Spatiotemporal variability of the gas transfer coefficient ( K CO 2 ) in boreal streams: Implications for large scale estimates of CO 2 evasion

Boreal streams represent potentially important conduits for the exchange of carbon dioxide (CO 2 ) between terrestrial ecosystems and the atmosphere. The gas transfer coefficient of CO 2 ( K CO 2 ) is a key variable in estimating this source strength, but the scarcity of measured values in lotic systems creates a risk of incorrect flux estimates even when stream gas concentrations are well known. This study used 114 independent measurements of K CO 2 from 14 stream reaches in a boreal headwater system to determine and predict spatiotemporal variability in K CO 2 . The K CO 2 values ranged from 0.001 to 0.207 min −1 across the 14 sites. Median K CO 2 for a specific site was positively correlated with the slope of the stream reach, with higher gas transfer coefficients occurring in steeper stream sections. Combining slope with a width/depth index of the stream reach explained 83% of the spatial variability in K CO 2 . Temporal variability was more difficult to predict and was strongly site specific. Variation in K CO 2 , rather than p CO 2 , was the main determinant of stream CO 2 evasion. Applying published generalized gas transfer velocities produced an error of up to 100% in median instantaneous evasion rates compared to the use of actual measured K CO 2 values from our field study. Using the significant relationship to local slope, the median K CO 2 was predicted for 300,000 km of watercourses (ranging in stream order 1–4) in the forested landscape of boreal/nemoral Sweden. The range in modeled stream order specific median K CO 2 was 0.017–0.028 min −1 and there was a clear gradient of increasing K CO 2 with lower stream order. We conclude that accurate regional scale estimates of CO 2 evasion fluxes from running waters are possible, but require a good understanding of gas exchange at the water surface.