Spatiotemporal variability of the gas transfer coefficient (KCO2) in boreal streams: implications for large scale estimates of CO2 evasion

摘要

Boreal streams represent potentially important conduits for the exchange of carbon dioxide (CO2) between terrestrial ecosystems and the atmosphere. The gas transfer coefficient of CO2 (KCO2) 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 evenudwhen stream gas concentrations are well known. This study used 114 independent measurements of KCO2 from 14 stream reaches in a boreal headwater system to determine and predict spatiotemporal variability in KCO2. The KCO2 values ranged from 0.001 to 0.207 min−1 across the 14 sites. Median KCO2 for a specific site was positively correlatedudwith 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 reachudexplained 83% of the spatial variability in KCO2. Temporal variability was more difficult to predict and was strongly site specific. Variation in KCO2, rather than pCO2, was theudmain determinant of stream CO2 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 KCO2 values from our field study. Using the significant relationship to local slope, the median KCO2 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 KCO2 was 0.017–0.028 min−1udand there was a clear gradient of increasing KCO2 with lower stream order. We conclude that accurate regional scale estimates of CO2 evasion fluxes from running waters are possible, but require a good understanding of gas exchange at the water surface.