Representative regional sampling of carbon dioxide and methane concentrations in hemiboreal headwater streams reveal underestimates in less systematic approaches

Abstract Boreal headwater streams have been identified as hot spots for evasion of greenhouse gases (GHGs). This study was the first to systematically determine the concentrations of CO2 and CH4 in hemiboreal headwater streams. The use of a headspace sampling method focusing on GHGs in combination with a statistically representative selection of more than 200 streams across two regions in Sweden was the basis for defining the base flow concentrations of CO2 and CH4. All streams were supersaturated relative to the atmosphere in CO2 and the majority in CH4 for the 82% of streams in which CH4 was detected. The spatial variability in both CO2 and CH4 was high but positively related to total organic carbon, mean annual temperature, and proportion of peatland in the catchment. There were, however, regional differences in the spatial controls, which are something that predictive models need to consider. The data set allowed for comparison between a headspace and an alkalinity‐based method for determining CO2. More than 50% of the streams contained no alkalinity which made the alkalinity‐based determination of CO2 impossible. In addition, half of the streams with alkalinity had alkalinities low enough (<0.07 meq L−1) to make the CO2 determination very uncertain. The streams with low pH and no alkalinity contained median CO2 concentrations that were 45% higher than the streams containing alkalinity. Therefore, large‐scale generalizations about CO2 in such headwaters will be significantly underestimated if (1) headwaters are underrepresented and (2) the headwaters are sampled but CO2 is calculated from their alkalinity.