Spatiotemporal Dynamics and Drivers of Stream pCO2 in a Headwater Mountain Catchment in the Cascade Mountains, Oregon

We examined the spatial and temporal variability of stream carbon dioxide (CO2) and the drivers of these variations in a headwater catchment. To examine temporal variation and drivers, we measured stream and hyporheic pCO2 at high temporal resolution over 11 months in a 95.9-ha forested headwater catchment in the Western Cascades of Central Oregon, USA. Stream and hyporheic pCO2 showed high seasonal and event-scale variability with distinct stream and hyporheic dynamics during storm discharge events. Hyporheic exchange flow exported 37.5 kg-C yr-1 per watershed hectare (confidence interval 4.0–122.3 kg-C ha-1 yr-1) from the riparian zone to the stream. Summing CO2 evasion and downstream advection suggests that one third of inorganic carbon export originated in the hyporheic zone. Hyporheic exchange flow had greatest influence over stream pCO2 during low and high baseflow, while CO2 evasion had greatest influence during storm discharge events. These findings suggest that the hyporheic zone actively participates in carbon cycling in this headwater stream and continuously replenishes stream CO2.

To examine spatial variation and drivers, we measured stream CO2 at monthly intervals from July 2013 through July 2014 at 38 locations across the 6400-ha HJ Andrews Experimental Forest. Stream pCO2 was consistently supersaturated with respect to atmospheric concentrations. Stream pCO2 ranged from atmospheric (about 400 micro-atm) to 20 times atmospheric concentrations (8150 micro-atm) and exhibited strong spatial and temporal variability. The distribution of pCO2 over the study period was different in small and large streams within the drainage network. At the watershed scale, pCO2 decreased with distance downstream. At the reach scale, we did not detect clear patterns in the downstream direction. However, individual transects displayed persistent profile shape, with consistent high and low pCO2 locations. We found negative relationships between stream pCO2 and stream discharge, mean velocity and the carbon dioxide gas transfer velocity. Stream pCO2 exhibited changes over short distances, with large changes in pCO2 over less than 50 m. Longitudinal variability indicates spatial variability of in-stream controls on pCO2 at this scale. Stream pCO2 shows generally higher concentrations during the summer and lower concentrations in the winter, with considerable intrannual variability.