Relative importance of groundwater and sediment-produced methane in stream emissions of two boreal catchments

Inland waters are important sources of methane (CH₄) to the atmosphere. Significant quantities of CH₄ are shown to be emitted from stream networks, despite their small areal coverage. Considerable gaps and uncertainties exist in the knowledge on the regulation of stream CH₄ emissions, and their contribution to landscape scale C emissions. When the CH₄ input from groundwater/surface runoff or from sediment production reaches the discharge areas, CH₄ can either be microbially oxidised to carbon dioxide or emitted to the atmosphere. The relative importance of these sources and fates has implications for modelling and assessing long-term ecosystem CH₄ balances. In the existing body of literature, there is a clear lack of data on the share of groundwater CH₄ and sediment-produced CH₄ to the total CH₄ input in streams, the extent of CH₄ oxidation or emission of these sources and the spatial variability over whole-catchment scales. Here we present a study on the fates of ground water and sediment-produced CH₄ reaching stream environments in two different boreal catchments in Sweden. A combination of measurements, including CH₄ concentration gradients below stream beds, stable carbon isotope gradient measurements, high resolution stream flux and discharge assessments, were used to follow the transport of CH₄ below the stream bed to the stream water surface using inverse mass-balance modelling. The measurements covered all parts of the stream network in both catchments to include spatial variability. We show that around half of the total CH₄ entering the streams were from groundwater. Almost all the groundwater and sediment-produced CH₄ were oxidised (> 97%) before reaching atmosphere. Emissions to the atmosphere only represented a small fraction of the groundwater and sediment-produced CH₄ reaching the stream (< 3%), indicating that CH₄ oxidation is a major sink in the studied streams. Our data also reveals large spatial variability in surface water CH₄ concentrations, concentration gradients below the stream beds, CH₄ inputs, oxidation, and emission related to morphometry and presumably soil characteristics. We emphasize the importance of including spatial variability in stream networks to constrain the uncertainties in stream CH₄ budget studies.