Zooplankton Size Structure and Energy Transfer Characteristics under the Influence of Mesoscale Eddies in the Northern South China Sea during Spring: Insights from ZooScan Imaging

Mesoscale eddies are key oceanographic features influencing zooplankton community structure and ecosystem function. However, the vertical impacts of cyclonic and anticyclonic eddies on zooplankton energy transfer efficiency remain unclear in the northern South China Sea (SCS). We conducted a field survey in April 2023, collecting zooplankton samples with a multi-net system and analyzing them via ZooScan imaging technology. Size-based and trophic indicators—including the normalized biovolume size spectrum (NBSS), size diversity, and average equivalent spherical diameter (ESD)—were used to assess energy transfer efficiency across depth layers and eddy types. Results indicated significantly higher zooplankton total abundance, biovolume, and carbon biomass within cyclonic eddies (mean ± SD: 93.2 ± 25.7 ind./m³, 45.4±20.9 mm³/m³, 2.9±1.5 mg C/m³) compared to anticyclonic eddies (mean ± SD: 82.2±23.0 ind./m³, 37.8±14.0 mm³/m³, 2.4±0.9 mg C/m³) in the upper 300 m. Small copepods dominated all depth layers in both eddy types, comprising over 70% of the total abundance. Functional indicators, including the NBSS slope, size diversity, and average ESD, indicated higher energy transfer efficiency in cyclonic eddies within the upper 300 m. However, at the 0–25 m depth layers, anticyclonic eddies exhibited flatter NBSS slopes and higher size diversity than cyclonic eddies. Zooplankton productivity declined consistently with depth, while energy transfer efficiency to higher trophic levels showed a fluctuating vertical pattern and tended to rebound in deeper layers. Our findings highlight the crucial role of mesoscale eddy dynamics in structuring zooplankton communities and regulating energy flow in pelagic ecosystems of the northern SCS.