Variations of transparency derived from GOCI in the Bohai Sea and the Yellow Sea

Secchi disk depth (Z_sd), represents water transparency which is an intuitive indicator of water quality and can be used to derive inherent optical properties, chlorophyll-a concentrations, and primary productivity. In this study, the Z_sd was derived from the Geostationary Ocean Color Imager (GOCI) data over the Bohai Sea (BHS) and the Yellow Sea (YS) using a regional tuned model. To validate the GOCI derived Z_sd observations, in situ data, were collected for the BHS and YS regions. Results showed a good agreement between the GOCI derived Z_sd observations and in situ measurements with a determination coefficient of 0.90, root mean square error of 2.17 m and mean absolute percent error of 24.56%. Results for diurnal variations showed an increasing trend of Z_sd at the first and then decreasing, and all the maxima of Z_sd in the central areas of the BHS and YS were found in the midday. For seasonal variations, higher values of Z_sd, both in range and intensity, were observed in summer compared with those in winter. The reasons to explain the variations of Z_sd have also been explored. Solar zenith angle (SOLZ) has an impact on the daily dynamics of Z_sd, due to the influence of SOLZ on the attenuation of light radiation in water. The influence level of SOLZ on Z_sd is largely determined by the water bodies’ composition. The significant seasonal variations are mainly controlled by the stability of the water column stratification, because it can lead to the sediment resuspension and influence the growth and distribution of phytoplankton. Runoff and sediment discharge are not the main factors that impact the seasonal dynamics of Z_sd. Tidal currents and mean currents may have influences on the variations of Z_sd. However, due to the lack of in situ measurements to support, further studies are still needed.