Spatiotemporal variability of gas transfer velocity in a tropical high-elevation stream using two independent methods

Streams in high-elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO2) to the atmosphere by transforming terrestrial carbon to gaseous CO2. Studies of these environments are scarce and estimates of CO2 fluxed are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO2 evasion and other biogeochemical processes. The first method, ‘Kinematic k600’ (k600-K), is derived from an empirical relationship between temperature-adjusted k (k600) and the physical characteristics of the stream. The second method, ‘Measured k600’ (k600-M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO2 (pCO2) and CO2 evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5-week period during the wet season of a peatland-stream transition within a páramo ecosystem located above 4,000m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250-m reach on five occasions, and both methods showed a wide range of variability in k600 at small spatial scales. Values of k600-K ranged from 7.42 to 330 m d-1 (mean =116 ± 95.1 m d-1), whereas values of k600-M ranged from 23.5 to 444 m d-1 (mean = 121 ± 127 m d-1). Temporal variability in k600 was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L s-1). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L s-1). Our study provides the first estimates of k600 values in a high elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co-regulating gas transfer velocities in páramo streams.