Seasonal Variation and Controlling Mechanisms of Dissolved Inorganic Carbon Cycling and Fluxes in a High‐Turbidity River Reservoir Under Different Regulation Regimes

Abstract The operation of reservoirs significantly impacts the cycling of dissolved inorganic carbon (DIC) in rivers, and yet such effects in highly turbid river reservoirs remain poorly understood. This study collected water samples from the Xiaolangdi Reservoir (XLDR) located in the lower reaches of the Yellow River, China, during four distinct periods: June and December 2017, and April and August 2018. The DIC concentration, isotopic composition, and total alkalinity were analyzed to investigate the seasonal variations and controlling mechanisms of DIC cycling and fluxes under different regulation regimes within this turbid reservoir. Artificial regulation affects the hydraulic residence time of the reservoir, which varies from 22 days in summer to 99 days in winter. This variation leads to a transition from thermal stratification to homogeneous mixing, resulting in changes in physicochemical factors. Consequently, there is a pronounced seasonal variation in DIC flux and storage. During the spring release period, DIC storage was second only to winter, despite the net output flow peaking at 8.0 × 10 4 t/month. In contrast, during the summer flood control period, DIC output flow reached its maximum while DIC storage in the reservoir was at its lowest, measuring 4.3 × 10 4 t. Major factors influencing DIC cycling in the XLDR include photosynthesis, organic matter decomposition, and carbonate precipitation and dissolution, exhibiting regional and depth‐dependent variations. These findings highlight the relationship between artificial regulation and biogeochemical processes, positioning the XLDR as a valuable natural laboratory for studying carbon cycling in inland waters in the Anthropocene.