An account of the key diatom frustules from the surface sediments of the Central and Eastern Arabian Sea and their biogeochemical significance.

Abstract

The Arabian Sea, a productive oceanic region in the North Indian Ocean, is under the direct influence of monsoon winds that impact the surface ocean processes. High biological productivity occurs due to natural nutrient enrichment events via coastal and open ocean upwelling (summer monsoon) and convective mixing (winter monsoon). Ample studies from this basin addressed the diatom community from the surface ocean, yet the key contributing diatom frustules to sedimentary phytodetritus has been overlooked. These microscopic biosilcifiers play an important role in the biological carbon pump by exporting significant organic carbon from the surface waters to the deep sea due to their ballasted silica shell (frustule). Hence, this is imperative to document the diatom genera that are transported efficiently to the sediment. The present study analyzed diatom frustule abundance (valves g-1) and identified the major diatom genera in core top sediments (0.5cm) of 10 locations from the Central (21, 19, 15, 13, and 11 °N along 64 °E) and Eastern Arabian Sea (21, 17, 15, 13, and 11 °N at 200 m isobath).  This is the first of this kind and found a comparable frustule distribution from the surface sediments of both Central (av. 5.16±1.23×104 valves g-1) and Eastern Arabian Sea (av. 5.80±7.14×104 valves g-1). Size-based classification revealed that the contributions of medium-sized (30-60 µm) frustules from both the central (49 %) and eastern (51%) Arabian Sea were quite high. And the contribution of large-sized frustules (>60 µm) was higher in the central Arabian Sea (39%) compared to the eastern part (19%). A total of 40 diatom genera with 18 in common from both locations were detected from the sedimentary phytodetritus with Coscinodiscus and Thalassiosira being the dominating ones. In the north-central (21, 19, 15 °N) Arabian Sea, the prevalence of large-sized diatoms (Coscinodiscus) was attributed to open ocean upwelling as well as convective mixing during summer and winter monsoons, respectively. Such large species can easily escape grazing and sink rapidly due to higher ballasting. Further, the presence of the oxygen minimum zone at the intermediate depth in this region might reduce the remineralization and grazing pressure within the mesopelagic during their transport to the abyss. Whereas relatively smaller diatoms (Thalassiosira, Pseudo-nitzschia, Fragilaria, Nitzschia) were in high abundance towards the south-central (13, 11 °N) that area remains nutrient-poor. In the Eastern Arabian Sea, Thalassiosira was noticed in high abundance towards the southeast (15, 13, 11 °N), whereas the northeast (17, 21 °N) was dominated by Coscinodiscus and mostly due to the prevalence of coastal upwelling and convective mixing, respectively. Likely, these diatoms (Coscinodiscus, Thalassiosira, Pseudo-nitzschia, Fragilaria, Nitzschia) play a key role in transferring the organic matter from the surface to sediments and thus actively contribute to carbon capture, elemental cycling, and supplying food source for the benthic biota. This study highlights for the first time the biogeochemical significance of these diatoms from this highly productive oceanic province.