Remote sensing of transparency in the China seas from the ESA-OC-CCI data

Water transparency, often measured by Secchi disk, is an intuitive indicator of water quality and plays an important role in aquatic environments. The European Space Agency (ESA) Ocean Colour Climate Change Initiative (OC-CCI) datasets provide daily and monthly remote-sensing reflectance over the period 1997–2019, which can be used to estimate Secchi disk depth (Zsd) with an innovative mechanistic model developed by Lee et al. (2015). In this study, the analytically-derived Zsd was validated by in situ Zsd measurements ranging from 0.1 to 38 m. The in situ Zsd measurements were collected in the first large-scale systematic survey on the marine environment in the China's offshore waters. Results indicated a good agreement between the in situ and satellite measurements. Based on the validated OC-CCI Zsd products, the spatio-temporal patterns and long-term changes of transparency in the China seas over the period 1997–2019 were studied. Regions with significant increase and decrease in Zsd on a long-term scale were also identified. Results of this work demonstrate the importance of both the OC-CCI Zsd products and remote sensing techniques in accessing water quality from long-term observations.