Seasonal shifts in zooplankton community composition and size-based functional proxies in the Yellow Sea

The Yellow Sea Cold Water Mass (YSCWM) is a major hydrographic feature of the Yellow Sea, but seasonal changes in zooplankton community composition and trophic-transfer-related patterns associated with the YSCWM remain insufficiently resolved. We conducted field surveys in April and July 2024 and used ZooScan imaging to quantify zooplankton taxonomic composition, biomass, size structure, and size-based functional proxies, including the normalized biovolume size spectrum (NBSS) slope, size diversity, mean body size, and trophic ratios. Zooplankton abundance, biovolume, and carbon biomass were significantly higher in July than in April, particularly in the cold water mass (CWM) region, and copepods dominated these metrics across seasons and regions. In the CWM region, zooplankton communities in July were characterized by steeper NBSS slopes, lower size diversity, smaller mean body size, and higher proportions of taxa classified as carnivorous than in April, suggesting a shift toward a smaller-bodied trophic structure with lower potential for efficient energy transfer. Specifically, the NBSS slope became steeper from -1.16 ± 0.16 in April to -1.19 ± 0.24 in July, while size diversity declined from 2.05 ± 0.21 to 1.73 ± 0.20; both indicators were positively correlated with the zooplankton-to-phytoplankton biomass ratio (Z/P) in July. Redundancy analysis showed clear seasonal differences in community-environment relationships: zooplankton community structure was associated with both surface and bottom hydrographic variables in April, but closely related to bottom-water temperature, salinity, and dissolved oxygen in July. These findings suggest a seasonal reorganization of zooplankton size structure and trophic-transfer-related proxies in the YSCWM region.