Interannual variability in particulate organic matter associated with physical forcing in the central Arabian Sea assessed from (stable) carbon and nitrogen isotopes.

The biogeochemistry of the Arabian Sea, the northwestern part of the Indian Ocean, is directly impacted by monsoonal wind reversal and is an area of strong ocean-atmospheric interaction. During the summer monsoon, coastal as well as open ocean upwelling occurs in the western, southeastern, and central parts of the Arabian Sea. The highest primary productivity rates are documented in these areas compared to the global oceans. Phytoplankton-derived particulate organic matter (POM) [Particulate organic carbon (POC) and nitrogen (PN)] play a central role in supporting the food chain as well as carbon export flux to the deep sea. Hence understanding the dynamics of POM concentrations and its stable carbon (δ¹³C_POC) and nitrogen (δ¹⁵N_PN) isotopic ratios are important in delineating its sources and recycling. However, such studies are scarce from the Indian Ocean region. Here we present the first study describing the POM dynamics during the summer monsoon from the central Arabian Sea, addressing the interannual variability. We studied the monsoonal changes in POM and its isotopic signatures in the central Arabian Sea (21–11°N; 64°E) during August 2017 and 2018. A strong, low-lying atmospheric jet (Findlater Jet) blows across the basin during the southwest (SW) monsoon. Positive wind stress curl resulted in “open ocean upwelling” to the north of the jet’s axis, characterized by substantially shallower Mixed Layers Depths (MLDs) and higher POM contents relative to the jet’s axis and its south. The highest wind speeds were observed in the center of the transect due to the presence of the jet’s axis. And the negative curl to the south of the jet’s axis resulted in downwelling and, consequently, the deepest MLDs. The molar ratio between POC and PN (6.2 ± 1.9 in 2017; 6.4 ± 0.9 in 2018) was close to the canonical Redfield ratio (6.63). The δ¹³C_POC values (−26.3 ± 1.4‰ in 2017; 25.5 ± 1.4‰ in 2018) exhibited typical marine signature and a noticeable inter-annual difference. Relatively higher δ¹⁵N_PN values in the north (7.68 ± 2.6‰ in 2017; 9.24 ± 3‰ in 2018) indicated the uptake of regenerated dissolved inorganic nitrogen from the oxygen minimum zone (OMZ). The lower δ¹⁵N_PN values along the jet’s axis and to its south were attributed to the eastward advection of upwelled waters from the western Arabian Sea. Higher wind speeds and jet-induced wind stress curl in 2018 resulted in lower sea surface temperatures (SST) and higher nutrient concentrations. Despite the higher nutrient availability in 2018, POC contents did not exceed the values in 2017. However, considering the total nitrogen consumption (according to C:N: P = 106:16:1), the potential POC development in 2018 could be double the value in 2017. The interannual differences in SW monsoon onset and wind speeds seemed to directly control the nutrient supply, affecting plankton community structure and POM variability. Thus, any future changes in the physical forcing may directly influence the POC pool and consequent export flux to the mesopelagic.