Hydraulic residence time thresholds and seasonal regimes governing DOM transformation in upper Yangtze River cascading reservoirs.

The molecular characteristics and biogeochemical transformations of riverine dissolved organic matter (DOM) have a significant impact on carbon cycling in freshwater ecosystems. Although individual dams influence DOM dynamics, the cumulative effects of sequential reservoir systems remain unclear. This study investigates five consecutive reservoirs along the Yangtze River through seasonal sampling (2021-2022). We introduce the concept of cumulative hydraulic residence time (cHRT), which represents the sequential water retention in a low-interference cascade reservoir. Our findings demonstrate that cascade damming amplifies the effects of individual reservoirs. Specifically, in longer cHRT with cooler water, preferential degradation of labile and oxygen-rich components leads to refractory DOM accumulation. Conversely, in shorter cHRT with warmer water, biosynthesis predominates, resulting in the production of nitrogen-/phosphorus-rich DOM with higher hydrogen-to-carbon ratios. System equilibria occur at critical cHRT thresholds (∼10 days) in low-interference environments. Contrastingly, the Three Gorges Reservoir exhibits altered dynamics under significant anthropogenic pressures, where energy-intensive processes accumulate low-molecular-weight and sulfur-rich saturated DOM. Our results reveal three operational regimes: 1) biodegradation-driven buildup of refractory compounds during prolonged cold retention, 2) productivity-driven formation of heteroatom-rich components under warm transient conditions, and 3) anthropogenic signals dominance in heavily modified systems. The cHRT framework quantitatively identifies the critical transition points of DOM evolution in reservoir cascades, advances predictive understanding of carbon cycling in regulated river networks, and informs sustainable dam management.