Dissolved inorganic carbon in Chongqing urban section of Yangtze River, China

Understanding carbon cycling in reservoir ecosystems during hydrological regulation is essential for assessing inland water contributions to the global carbon cycle. Dissolved inorganic carbon (DIC) represents the dominant carbon pool in reservoir waters. In this study, Chongqing main urban section of the Yangtze River basin was selected as the research area. Low-water and reservoir periods were chosen, and the proportion and content of DIC in the reservoir water, as well as δ13C-DIC, were measured. Carbon stable isotope technology and a mass balance model were used to analyse the main sources of DIC and their influencing factors. Results indicate that DIC dominated the carbon pool in the main stream, primarily originating from soil carbon dioxide (CO2) inputs and carbonate weathering. Thermal stratification during the reservoir period significantly influenced vertical carbon distribution and accumulation processes. In summer, δ1³C-DIC exhibited significant negative correlations with temperature and pCO2 (p < 0.001) and positive correlations with pH and total particulate matter (p < 0.05), reflecting coupled physical and biogeochemical controls. In autumn, a strong negative relationship between DIC concentration and δ1³C-DIC was observed. These findings highlight the regulatory role of reservoir operation and stratification in shaping DIC dynamics along the river–reservoir continuum.