Dynamics of Dissolved Inorganic Carbon and CO2 Emission Controlled by Reservoir Regulation in the Yellow River

Reservoir construction has significantly modified the export and residence of riverine carbon in global rivers; however, various strategies of reservoir operation also introduce great uncertainties into aquatic carbon transformation and associated ecological effects. The material transport in the Yellow River (YR) is currently manipulated by water-sediment regulation scheme (WSRS) of Xiaolangdi Reservoir (XLDR), an effective strategy for managing sediment-laden rivers worldwide. Here, we investigated the spatiotemporal variability of water chemistry and dissolved inorganic carbon (DIC), from within XLDR to downstream YR. The results revealed that during the water regulation of XLDR, downstream DIC export was controlled by carbonate weathering but influenced by enhanced oxidation of dissolved organic carbon and soil CO₂ flushing. In contrast, during the sediment regulation, XLDR-released particulate organic carbon (POC) underwent significant mineralization within ∼400 km transport range, resulting in water acidification, hypoxia and extremely high CO₂ partial pressure. Furthermore, the substantial CO₂ production markedly intensified the carbonate weathering of XLDR-released sediments. Major cation and isotopic analyses indicated that 81%–82% of the downstream DIC production originated from OC mineralization, while 18%–19% contributed by carbonate mineral dissolution. As a strong CO₂ source, the WSRS significantly accelerated the CO₂ evasion along the downstream YR, which was estimated at 0.27 ± 0.05 Tg C within a month, corresponding to a 27% increase in annual downstream CO₂ efflux. The CO₂ evasion was primarily driven by the reservoir sediment release and OC mineralization. These findings highlight the crucial role of reservoir regulation in modulating riverine carbon transformation and emissions.