Geological, land use and biological influences on carbon cycling and CO2 degassing in the Danube River

Abstract River systems form critical interfaces in the terrestrial-atmospheric carbon cycle, yet basin-scale constraints on the spatial and seasonal controls of inorganic carbon dynamics and CO 2 exchange remain limited. We present a multi-seasonal longitudinal assessment of dissolved inorganic carbon (DIC), its stable isotopes ( δ 13 C DIC ), and aqueous CO 2 partial pressure ( p CO 2(aq) ) along the entire Danube River. Five sampling campaigns captured seasonal variability along the main stem and major tributaries of this geologically diverse basin. DIC comprised > 90% of total carbon, showing limited longitudinal and seasonal variability (0.3 to 5.3 mmol L − 1 ), with highest concentrations in the upper Danube due to groundwater inputs and carbonate weathering, followed by downstream homogenization. In contrast, δ 13 C DIC increased downstream from − 13.3 to -8.7‰, indicating cumulative CO 2 degassing, with deviations linked to tributary inflows and biological activity. C 4 plant inputs may have contributed up to 27.2% to the δ 13 C DIC . River p CO 2(aq) ranged from ∼260 to 3,770 µatm, predominantly supersaturated, with CO 2 fluxes of 0.975–1.993 Gg C d⁻¹, confirming the Danube as a persistent CO 2 source. Near-equilibrium conditions in spring and summer coincided with enriched δ 13 C DIC , consistent with episodic photosynthetic uptake. These results reveal how groundwater inputs, in-stream photosynthesis, and air-water exchange regulate inorganic carbon dynamics and CO 2 emissions in large rivers.