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 (