Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Ebullition transports large amounts of the potent greenhouse gas methane (CH<jats:inline-formula><jats:alternatives><jats:tex-math>$$_4$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula>) from aquatic sediments to the atmosphere. River beds are a main source of biogenic CH<jats:inline-formula><jats:alternatives><jats:tex-math>$$_4$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula>, 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<jats:inline-formula><jats:alternatives><jats:tex-math>$$_4$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula> 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<jats:inline-formula><jats:alternatives><jats:tex-math>$$_4$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula> in the surface water. The data revealed highest activity with gas fluxes above 1000 ml m<jats:inline-formula><jats:alternatives><jats:tex-math>$$^{-2}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula> d<jats:inline-formula><jats:alternatives><jats:tex-math>$$^{-1}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula> in the center of the stream, sustained ebullition during winter, and a larger contribution of ebullitive compared to diffusive CH<jats:inline-formula><jats:alternatives><jats:tex-math>$$_4$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula> 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<jats:inline-formula><jats:alternatives><jats:tex-math>$$_4$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula> transported diffusively was oxidized. Predictors like temperature, air pressure drop, discharge, or precipitation could not or only poorly explain temporal variations of ebullitive CH<jats:inline-formula><jats:alternatives><jats:tex-math>$$_4$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
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</mml:math></jats:alternatives></jats:inline-formula> fluxes.</jats:p>