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

Abstract

Evasion of carbon dioxide (CO 2 ) 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 CO 2 evasion from streams, and particularly headwater streams with steep gradients, complex morphologies, and challenging terrain. CO 2 source dynamics coupled with turbulence conditions control gas transfer velocities of CO 2 (k CO2 ) and therefore drive CO 2 evasion. We present estimates of k CO2 and CO 2 evasion from a steep, turbulent headwater stream in southwestern British Columbia, Canada, collected using an automated in situ CO 2 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 CO 2 emissions occurring when discharge was higher than Q 50 , the median discharge (92.6 L/s). Widely used models overestimated gas transfer velocities with a mean relative error of 24% but underestimated k 600 values above 165 m/day. Our determinations of gas transfer velocities for a range of streamflow suggest that CO 2 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 CO 2 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 (CO 2 ), a potent greenhouse gas. Headwater streams are particularly active sites of CO 2 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 CO 2 evasion from a headwater stream and suggests that high streamflow events, such as storms, drive CO 2 evasion. This study also outlines a new technique for measuring CO 2 evasion from streams, which will allow researchers to collect high-quality data more frequently.

Gas exchange in streams and rivers

“…This difference is because within a stream, stream discharge (Q) controls k as stream flow velocity and stream depth varies with changing stream discharge, but with constant channel shape. Generally, k may increase with increasing stream discharge as velocity increases (Maurice et al, 2017;McDowell & Johnson, 2018;Roberts et al, 2007;Ulseth et al, 2019). These k~Q relationships tend to be site specific and even idiosyncratic (Genzoli & Hall, 2016).…”

Variations in dissolved greenhouse gases (CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity

“…However, there can be large errors associated with the computation of the dissolved CO 2 concentration from pH and total alkalinity (TA) for low alkalinity waters in particular so that high-quality direct measurements of dissolved CO 2 concentration are preferred (Abril et al, 2015), although scarce. In the tropics, research on GHGs in rivers has mainly focussed on South American rivers and on the central Amazon in particular (Richey et al, 1988(Richey et al, , 2002Melack et al, 2004;Abril et al, 2014;Barbosa et al, 2016;Scofield et al, 2016), while until recently African rivers were nearly uncharted with a few exceptions (Koné et al, 2009(Koné et al, , 2010.…”

Differential Response of Greenhouse Gas Evasion to Storms in Forested and Wetland Streams

“…N 2 O discharge dynamics are far less studied. In regard to gas evasion, discharge has been shown to be positively correlated with gas transfer velocities in headwater streams (Billett & Harvey, 2013;McDowell & Johnson, 2018). As more frequent and intense storms are predicted as a result of climate change (U.S.…”