Exploring the Sources of Unexpected High Methane Concentrations and Fluxes From Alpine Headwater Streams

The dynamics of methane generation and evasion from well-oxygenated, oligotrophic streams have been traditionally neglected. We estimated evasion of methane and assessed its sources and production pathways using a stable isotope approach in 16 oxygen-rich and C-poor (dissolved organic carbon: 55.32 ± 57.56 μmol/L) Alpine headwater streams. Methane was often supersaturated relative to the atmosphere (0.093 ± 0.179 μmol/L). Fluxes (0.87 ± 1.34 mmol·m⁻²·day⁻¹) were unexpectedly high and comparable to those from high-latitude lakes and reservoirs. Our findings suggest that methane in the streambed was largely produced from carbon dioxide reduction, whereas acetoclastic pathways and major deliveries from adjacent soils, assessed from a mass balance, may have contributed to stream water methane. This study sheds new light on high-alpine streams as a hitherto unaccounted source of methane to the atmosphere.