Influence of mesoscale eddies on the distribution of phytoplankton chlorophyll-a concentration in the East China Sea

The main purpose of this study is to investigate the influence of mesoscale eddies on the distribution of phytoplankton chlorophyll-a (Chl-a) concentration in the East China Sea. Mesoscale eddies (radius > 20 km) were identified using sea current data from the Copernicus Marine Environment Monitoring Service (CMEMS) GLORYS12V1 reanalysis (0.08° grid, 24-h intervals) via a vector geometry algorithm, and validated against sea level anomaly (SLA) assembled by Haiyang-2 (HY-2) altimeters and Chl-a from Haiyang-1C (HY-1C) Level-3 (L-3) (2023–2024). The CMEMS-based eddies show good consistency with SLA data for radii >100 km. Seasonal analysis of SLA/Eddy Kinetic Energy (EKE) and Chl-a cross-correlations indicate the strongest negative relationships in winter, suggesting that lower sea surface temperatures enhance phytoplankton sensitivity to eddy-induced dynamics. Results further reveal that mixed layer depth (MLD) strongly regulates Chl-a in cyclonic eddies (CEs), while its effect is weaker inside anticyclonic eddies (AEs) but more variable at their peripheries. Four interaction mechanisms were analysed – eddy stirring, eddy trapping, eddy-induced Ekman pumping and eddy intensification – classified as horizontal or vertical processes. Vertical mechanisms exhibit stronger links to Chl-a anomalies than horizontal ones. Notably, AEs exhibit peak Chl-a concentrations and vorticity in the upper-right quadrant, whereas CEs show maxima in the upper-left quadrant. Overall, the findings highlight eddy polarity, seasonality and vertical coupling as key drivers of phytoplankton distribution, underscoring the utility of high-resolution reanalysis for resolving ecological processes