Decoupling of carbon dioxide and dissolved organic carbon in boreal headwater streams

Streams and rivers emit large quantities of carbon dioxide (CO 2 ) to the atmosphere. The sources of this CO 2 are in‐stream mineralization of organic carbon (OC) and CO 2 input via groundwater inflow, but their relative importance is largely unknown. In this study, we quantified the role of in‐stream OC mineralization as a source of CO 2 in a number of nested boreal headwater streams. The results showed that mineralization of stream OC contributed 3% of CO 2 supersaturation at time scales comparable to the estimated water travel times in the streams (<24 h). Mass balances showed that downstream losses of OC were ≤3% in low‐order streams, whereas up to 16% of the OC was lost in the largest (fourth order) streams. In contrast, 85% of the CO 2 was lost along the stream network (longest total stream length = 17 km). Under the assumption that in‐stream OC mineralization was the main source of stream CO 2 , higher rates of OC mineralization (6% of OC) than those reported across the literature (≤0.7% of OC) would be required to sustain observed CO 2 supersaturation. Further, model results indicated that groundwater inflows were sufficient to sustain observed stream CO 2 concentrations. We hence conclude that in‐stream OC mineralization was a minor source of CO 2 in these boreal headwater systems and that the main source of stream CO 2 was inflowing groundwater transporting CO 2 originating from soil respiration.