Discharge Modulates the Dominance of Downstream Carbon Export Over Evasion in a Boreal Headwater Stream

Carbon dioxide (CO 2 ) evasion and downstream export of carbon (C) from headwater streams represent important fluxes in the global C cycle. Yet, these fluxes are generally studied in isolation, leaving gaps in the understanding of the overall role of streams in the C cycle. In this study, we carried out high resolution measurements of dissolved inorganic and organic C to estimate CO 2 evasion and C export along a 400 m reach of a boreal headwater stream to assess the magnitude and control of the C evasion:export ratio. Higher downstream C export (3.1–74.0 kg C d −1 ) compared to CO 2 evasion rates (0.53–2.56 kg C d −1 ) for the full stream network over the open water season resulted in an average C evasion:export ratio of 0.23, which corresponds to a 17% loss of C entering the stream through CO 2 evasion. The temporal variation in C evasion:export ratios (0.03–0.60) was mainly driven by stream discharge, largely through its strong influence on downstream C export. Further, CO 2 evasion showed high spatial variability, and we demonstrate that using only data of a subset of the stream reach would lead to a wide range in the overall C evasion:export ratios upscaled to the whole stream network. Resolving the processes controlling spatial and temporal variation in C fluxes and understanding the importance of discharge for the fate of C routed through streams is crucial for predicting the terrestrial C sink capacity at high latitudes under a changing climate.