Spatiotemporal variability of gas transfer velocity in a tropical high‐elevation stream using two independent methods
Keridwen M. Whitmore, Nehemiah Stewart, Andrea C. Encalada, Esteban Suárez, Diego Riveros‐Iregui
Abstract
Abstract Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO 2 ) to the atmosphere by transforming terrestrial carbon to gaseous CO 2 . Studies of these environments are scarce, and estimates of CO 2 fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity ( k ), a critical variable in the estimation of CO 2 evasion and other biogeochemical processes. The first method, kinematic k 600 ( k 600‐K ), is derived from an empirical relationship between temperature‐adjusted k ( k 600 ) and the physical characteristics of the stream. The second method, measured k 600 ( k 600‐M ), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO 2 (pCO 2 ) and CO 2 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 4000 m 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 k 600 at small spatial scales. Values of k 600‐K ranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values of k 600‐M ranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability in k 600 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). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates of k 600 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.