Spatiotemporal dynamics of carbon isotope ratios and phytoplankton composition in the eastern Arabian Sea

This study investigated major contributors of the particulate organic matter (POM) using stable isotope ratios of particulate organic carbon (δ¹³C_POC) and its relationship with phytoplankton composition during three seasons across six coast-offshore transects in the eastern Arabian Sea (EAS). Results revealed significant spatiotemporal variations, with elevated δ¹³C_POC in coastal waters during the winter and summer monsoon (-22.40 ± 1.76‰), linked to increased phytoplankton biomass (2.26 ± 2.21 μg l^-1) and dominance of diatoms (micro-phytoplankton) (65.35 ± 30.98%). In contrast, lower δ¹³C_POC during the spring inter monsoon (SIM) and in offshore waters during WM and SM (-23.31 ± 1.32‰) were associated with cyanobacteria (pico-phytoplankton) dominance (78.27 ± 22.75%). Positive correlations of POC with Chlorophyll a (Chla) and particulate nitrogen (PN) (r > 0.5, p < 0.05), along with their ratios (POC:Chla < 200 and POC:PN < 12), indicated the autochthonous nature of POM. Distinct δ¹³C_POC ranges were identified based on phytoplankton composition; diatoms (micro-phytoplankton) dominated waters exhibited δ¹³C_POC values between -23.83 and -19.19‰, while nano and/or pico-phytoplankton dominated showed values ranging from -26.00 to -22.15‰. This study highlights spatiotemporal variability in the POM composition and emphasises the importance of δ¹³C_POC in understanding its relationship with phytoplankton composition. Baseline information on δ¹³C_POC will aid in developing stable isotope isoscapes of the EAS, enhancing our understanding of trophodynamics and animal migratory patterns.