Fen Connectivity and Snowmelt-driven Hyporheic Dynamics Drive Carbon Dioxide Budgets in Mountain Headwaters

Rivers and streams process and transport significant quantities of terrestrial carbon into global oceans and Earth’s atmosphere annually. However, determining stream carbon sources via quantitative budgets is difficult due to complex hydrologic and biogeochemical interactions at the land-water interface. Here, we develop carbon dioxide (CO2) budgets from two mountainous headwater streams in the East River watershed: one flowing through a high-elevation fen and one flowing through an alpine meadow. We performed three resazurin, chloride, and argon tracer release experiments, targeting comparisons between snowmelt and baseflow dynamics at one site as well as inter-site variability during baseflow. We also monitored stream geochemical conditions and dissolved organic carbon (DOC) of water samples. CO2 emissions were 5.5x higher during spring snowmelt than during baseflow and 1.5x higher at the site with greater fen extent during baseflow. During snowmelt, stream-corridor CO2 production supplied 72% of evasion from the stream, likely supported by discharge-mediated increases in exchange with the microbially active hyporheic zone. Stream-corridor production was ~14x greater during snowmelt than during baseflow, whereas groundwater CO2 inputs were only ~6x greater. The stream with greater fen extent featured ~2.5x higher CO2 concentrations and ~2x higher DOC concentrations during baseflow, indicating greater accessibility to carbon sources via the fen. These results demonstrate that both the source distribution and magnitude of in-stream CO2 production are highly dynamic, even in headwater systems which are often assumed to reflect solely groundwater contributions. Further, wetland connectivity to streams may sustain carbon inputs even during low flow conditions.