Monitoring Water Transparency in Shallow and Eutrophic Lake Waters Based on GOCI Observations.

Water transparency represented by the Secchi disk depth (Z_sd) plays an important role in understanding water ecology environment variations, especially for optically complex and shallow lake waters. In this study, using in situ measured remote sensing reflectance (R_rs), diffuse attenuation coefficient (K_d), and Z_sd data collected in Lake Taihu (China), a regional algorithm for estimating K_d from R_rs was designed, and the semi-analytical model proposed by Lee et al. (2015) (hereafter called Lee_2015 model) was refined using a linear scaling correction for remote sensing of Z_sd. The results showed that a good agreement between the derived K_d and in situ measured data (mean absolute percentage error (MAPE) = 26% for K_d(490); MAPE < 5% for K_d at 443, 555, and 660 nm). The in situ R_rs-derived Z_sd results using the refined Lee_2015 model compared well with the in situ measured Z_sd (R² = 0.72 and MAPE = 36%), which was an obvious improvement over the Lee_2015 model in our study region. Subsequently, the refined Lee_2015 model was applied to the geostationary ocean color imager (GOCI) observations between 2012 and 2018 to yield the spatial and temporal variations of water transparency in the Lake Taihu waters. The long-term mean distribution of Z_sd revealed that water transparency values in the northeastern Lake Taihu were generally higher than those in the southwest part. Monthly climatological Z_sd patterns suggested that the Z_sd distributions had large temporal variability, and distinct monthly patterns of Z_sd existed in different subregions of Lake Taihu. The significant interannual variations of Z_sd in Lake Taihu are probably affected by a combination of the water column stability mainly caused by wind, water temperature, human activity, and riverine discharge. The present study can provide a new approach for quantifying water visibility and serve for water-color remote sensing of optically complex and highly turbid waters. [ABSTRACT FROM AUTHOR]