Hydrology and Bloom Shapes Dissolved and Particulate Organic Carbon and DOC Fluxes in Danjiangkou Reservoir

Reservoirs are increasingly recognized as active components of the global carbon cycle, acting as both sinks and sources of organic carbon while providing essential drinking-water services. Yet, the spatiotemporal behavior of organic carbon fractions and their coupled controls by hydrological extremes, algal blooms, and reservoir operations remain poorly resolved in large, regulated water-supply reservoirs. Here, we quantified the distribution, drivers, and transport balance of organic carbon in Danjiangkou Reservoir (China) through monthly monitoring from March to December 2024 across the main reservoir, downstream reach, and major tributaries. Dissolved organic carbon (DOC), particulate organic carbon (POC), and total organic carbon (TOC) were measured together with key water-quality parameters and hydrological indicators to identify the dominant controls. Across the reservoir–tributary network, DOC, POC, and TOC ranged from 1.03 to 13.17 mg/L, 0.03 to 45.40 mg/L, and 1.34 to 48.57 mg/L, respectively, with tributaries generating the largest extremes. Reservoir waters exhibited substantially lower concentrations than tributaries and showed pronounced seasonal heterogeneity, with DOC generally higher in autumn than in summer and spring. Event-scale analyses revealed two distinct regimes: precipitation-driven runoff primarily intensified POC and TOC through terrestrial particle inputs, whereas bloom periods increased DOC and, under severe conditions, elevated TOC. POC and TOC were positively correlated with inflow, indicating that inflow dynamics govern the delivery of particulate carbon. The annual DOC transport flux was about 61 kt, with seasonal alternation between sink and source months, indicating net interception of upstream DOC under specific hydrological and operational conditions. These findings clarify how tributary forcing and reservoir regulation jointly govern carbon routing in large drinking-water reservoirs, with implications for carbon budgeting and protection of source water.