High-resolution in situ imaging reveals zooplankton continuous vertical distributions and eddy-driven community variations in the South China Sea

Mesoscale eddies are ubiquitous oceanographic features that play a pivotal role in regulating marine ecosystems by altering water column structure and redistributing biological resources. To examine their ecological effects on zooplankton communities in the South China Sea (SCS), we conducted in situ imaging observations using the PlanktonScope and Underwater Vision Profiler 6 systems within representative cyclonic eddy (CE) and anticyclonic eddy (AE) in April 2023. Integrating satellite remote sensing with in situ environmental measurements, we analyzed eddy-induced variations in zooplankton abundance, community composition, vertical distribution, and diel vertical migration (DVM) behavior. Zooplankton abundance within the upper 300 m was consistently higher in the CE, showing strong aggregation in the eddy core, whereas in the AE, abundance peaked at intermediate depths (50–100 m) near the periphery, forming a characteristic ring-shaped pattern. Seawater temperature and dissolved oxygen were identified as the dominant environmental drivers regulating zooplankton community structure, while nutrient enrichment at 100–150 m in the CE reflected upwelling-driven enhancement of bottom-up control. Zooplankton also exhibited more pronounced DVM amplitudes within the CE than in the AE, indicating that mesoscale hydrodynamic conditions strongly modulate vertical behavioral dynamics and trophic connectivity. Overall, mesoscale eddies exert a profound influence on zooplankton spatial ecology in the SCS by reshaping hydrographic structure, nutrient availability, and behavioral patterns, providing new insights into the coupling between mesoscale physical processes and biological responses in tropical ocean ecosystems.