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

Streams are significant emitters of carbon dioxide (CO₂) to the atmosphere that are influenced by diel CO₂ dynamics. However, we know little about diel CO₂ variability within streams, the diel dynamics of CO₂ 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 CO₂ partial pressures (pCO₂) for 2–4 d. All streams and sites showed increased pCO₂ at night and decreased pCO₂ during the day, however, with fourfold higher diel amplitudes for atmospheric pCO₂ compared to the water. Calculating diffusive CO₂ fluxes with fixed compared to dynamic measured atmospheric CO₂ resulted in negligible to 431% lower estimates. We might thus currently overestimate fluvial CO₂ emissions and should include diel water and air CO₂ variability to more accurately assess stream CO₂ emissions.