Hydrographic regimes structure zooplankton communities in the eastern South Yellow Sea

This study examined how seasonal stratification and hydrographic variability structure zooplankton communities in the eastern South Yellow Sea during summer and autumn of 2020-2021 (15 stations). Using CTD-derived density stratification (vertical stratification index; VSI) together with temperature, salinity, nutrients, and chlorophyll-a (Chl-a), we examined relationships between community composition, abundance, biomass, and β-diversity. Zooplankton abundance and biomass showed clear seasonal contrasts, with higher values in summer (2134 ± 1215 to 3151 ± 1989 ind. m^-3; 43.6 ± 45.5 to 58.8 ± 40.9 mg DW m^-3) than in autumn (684 ± 347 to 1222 ± 936 ind. m^-3; 24.8 ± 26.1 to 34.1 ± 14.1 mg DW m^-3). β-diversity partitioning revealed higher among-station similarity in summer (>70%), whereas autumn assemblages showed reduced similarity and increased species turnover. Stratification-regime contrasts differed between years: in 2020, strongly stratified offshore stations supported higher biomass with increased contributions of cold-temperate copepods (e.g., Calanus sinicus), whereas in 2021, expanded low-salinity surface waters were associated with a shift toward cladocerans (Penilia avirostris) and altered dominance patterns. Indicator species analysis identified season-specific taxa associated with hydrographic regimes, and redundancy analysis (RDA) linked indicator assemblages to gradients in stratification, temperature, and nutrient conditions. Collectively, these results suggest that VSI and indicator taxa provide an operational framework for tracking community shifts driven by stratification, mixing, and freshwater-related hydrographic variability in the eastern South Yellow Sea.