Drivers and controls of greenhouse gas dynamics in subarctic, alpine catchments

Headwater catchments are known to substantially contribute to global carbon and nitrogen cycles through transport, storage, and direct emissions of greenhouse gases (GHGs). Despite extensive research on GHG dynamics in headwater systems, their drivers and controls remain elusive, particularly in cold region environments that are undergoing rapid transformations. Cryospheric changes, such as alterations in snowpack mass, are known to be strongly coupled with the hydrological cycle. However, we have limited insight into the nexus between snow cover changes, source water contributions (e.g., groundwater and glacial meltwater) to surface waters and associated biogeochemical cycling. To better understand the hydrological and biogeochemical changes in cold regions, we obtained field- and satellite-derived data from two sub-arctic catchments (one glaciated, one non-glaciated) in the north-western part of the Hardangervidda mountain plateau (South Central Norway). With this work, we aim to obtain an improved understanding of the impact of snow cover on GHGs dynamics in high-latitude, alpine catchments. During late summers in 2020 and 2021, we analysed various water sources including streams, lakes, groundwater, snow and ice for environmental tracers (major ions, stable water isotopes, radon-222) and GHGs  (CO2, CH4 and N2O). The combination of environmental tracer data with a Bayesian end-member mixing model allowed us to partition water source contributions to streams and lakes. To estimate snow cover anomalies between 2020 and 2021 compared to a five-year mean, we retrieved fractional snow cover durations (FSCDs) from 2016 to 2021 by applying a spectral unmixing algorithm to merged Sentinel-2 and Landsat 8 imagery over Finse. According to the satellite-derived data, 2020 was exceptionally snow-rich, while 2021 was a normal year. Our results indicate that GHG saturations distinctively differ among different water sources (e.g., lakes and streams), of which most are supersaturated. Thus, surface waters act as net sources for GHGs to the atmosphere, at least for the time windows of our sampling campaigns. Gas saturations distinctively differed between the glaciated and the non-glaciated catchments as well as between snow-rich and normal snow conditions. Groundwater is the most CO2 and CH4 supersaturated water source. However, groundwater only marginally contributed to surface waters and is thus not a major driver of GHGs emissions. Consequently, we hypothesise that snow cover, glacial meltwater, and resulting differences in subsurface water routing control GHGs dynamics at our study site. These findings provide new insights into the linkage between snow cover and the associated different hydrologic conditions and GHGs dynamics in high-latitude and alpine inland waters.