Quantifying coastal water quality responses to pandemic-induced human mobility: Insights from Bohai Bay

The COVID-19 lockdown presented an unprecedented opportunity to address the intricate interplay of drivers complicating coastal pollution sources during pandemic-induced human mobility. This study employed a quasi-analytical algorithm (QAA) to retrieve water transparency (Z_SD, in meters) from Sentinel-2 satellite imagery over Bohai Bay (BHB), China. The spatiotemporal and seasonal distribution of coastal waters reflected the combined effects of natural effects and human activities. Random Forest and Partial least square-path modeling (PLS-PM) demonstrated the mechanisms and effects of each factor (natural effects, anthropogenic pressure, and policy management) through different pathways, with industrial emission and marine policies incorporated to support coastal NPS pollution control. The major findings were as follows: (1) Water transparency improved significantly by 52.4% during the lockdown period (2020-2022) compared to 2017-2019. This improvement continued into 2023 and revealed spatial heterogeneity in water quality responses. (2) Port transportation ranked highly in feature importance, as it continued operating under government support, whereas urban transportation was restricted by lockdown measures. (3) It revealed that natural effects exerted indirect regulation on NPS pollution when anthropogenic disturbances were suppressed. Industrial data and marine policies further suggested the effectiveness of government interventions in coastal environment management. Overall, this study quantified the impact of natural and human factors during lockdown management, providing valuable insights into the sources of coastal water pollution.