Absorption spectral characteristic dynamics of dissolved organic matter (DOM) from a typical reservoir lake in inland of Three Gorges Reservoir areas: Implications for Hg species in waters

Dissolved organic matter (DOM) is an important component in lake eco-systems. It plays a crucial role in the environmental fate of pollutants such as mercury (Hg), because of its specific characteristics and functional structures. In this study, a typical reservoir lake from inland of Three Gorges Reservoir areas, Changshou Lake was selected to track the changes of DOM geochemical properties for one year by using UV-Vis absorption spectroscopy technique. Meanwhile, based on observed Hg data in Changshou Lake, it further validated the key environmental implications of DOM for Hg distributions in lake. The results showed that as compared to January, other months including April, June and September had significantly higher CDOM, but also higher DOC with a slightly decreasing significance. Dynamics of chromphoric component is an important reason to explain the seasonal changes of DOM concentration in Changshou Lake. Chromphores of DOM were mainly derived from high molecular weight (MW) materials with high aromaticity. Also, three wavelengths fitting model of CDOM could be used for inversion of DOC concentration in annual monitoring. Meanwhile, seasonal differences of SUVA254 and S275-295 were significant. Lowest aromaticity and MW size were observed in January following an obvious increase from April. In contrast to other types of lakes, DOM aromaticity and MW size in Changshou Lake were lower than forest lakes, but higher than plateau lakes. Eco-system and land use types surrounding lakes could have a heavily impact on the heterogeneous properties of DOM. Importantly, no clear differences between concentrations of Hg species and SUVA254 and DOC respectively were observed, however chromphoric component and MW size controlled the dissolved Hg and reactive Hg in lake. Additionally, enrichment and migration of organic matter resulted from primary productivity in lake may be a substantial reason to explain the methylmercury changes.