Gas Transfer Velocities Evaluated Using Carbon Dioxide as a Tracer Show High streamflow to Be a Major Driver of Total CO2 Evasion Flux for a Headwater Stream

Evasion of carbon dioxide (CO2) from headwater streams is a dominant process controlling the fate of terrestrially derived carbon in inland waters. However, limitations of sampling techniques inhibit efforts to accurately characterize CO2 evasion from streams, and particularly headwater streams with steep gradients, complex morphologies, and challenging terrain. CO2 source dynamics coupled with turbulence conditions control gas transfer velocities of CO2 (k(CO2)) and therefore drive CO2 evasion. We present estimates of K-CO2 and CO2 evasion from a steep, turbulent headwater stream in southwestern British Columbia, Canada, collected using an automated in situ CO2 tracer technique. Gas transfer velocities scaled positively with discharge, with a median k(CO2) of 36.8 m/day and a range of 13.5 to 169 m/day. Gas transfer velocities were highest during high-flow events, with 84% of all CO2 emissions occurring when discharge was higher than Q50, the median discharge (92.6 L/s). Widely used models overestimated gas transfervelocities with a mean relative error of 24% but underestimated k600 values above 165 m/day. Our determinations of gas transfer velocities for a range of streamflow suggest that CO2 evasion may be higher than previously estimated from direct measurements or models, particularly during high-flow events. These findings illustrate the need for direct, frequent, in situ determinations of K-CO2 to accurately characterize CO2 evasion dynamics in steep headwater streams. Plain Language Summary Characterizing the global carbon cycle is crucial for many aspects of earth science. Inland waters, such as streams, rivers, lakes, and estuaries, play a major role in the carbon cycle by carrying carbon from the land into water bodies and processing it, such that much of the carbon that enters an inland water body is either stored in sediments or evades into the atmosphere as carbon dioxide (CO2), a potent greenhouse gas. Headwater streams are particularly active sites of CO2 evasion, but a lack of data and limited sampling techniques inhibit our ability to accurately estimate and characterize this important system. This study provides new data on CO2 evasion from a headwater stream and suggests that high streamflow events, such as storms, drive CO2 evasion. This study also outlines a new technique for measuring CO2 evasion from streams, which will allow researchers to collect high-quality data more frequently.