Hydraulic regulation reshapes aquatic ecological quality of river: Spectroscopic evidence from DOM compositional...

Hydraulic regulation profoundly alters hydrological regimes and biogeochemical processes in river-reservoir systems, potentially reshaping aquatic ecological quality by modifying the composition and transformation of dissolved organic matter (DOM); however, the underlying mechanisms and pathways remain poorly understood. This study investigates how regulation and seasonal hydrodynamics jointly influence the composition, transformation, and ecological functions of DOM in the Puhe River, Northeast China. By integrating excitation-emission matrix spectroscopy, PARAFAC modeling, and structural equation modeling, we characterized the spatial and seasonal heterogeneity of DOM and identified its primary environmental drivers. Results indicate that regulated regions were dominated by humic-like, refractory DOM associated with improved water quality, whereas unregulated regions contained higher proportions of labile, protein-like fractions linked to nutrient runoff and microbial activity. During the snowmelt-driven high-flow period, reservoirs effectively buffered DOM inputs and maintained compositional stability, whereas unregulated regions exhibited rapid DOM mobilization and increased ecological vulnerability. Structural equation model further revealed that land use, nutrient dynamics, and physicochemical conditions indirectly regulated DOM composition through cascading pathways. These findings provide spectroscopic evidence for the linkages between hydraulic engineering and carbon-nutrient interactions, providing a scientific basis for optimizing watershed regulation and mitigating ecological risks in high-latitude, snowmelt-influenced river basins under increasing human pressures.