Zooplankton trophic dynamics in response to upwelling phases in the eastern arabian sea: Insights from stable...

The study investigated the influence of upwelling-driven changes in physicochemical conditions and phytoplankton structure on the carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope composition of particulate organic matter (POM) and major zooplankton groups (crustaceans, chaetognaths, and fish larvae) in the eastern Arabian Sea (EAS) during the 2018 summer monsoon (SM). Enhanced upwelling in the early and peak phases stimulated diatom-dominated blooms (73-91 %) resulting in elevated Chla concentrations (1.39 ± 1.09 and 3.94 ± 3.38 μg L^-1). In contrast late-phase upwelling conditions were characterised by a deeper nitracline, leading to a shift towards smaller phytoplankton and dinoflagellates. The δ¹³C of POM (-21.8 ± 1.9 ‰) remained relatively enriched throughout, reflecting a sustained contribution from micro-phytoplankton. In contrast, δ¹⁵N of POM increased during late-monsoon (9.0 ± 2.5 ‰), consistent with nitrogen loss processes under low-oxygen conditions. Zooplankton δ¹³C and δ¹⁵N values exhibited similar phase-dependent pattern, indicating strong coupling with POM. Group-level isotopic signatures and trophic positions (TPs) revealed distinct feeding strategies: crustaceans exhibited herbivorous diets and lower TPs, whereas chaetognaths, and fish larvae occupied higher TPs indicative of carnivory. These findings underscore the dominant role of micro-phytoplankton (diatoms and dinoflagellates) in sustaining a shorter, classical food-chain during upwelling. This study provides the first regional-scale isotopic baseline for the EAS SM, with implications for isoscape development and trophodynamic modelling.