Downstream export dominates the fate of groundwater-derived CO₂ in a boreal stream

Abstract. Groundwater inflow is increasingly recognized as a major source of carbon dioxide (CO₂) to streams. Yet, its fate – whether it is emitted to the atmosphere or exported downstream – remains poorly characterized, partly due to the challenges of quantifying groundwater inflow rates at high spatial (meter) and temporal (days) resolutions. In this study, we assessed the fate of groundwater-derived CO₂ in a 400 m boreal headwater stream reach by combining fine-scale measurements of groundwater inputs, emissions and downstream export of CO₂. Spatial patterns in groundwater-derived CO₂ inputs were primarily driven by the magnitude of groundwater inflows, which were controlled by catchment characteristics, such as stream slope and localized aquifer properties. Temporally, peaks in groundwater CO₂ inputs during snowmelt were primarily driven by increased groundwater discharge rather than elevated CO₂ concentrations in the groundwater, whereas peaks during summer and early autumn were associated with rainfall events and higher CO₂ concentrations in groundwater, likely resulting from enhanced soil respiration. Overall, groundwater CO₂ inputs exceeded CO₂ emissions by up to fourfold, with 40–60 % of terrestrial CO₂ transported downstream. This indicates that a substantial portion bypasses immediate atmospheric emission and may contribute to CO₂ emission further along the stream network or be cycled through in–stream processes downstream. Our results demonstrate how and to what extent groundwater inflows contribute to the variability of CO₂ fluxes from headwater streams. These findings highlight the importance of integrative assessments of CO₂ fluxes (i.e. groundwater inputs, emissions, and downstream export), which consider both in-stream processes and catchment-scale dynamics. This is particularly important in the context of climate-driven changes in hydrology and terrestrial carbon cycling.