Seasonal dynamics and source attribution of dissolved organic carbon in a representative river-reservoir system in...

Reservoir systems serve as critical conduits for terrestrial carbon transfer to aquatic ecosystems. However, research on the dynamics and sources of dissolved organic carbon (DOC) in river-reservoir systems within plateau regions remains incomplete. This study centered on the upper Yellow River's terraced hydropower section (Longyangxia-Lijiaxia-Liujiaxia) and employed spectral analysis and stable carbon isotope techniques to examine DOC's spatial-temporal distribution and source allocation. Results showed: (1) DOC concentrations exhibited significant spatial heterogeneity and seasonal variations, lowest in fall and highest in summer. This variation is likely closely related to precipitation patterns; spatially, the DOC concentration decreases and then increases along the course, with downstream DOC transport patterns altered due to the dam's blocking effect. (2) UV-Vis and EEM-PARAFAC analysis revealed low dissolved organic matter (DOM) humification, with endogenous sources predominating. Four types of fluorescent components were identified within the watershed: humic-like (C1, C2), fulvic-like (C3), and tryptophan-like (C4). (3) MixSIAR modeling results indicate that endogenous sources (e.g., algae) dominate contributions at 49.4 %, supplemented by exogenous terrestrial C3/C4 plant inputs at 29.3 %. Notably, the contribution from endogenous sources is more pronounced in summer, fall, and winter. This study elucidates the biogeochemical behavior of DOC in plateau rivers, providing evidence for carbon cycle research and the promotion of the "spectrum-isotope" combined method.