Chasing Bubbles: Toward a Standardized Approach for Quantifying Methane Ebullition Using Bubble Traps in Streams and Rivers

Ebullition transports large quantities of the potent greenhouse gas methane (CH4) from stream sediments into the atmosphere. However, the use of different sampling designs complicates comparisons of CH4 emissions estimates between studies. Here, we present the first systematic review of sampling designs used in the past for assessing fluvial CH4 ebullition rates. In addition, we conducted field experiments to estimate the optimal number of bubble traps and the length of the sampling period, and to assess uncertainties in determining CH4 content in the emitted bubbles. Our results demonstrate considerable spatial variability in bubble fluxes (0-4.8 L m-2 d-1) and CH4 content in the bubbles (0.01-67%), necessitating a larger number of bubble traps than typically deployed in streams. Using fewer traps leads to greater deviations in CH4 ebullition estimates than shorter deployment times. Therefore, the robust sampling design should account for strong small-scale heterogeneity in streams through an adequate number of bubble traps and link bubble fluxes to trap-specific gas content. It should also consider the risk of CH4 concentration decreases in accumulated bubbles over a few days, primarily due to redissolution, followed by oxidation and leakage.