Cascading river damming amplifies photosynthetic organic matter production and DOC transport

Large-scale damming of global rivers has fundamentally altered hydrological connectivity and carbon cycling dynamics. However, cascade damming effects on the transport and transformation processes of dissolved organic matter (DOM) remain unclear. To address this, we integrated water chemistry, nutrient concentrations, optical properties, dual carbon isotopes (δ¹³C and Δ¹⁴C), and structural equation modeling (SEM) to elucidate organic carbon dynamics and underlying controlling mechanisms across the Wujiang cascade reservoirs. Results demonstrated intensified photosynthetic OM production during the thermally stratified period (April-October), evidenced by higher epilimnetic chlorophyll-a (8.0 ± 8.5 vs 1.0 ± 0.7 µg L^-1) and increased DOC concentrations (1.3 ± 0.5 vs. 0.9 ± 0.1 mg L^-1) compared to the vertical mixing period (January). Concurrently, isotopic evidence from enriched δ¹³C_DIC values and modern Δ¹⁴C_POC signatures in the epilimnion confirmed intensification of the biological carbon pump (BCP) effect, coinciding with dissolved inorganic carbon (DIC) uptake (29.0 ± 2.5 vs. 26.5 ± 5.2 mg L^-1) and nutrient assimilation (nitrate, 3.2 ± 0.6 vs. 2.7 ± 1.1 mg L^-1; phosphate, 92.1 ± 61.2 vs. 65.8 ± 76.3 µg L^-1). The SEM analysis identified DIC as the primary regulator of BCP strength across the cascade reservoirs (r = -0.679, p < 0.001). It is estimated that the annual DOC flux increased 17-fold from upstream (1076 ± 142 t yr⁻¹) to downstream reservoirs outlet (18,476 ± 667 t yr⁻¹), primarily governed by riverine DOM dynamics and hydrological regulation. Our findings reveal that cascade reservoirs profoundly alter biogeochemical processes related to DOM. With the increasing number of global reservoirs, elucidating the organic carbon dynamics of cascade reservoirs is crucial for accurate carbon budgeting and science-based reservoir management.