Zooplankton community structure and vertical distribution in the South China Sea during the summer monsoon

To investigate the community structure and spatial distribution characteristics of zooplankton during the summer monsoon in the South China Sea, this study analyzed zooplankton samples collected from May to June 2021, focusing on species composition, abundance, and wet weight biomass. A total of 353 species/taxa were identified, with copepods being the most abundant group (167 species), followed by cnidarians (32 species) and planktonic larvae (30 species/taxa). In total, 15 dominant species were identified, all of which were classified as copepods, with Oithona similis being the only common dominant species across all water layers. Regions of high zooplankton abundances and wet weight biomass were concentrated in the northern SCS and were highly influenced by cold eddies. Considering vertical variation, the highest abundances and biomass were observed in the 0-200 m layer and these values decreased with depth. Although spatial distribution patterns differed from those of evenness index values, diversity indices were similarly highest in the 0-200 m layer. Cluster analysis revealed the presence of three zooplankton community groups including those in the 200 m layer at the cold eddy-influenced stations, the 200 m layers at other stations, and all other water layers. Redundancy analysis (RDA) modeling and BIO-ENV analysis indicated that seawater temperature, salinity, dissolved oxygen, and chlorophyll fluorescence concentration were significantly associated with zooplankton community structures and spatial distribution. These results provide valuable insights into the ecological responses of zooplankton to monsoonal dynamics and mesoscale eddies, thereby contributing to marine environmental management and resource conservation.