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 (δC) 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 δC in coastal waters during the winter and summer monsoon (-22.40 ± 1.76‰), linked to increased phytoplankton biomass (2.26 ± 2.21 μg l) and dominance of diatoms (micro-phytoplankton) (65.35 ± 30.98%). In contrast, lower δC 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 δC ranges were identified based on phytoplankton composition; diatoms (micro-phytoplankton) dominated waters exhibited δC 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 δC in understanding its relationship with phytoplankton composition. Baseline information on δC will aid in developing stable isotope isoscapes of the EAS, enhancing our understanding of trophodynamics and animal migratory patterns.