Sources and variability of CO₂ in a prealpine stream gravel bar

Gravel bars (GBs) contribute to carbon dioxide (CO 2 ) emissions from stream corridors, with CO 2 concentrations and emissions dependent on prevailing hydraulic, biochemical, and physicochemical conditions. We investigated CO 2 concentrations and fluxes across a GB in a prealpine stream over three different discharge‐temperature conditions. By combining field data with a reactive transport groundwater model, we were able to differentiate the most relevant hydrological and biogeochemical processes contributing to CO 2 dynamics. GB CO 2 concentrations showed significant spatial and temporal variability and were highest under the lowest flow and highest temperature conditions. Further, observed GB surface CO 2 evasion fluxes, measured CO 2 concentrations, and modelled aerobic respiration were highest at the tail of the GB over all conditions. Modelled CO 2 transport via streamwater downwelling contributed the largest fraction of the measured GB CO 2 concentrations (31% to 48%). This contribution increased its relative share at higher discharges as a result of a decrease in other sources. Also, it decreased from the GB head to tail across all discharge‐temperature conditions. Aerobic respiration accounted for 17% to 36% of measured surface CO 2 concentrations. Zoobenthic respiration was estimated to contribute between 4% and 8%, and direct groundwater CO 2 inputs 1% to 23%. Unexplained residuals accounted for 6% to 37% of the observed CO 2 concentrations at the GB surface. Overall, we highlight the dynamic role of subsurface aerobic respiration as a driver of spatial and temporal variability of CO 2 concentrations and evasion fluxes from a GB. As hydrological regimes in prealpine streams are predicted to change following climatic change, we propose that warming temperatures combined with extended periods of low flow will lead to increased CO 2 release via enhanced aerobic respiration in newly exposed GBs in prealpine stream corridors.