Mesoscale eddies are prevalent physical oceanographic phenomenon that significantly influence marine primary production and biogeochemical cycling. While previous studies have detailed the temporal progression of eddies, the impact of decaying eddies on phytoplankton communities has received less attention. To fill this gap, our comparative study, utilizing high-resolution regional field (10-30 km) and satellite observations in the northwest South China Sea, uncovers distinct differences in phytoplankton communities. In the center of the cyclonic eddy (CEC), where the dinoflagellate genus Gyrodinium predominated, the community was notably dissimilar to others (SIMPER dissimilarity: 68.57 ± 2.35%). The weakened upwelling in CEC decreased nutrient supply to the euphotic zone, intensifying species competition and leading to lowest diversity and community stability. Conversely, the cyclonic eddy edge (CEE), with higher chlorophyll a concentrations and diatom dominance under adequate nutrient conditions, exhibited the highest community stability. In the anticyclonic eddy center (AEC), the community was primarily composed of chlorophyta and dinoflagellata, with significant picophytoplankton Chloroparvula pacifica in the DCM layer (23.98 ± 4.52%), influenced by strong downwelling. However, at the edge of the anticyclonic eddy (AEE), diatoms and dinoflagellate co-dominated the community due to the sub-mesoscale upwelling, which provided relatively sufficient nutrients and minimized inter-species competition. Overall, we found a patchy distribution of phytoplankton and nutrients within mesoscale eddies, influenced by fine-scale physical structures. This study provides new insights into how different stage of mesoscale eddies shape phytoplankton communities and highlights the need to consider fine-scale physics to resolve the mechanisms behind phytoplankton distribution patterns.
 

Phytoplankton,mesoscale,submesoscale,eddy,the South China Sea