The impact of COVID-19 lockdown on the variation of sea surface chlorophyll-a in Bohai Sea, China

To prevent the pandemic of COVID-19, Chinese government implemented restrict city lockdown policy, which directly changed human daily habitats and customs, and further influence environment. Using the GOCI-derived sea surface chlorophyll-a Level-2 products, the impacts of city lockdown policy on sea surface chlorophyll-a in the coastal regions of Bohai Sea were evaluated by difference-in-differences (DiD) model. To improve the spatiotemporal coverage of the remote sensing data, data interpolating empirical orthogonal functions were employed to reconstruct the sea surface chlorophyll-a data. The results of DiD model showed that the city lockdown policy will contribute to a significant increment (with the range 1.12–1.14 mg/m3) of sea surface chlorophyll-a within 15 km from the coastline; meanwhile, the impact strength weakens with the distance to coastline increases. Applying the DiD model for the cities with human development index larger than 0.77, the inference is more prominent and significant with the increment (1.15–1.29 mg/m3) of sea surface chlorophyll-a; while non-significant increment of sea surface chlorophyll-a was detected for the cities with a human development index smaller than 0.77. The detected 15 km delimiter can be regarded as a guidance for environmental manager to make sampling strategies.