Snow-rich conditions coincide with higher CO₂ saturation, while CH₄ is generally oversaturated and N₂O near equilibrium in alpine sub-Arctic headwaters

Abstract. Headwater streams and lakes exchange greenhouse gases with the atmosphere, but how their gas saturation varies between years with contrasting snow conditions remains poorly understood in cold regions. Groundwater discharge may contribute to this variability by transporting dissolved greenhouse gases to surface waters. At an alpine sub-Arctic site in Norway, we sampled streams, lakes, groundwater and snow in summer 2020 and 2021. We measured carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in surface waters and groundwater to calculate saturation relative to atmospheric equilibrium. A Bayesian mixing model estimated groundwater contributions from water chemistry, and separate regression models assessed their relationships with gas saturation. Our results show that 2020 was an extremely snow-rich year, with the longest snow-cover duration in the 21-year satellite record, whereas snow-cover duration in 2021 was close to the long-term average. CO₂ saturation was higher in 2020 than in 2021, both overall (medians 306 % and 221 %) and at most sites sampled in both years. Higher aquatic CO₂ saturation coincided with reduced terrestrial carbon uptake reported for 2020 in the companion study by Pirk et al. (2023). Median CH₄ saturation exceeded atmospheric equilibrium in both years (320 % and 198 %). CH₄ saturation differed among surface-water sources in 2021. N₂O medians were near equilibrium (109 % and 110 %), with no differences detected among surface-water sources. CO₂ saturation tended to increase with estimated groundwater contribution after accounting for year and surface-water source. However, after accounting for uncertainty in the groundwater estimates, no statistically significant relationship was detected for any of the three gases. Estimated groundwater contributions tended to be higher in 2021, when CO₂ saturation was lower. The clearest between-year contrast was observed for CO₂, whereas CH₄ and N₂O showed no consistent directional change. Multi-year observations linking snow conditions, hydrological pathways and direct gas fluxes are needed to distinguish transport from biological processing and atmospheric exchange.