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 (similar to 1 km apart) with automatic logging stations that continuously recorded water and air CO2 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 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.