Contrasting CO 2 concentration discharge dynamics in headwater streams: A multi‐catchment comparison

Aquatic CO 2 concentrations are highly variable and strongly linked to discharge, but until recently, measurements have been largely restricted to low‐frequency manual sampling. Using new in situ CO 2 sensors, we present concurrent, high‐frequency (<30 min resolution) CO 2 concentration and discharge data collected from five catchments across Canada, UK, and Fennoscandinavia to explore concentration‐discharge dynamics; we also consider the relative importance of high flows to lateral aquatic CO 2 export. The catchments encompassed a wide range of mean CO 2 concentrations (0.73–3.05 mg C L −1 ) and hydrological flow regimes from flashy peatland streams to muted outflows within a Finnish lake system. In three of the catchments, CO 2 concentrations displayed clear bimodal distributions indicating distinct CO 2 sources. Concentration‐discharge relationships were not consistent across sites with three of the catchments displaying a negative relationship and two catchments displaying a positive relationship. When individual high flow events were considered, we found a strong correlation between both the average magnitude of the hydrological and CO 2 response peaks, and the average response lag times. An analysis of lateral CO 2 export showed that in three of the catchments, the top 30% of flow (i.e., flow that was exceeded only 30% of the time) had the greatest influence on total annual load. This indicates that an increase in precipitation extremes (greater high‐flow contributions) may have a greater influence on the flushing of CO 2 from soils to surface waters than a long‐term increase in mean annual precipitation, assuming source limitation does not occur.