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

Area of Science:

• Marine Ecology
• Oceanography
• Remote Sensing

Background:

• Sea surface chlorophyll-a (SSC) variability in the Seto Inland Sea (SIS) is influenced by stratification, riverine inputs, oceanic exchange, and extreme climate events.
• The SIS, Japan's largest semi-enclosed coastal system, exhibits complex physical-ecological interactions.
• Understanding SSC dynamics is crucial for coastal ecosystem health and management.

Purpose of the Study:

• To investigate seasonal cycles, long-term shifts, and responses to extreme thermal events of SSC in the SIS using satellite observations from 1998 to 2024.
• To identify distinct physical-ecological regimes within the SIS.
• To quantify the impacts of marine heatwaves (MHWs), marine cold spells (MCSs), and typhoons on SSC.

Main Methods:

• Analysis of satellite-derived sea surface chlorophyll-a (SSC) and sea surface temperature (SST) data from 1998 to 2024.
• Identification and characterization of two distinct physical-ecological regimes: stratification-retention (inner basins) and mixing/intrusion-paced (outer gateways).
• Spectral analysis to confirm annual cycles and monsoonal forcing; detection and analysis of MHWs, MCSs, and typhoon impacts on SSC.

Main Results:

• Two distinct regimes identified: inner basins with high SSC blooms and outer gateways with lower baseline SSC.
• A coherent annual cycle (∼1.01 cycles yr⁻¹) driven by monsoonal forcing was confirmed across all regions.
• Marine heatwaves suppressed SSC (-6% to -15%), while marine cold spells enhanced SSC (+15% to +86%). Typhoon impacts were distance-dependent, with post-storm cooling stimulating phytoplankton growth.

Conclusions:

• Basin confinement and hydrodynamic exchange dictate SSC sensitivity to thermal extremes, with warming-induced stratification limiting nutrients and cooling-induced mixing enhancing productivity.
• This study provides the first basin-wide quantification of distance-dependent storm effects and contrasting MHW-MCS responses in the SIS.
• Findings offer new insights for ecosystem prediction and adaptive coastal management under increasing climate variability.