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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 mu mol/L) Alpine headwater streams. Methane was often supersaturated relative to the atmosphere (0.093 +/- 0.179 mu mol/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.

Plain Language Summary The greenhouse gas methane is naturally produced and emitted from organic matter rich freshwater ecosystems. Normally, this gas is produced in places where oxygen is absent. However, recent studies have shown that methane is also emitted from oxygen-rich rivers and streams. But the knowledge and understanding of the underlying processes that cause this paradox is currently poor. In this light, we investigated organic matter-poor and oxygen-rich Alpine streams to better understand the methane dynamics in those systems. We found that methane emissions from these streams are similar to emissions from organic-rich lakes and reservoirs at high latitudes, probably due to high gas exchange velocities in these steep environments. Methane that is found in the streambed of these streams is likely produced from inorganic carbon in oxygen-depleted sediment pockets, while in the stream water it likely originates mainly from adjacent soils. Headwater streams are most numerous in river networks worldwide, often organic matter poor and oxygen rich. Together with the strong potential for methane emissions, it is clear that Alpine streams should be considered in future methane emission budgets and knowledge about methane sources should be improved for effective mitigation of methane emissions.