Response of Dissolved Organic Matter to Thermal Stratification and Carbon Sink Indication in Drinking Water Reservoirs: Insights from Molecular Weight, Spectroscopy, and Watershed Analysis

Dissolved organic matter (DOM) plays an important role in drinking water reservoirs. However, the responses of DOM sources and properties to thermal stratification in watersheds with different molecular weights are not well understood. Using the six reservoirs of Luan River, Daqing River, and Hutuo River watersheds as subjects, the distribution characteristics of DOM with different molecular weights and basins were investigated through spectroscopy. The results indicated that the thermoclines in the Wangkuai and Gangnan reservoirs were the thickest (14 m). Three humic-like substances (C1, C2, and C4) and one protein-like substance (C3) were also detected. The fluorescence intensity of the DOM in the epilimnion and thermocline layers was also higher. There was a significant correlation (P < 0.05) between the chemical and thermal stratification indices of the C3 and C4 components. There are significant differences in the stratification of low molecular weight C3 and C4, Chla, BOD5, NO3–, and TN. Environmental factors (T, pH, and DO) were the primary factors affecting the DOM composition in the vertical direction. N and Chla are factors that influence the concentration and sources of DOM in different watersheds. This conclusion enhanced our understanding of the distribution of DOM and carbon sinks.