Abstract: Dissolved organic matter (DOM) in water plays an important role in biogeochemical cycles in aquatic environment, which affects water environment quality in watershed. Reservoirs are recharged by rivers and make a significant contribution to the supplement of drinking water, particularly Northern China. Therefore, the quality of reservoir water would be impacted by river pollution. In this study, UV-vis spectroscopy and three-dimensional fluorescence techniques were used to analyze the dynamics and sources of DOM in river water during both high and low flow seasons in Yuqiao Reservoir, a project of Diverting Luanhe River to Tianjin, and its inflow rivers. The results showed that newly produced humic substances were the primary components of river DOM during high flow season, with mixed internal and external inputs. Precipitation and river input had significant impacts on humic-like DOM during high flow season. The molecular weight of humus DOM was regulated by photodegradation during river transport. The abundance of proteins-like DOM was higher during low flow season, which corresponded to the low humification levels. The fluorescence index (FI) was below 1.9, indicating mixed contribution from internal and external sources of DOM during low flow season. Endogenous sources were typically regulated by metabolic accumulation of DOM under limited hydrodynamic conditions. However, due to severe point source pollution, high-molecular-weight proteins primarily originated from sewage inputs, resulting in a large accumulation of macromolecular DOM in the water column during low flow season. The humification index (HIX) and FI of the reservoir were lower than those of the inflow rivers, and the abundance of aromatic protein components was increased, which was obviously different from that of the inflow rivers of the same period. There was a strong correlation between the optical parameters, which was consistent with seasonal variations of river DOM. These observations suggest that DOM optical characteristics can effectively reflect its sources and fate in rivers. In the context of water resource protection in China, future consideration should be given to hydrological regulation on aquatic DOM sources.

Key words: dissolved organic matter, optical parameter, parallel factor analysis, Yuqiao Reservoir