Spatiotemporal variations in sea surface chlorophyll-a in the Seto Inland Sea

Sea surface chlorophyll-a (SSC) variability in the Seto Inland Sea (SIS), Japan's largest semi-enclosed coastal system, reflects complex interactions among stratification, riverine inputs, oceanic exchange, and extreme climate events. Using satellite observations from 1998 to 2024, we investigated seasonal cycles, long-term shifts, and responses to extreme thermal events. Results reveal two distinct physical-ecological regimes. Inner basins (Regions 1-6 and 9) exhibit a stratification-retention regime characterized by positive SSC-SST and negative SSC-SSS relationships, with pronounced spring and autumn blooms and SSC frequently exceeding 7 mg m^-3 during productive periods. In contrast, outer gateways (Regions 7-8) follow a mixing/intrusion-paced regime linked to the meander of Kuroshio-driven saline intrusions, displaying lower baseline SSC (<0.5 mg m^-3) and inverse SSC-SST relationships. Spectral analysis confirms a coherent annual cycle (∼1.01 cycles yr^-1) across all regions, indicating monsoonal forcing as the dominant temporal driver, while regional amplitude differences reflect local nutrient pathways. Extreme thermal events (marine heatwaves (MHWs) and marine cold spells (MCSs)) exert asymmetric impacts. Among 462 MHWs and 395 MCSs detected, MHWs suppressed SSC in 61-83% of events (mean anomalies of -6% to -15%), particularly in inner basins, whereas MCSs enhanced SSC in 63-79% of events (mean anomalies of +15% to +86%) across all regions. Typhoon impacts were distance-dependent, with the strongest SSC perturbations confined within ∼100 km during passage and expanding to ∼275 km afterward, highlighting the role of post-storm cooling in stimulating phytoplankton growth. These findings demonstrate that basin confinement and hydrodynamic exchange structure SSC sensitivity to thermal extremes, with warming-driven stratification generally limiting nutrient resupply while cooling-induced mixing enhances productivity. This study provides the first basin-wide quantification of distance-dependent storm effects and contrasting MHW-MCS responses in the SIS, offering new insights for ecosystem prediction and adaptive coastal management under increasing climate variability.