Title:

Methane ebullition from headwater streams

Research has shown that streams and rivers are sources of methane, a potent greenhouse gas, to the atmosphere. However, not enough is known about the spatial and temporal variability of methane emissions across flowing waters to confidently scale to continental or global scales. While simple estimates of diffusive emissions of methane from flowing waters have been made, ebullitive (bubble) fluxes are completely ignored in these scaling exercises because of a scarcity of observations despite accounting for over 50% of total stream methane emissions in some studies. Here, we present the results of a two-year study of ebullitive methane emissions from four headwater streams of varying land use. We deployed passive bubble traps in multiple locations in each stream, providing a measure of in- and between-stream variability. On average, we observed significant ebullitive methane flux from three of the monitored streams. Land use did not appear to be a clear driver of differences in ebullitive flux. Temporal variability was driven largely by temperature, with warmer water associated with higher rates of ebullition. Spatial variability within the streams themselves was high with significant differences in patch-scale emission rates. Sediment characteristics weakly explained this variability. Finally, we compared ebullitive and diffusive emissions in two streams and found that ebullition comprised approximately 10% and 30% of total methane emissions. We conclude that ebullition should be included in most stream methane budgets. Moreover, this study demonstrates that spatial and temporal variability should be considered when monitoring ebullition within a stream; that is, multiple sites within a stream should be monitored at different parts of the year to most effectively capture the whole-stream ebullition rate.

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