Abstract
Ebullition transports large amounts of the potent greenhouse gas methane (CH 4 ) from aquatic sediments to the atmosphere. River beds are a main source of biogenic CH 4 , but emission estimates and the relative contribution of ebullition as a transport pathway are poorly constrained. This study meets a need for more direct measurements with a whole-year data set on CH 4 ebullition from a small stream in southern Germany. Four gas traps were installed in a cross section in a river bend, representing different bed substrates between undercut and slip-off slope. For a comparison, diffusive fluxes were estimated from concentration gradients in the sediment and from measurements of dissolved CH 4 in the surface water. The data revealed highest activity with gas fluxes above 1000 ml m -2 d -1 in the center of the stream, sustained ebullition during winter, and a larger contribution of ebullitive compared to diffusive CH 4 fluxes. Increased gas fluxes from the center of the river may be connected to greater exchange with the surface water, thus increased carbon and nutrient supply, and a higher sediment permeability for gas bubbles. By using stable isotope fractionation, we estimated that 12-44% of the CH 4 transported diffusively was oxidized. Predictors like temperature, air pressure drop, discharge, or precipitation could not or only poorly explain temporal variations of ebullitive CH 4 fluxes.