Distribution Characteristics and Sources of Dissolved Organic Matter in the River–Reservoir System of the Upper Yellow River

Dissolved organic matter (DOM) is the largest biologically available carbon pool in aquatic ecosystems and can mediate biogeochemical cycle processes. Its chemical nature and sources in marine environments and lakes have been extensively studied. However, DOM properties from river–reservoir systems in high altitude and severe cold climate regions remain unclear. In this study, the DOM components and sources in the upper “river–reservoir” system of the Yellow River were investigated using parallel factor analysis of fluorescence excitation–emission matrices. The factors influencing DOM florescence characteristics, especially the influence of cascade damming and frozen soil and freeze–thaw cycles, were explored. Our results demonstrated that the DOM fluorescence characteristics and sources of the river–reservoir system in the upper Yellow River had obvious temporal and spatial heterogeneity. Two humus-like substances (C1, C2) and one tryptophan-like substance (C3) were identified in the dry period. The fluorescence intensity of the cascade reservoir section (CR) was higher compared with the source region section (SR). External input and internal generation were the main sources of DOM, and the cascade reservoirs showed strong internal source characteristics. Two humus-like components (C1 and C2) were identified in the flood period, and the fluorescence intensity of the SR was higher compared with the CR. External input was the main source of DOM and the SR showed strong exogenous characteristics. The cascade reservoirs and seasonal freezing and thawing of frozen soil affected the fluorescence characteristics and sources of DOM. This study advances our knowledge of DOM florescence characteristics, sources, and influencing factors in river–reservoir systems in high altitude and severe cold climate regions.