Satellites reveal how extreme rainfall affects coastal water quality in the Bohai Sea: Implications for marine environmental management.

Extreme rainfall is intensifying globally as the climate continues to warm. Yet how such events affect coastal water quality and potentially influence assessments of water quality improvement remains poorly understood. This study uses time-series optical satellite observations to characterize the response and recovery of coastal water quality following the 2023 extreme rainfall event in the Bohai Sea and to evaluate how this short-term disturbance influences monthly assessments of water quality improvement. The event substantially increased suspended particulate matter (SPM), chlorophyll-a, colored dissolved organic matter, and Forel-Ule Index values, while reducing water transparency (Secchi disk depth, ZSD) and sea surface salinity. In particular, SPM increased by 173% relative to the pre-rainfall baseline. The proportions of highly turbid and diluted waters increased by 27 and 10 percentage points (pp), respectively. Across all indicators, the strongest changes were found in estuaries and nearshore waters. Responses peaked five days after the event, and all indicators returned to the pre-rainfall level within two weeks. Such dramatic and prolonged changes in water quality can alter monthly means, with the monthly ZSD assessment for August 2023 showing a sensitivity of 13 pp to disturbance-period observations. This finding emphasizes the importance to distinguish event-driven variability from longer-term water quality changes in marine environmental management.