus was dominant. The highest organic matter content (3.4%) and frustule abundance (18.46 × 10⁴ valves g^-1) were seen at 15° N despite low diatom valve flux (3.3 × 10³ valves cm^-2 yr^-1) and could be due to the influence of OMZ, where organic matter is well preserved. Contrarily, the upwelling-influenced station in the south (at 11° N) exhibited the highest diatom valve flux (10.14 × 10³ valves cm^-2 yr^-1), however low organic matter content (1.6 %) and frustule abundance (4.99 × 10⁴ valves g^-1) were attributed to faster mineralization. This study suggested that the preservation potential of organic matter varies across the shelf and is likely to control its recycling, impacting nutrient release and resources for the benthic community.