Characteristics and Source Analysis of Dissolved Organic Matter Components in the Dadu River Under Cascade Hydropower Development

Dissolved organic matter (DOM) plays a crucial role in aquatic carbon cycling and serves as a sensitive indicator of water quality changes, making it essential to understand its behavior in anthropogenically modified river systems. This study investigated the characteristics, sources, and influencing factors of DOM in the Dadu River cascade reservoir system. Through field sampling and laboratory analysis, key physicochemical parameters were measured, and DOM composition was characterized using three-dimensional fluorescence spectroscopy combined with the PARAFAC model. Three fluorescent components were identified: two terrestrial humic-like components (C1 and C2) and one autochthonous protein-like component (C3). Results suggest that the cascade reservoir system did not fundamentally alter the dual-source (allochthonous and autochthonous) of DOM inputs but induced spatial differentiation at individual reservoir. Reservoir retention enhanced the sedimentation and in situ transformation of particulate organic matter, leading to increased autochthonous contributions in quiescent zones. Conversely, discharge operations and construction phase disturbances modified DOM transport and composition patterns. Significant correlations were found between DOM components and nutrients. Specifically, TP correlated positively with C3 but negatively with C1 and C2, highlighting the close association between DOM composition and phosphorus dynamics. However, the FI showed no significant correlation with nutrients, suggesting lower sensitivity in mountainous rivers compared to plain river systems. This study demonstrates that cascade hydropower development regulates DOM dynamics through coupled physical (retention and disturbance) and biogeochemical (microbial degradation and photochemical transformation) processes.