Stream CO₂ emissions are overestimated without consideration of diel water and atmospheric CO₂ variability

Streams are significant emitters of carbon dioxide (CO 2 ) to the atmosphere that are influenced by diel CO 2 dynamics. However, we know little about diel CO 2 variability within streams, the diel dynamics of CO 2 in the air above streams, and the consequences for emission calculations. We studied five pre‐alpine streams by equipping three sites per stream in close proximity (~ 1 km apart) with automatic logging stations that continuously recorded water and air CO 2 partial pressures (pCO 2 ) for 2–4 d. All streams and sites showed increased pCO 2 at night and decreased pCO 2 during the day, however, with fourfold higher diel amplitudes for atmospheric pCO 2 compared to the water. Calculating diffusive CO 2 fluxes with fixed compared to dynamic measured atmospheric CO 2 resulted in negligible to 431% lower estimates. We might thus currently overestimate fluvial CO 2 emissions and should include diel water and air CO 2 variability to more accurately assess stream CO 2 emissions.