Estimation of terrestrial humic-like substances in inland lakes based on the optical and fluorescence characteristics of chromophoric dissolved organic matter (CDOM) using OLCI images

Abstract

Terrestrial humic-like substances (Peak C), the main constituent of chromophoric dissolved organic matter (CDOM), which is one of the optically active constituents present in water, is a good indicator of the ecological statuses of global aquatic ecosystems. In this study, the optical relationship between the absorption spectrum and the fluorescence spectrum of CDOM were investigated in a total of 168 samples collected from Lakes Taihu and Chaohu, and four components of the fluorescence spectrum were identified. There are significant positive correlations between the component arising from Peak C and the specific ultraviolet absorbance at 254 nm (SUVA), and the absorption coefficient of CDOM at 443 nm (a(443)), whereas the correlation between this component and the ratio of two absorption coefficients of CDOM (M) is negative. In addition, total nitrogen (TN), total phosphorus (TP), chlorophyll a and dissolved organic carbon (DOC) were shown to increase significantly with the increase in Peak C, highlighting the importance of the Peak C affecting the water quality in inland lakes. An optical algorithm was developed, which can be used to estimate the fluorescence intensity of Peak C based on Rrs(B6) and Rrs(B11) of the Ocean and Land Color Instrument (OLCI); the validation dataset showed that the mean absolute percentage error (MAPE) is 28% and the root mean square error (RMSE) is 0.13 QSU. This algorithm was successfully applied to OLCI images and used to analyze the distribution of Peak C in Lake Taihu. The resulting map indicated that the fluorescence intensity of Peak C was significantly higher near the lake entrance than in East Taihu, and the terrestrial input in the estuary area in the west is marked larger than other regions in Taihu Lake.