River systems form key interfaces between terrestrial landscapes and the atmosphere, where weathering-derived and groundwater-transported carbon is transformed, retained, or released as CO₂. To investigate these dynamics in a transboundary river system, we conducted five spatially and seasonally resolved sampling campaigns in 2023 and 2024 along the Danube River, Europe´s second-longest river. We quantified the dominant dissolved inorganic carbon fraction (DIC; >90 % of total carbon), its stable isotope composition (δ13C_DIC), and calculated aqueous CO₂partial pressure (pCO_2(aq)) along the Danube´s main stem and selected major tributaries.

The upper Danube near its two springs exhibited the highest DIC concentrations (up to 5.3 mmol L^-1), indicating strong contributions from groundwater discharge and carbonate weathering. Associated δ13C_DIC values became progressively enriched downstream (from -13.3 towards -8.7 ‰), consistent with preferential loss of ¹²CO₂ via outgassing. Correspondingly, pCO_2(aq) values were predominantly oversaturated relative atmospheric CO₂ (>421 µatm), reaching a maximum of 3770 µatm, demonstrating persistent outgassing across seasons. Temporary near-equilibrium conditions occurred only at two downstream locations during summer, likely linked to enhanced photosynthesis and reduced turbulence. Overall, the Danube functions as a groundwater-influenced, year-round net source of CO₂, with limited seasonal buffering capacity. Downstream evolution of the carbon system is dominantly shaped by initial groundwater input, followed by continuous degassing. This novel high-resolution dataset provides a robust baseline for improving carbon budget estimates in Europe and other large river systems worldwide. It further supports future integrations of groundwater-river CO₂ interactions into continental-scale monitoring and modeling frameworks.