Ebullition Controls on CH₄ Emissions in an Urban, Eutrophic River: A Potential Time-Scale Bias in Determining the Aquatic CH₄ Flux

Rivers and streams contribute significant quantities of methane (CH₄) to the atmosphere. However, there is a lack of CH₄ flux and ebullitive (bubble) emission data from urban rivers, which might lead to large underestimations of global aquatic CH₄ emissions. Here, we conducted high-frequency surveys using the boundary layer model (BLM) supplemented with floating chambers (FCs) and bubble traps to investigate the seasonal and diurnal variability in CH₄ emissions in a eutrophic urban river and to evaluate whether the contribution of bubbles is important. We found that ebullition contributed nearly 99% of CH₄ emissions and varied on hourly to seasonal time scales, ranging from 0.83 to 230 mmol m⁻² d⁻¹, although diffusive emissions and CH₄ concentrations in bubbles did not exhibit temporal variability. Ebullitive CH₄ emissions presented high temperature sensitivity (r = 0.6 and p < 0.01) in this urban river, and eutrophication might have triggered this high temperature sensitivity. The ebullitive CH₄ flux is more likely to be underestimated at low temperatures because capturing the bubble flux is more difficult, given the low frequency of ebullition events. This study suggests that future ebullition measurements on longer time scales are needed to accurately quantify the CH₄ budgets of eutrophic urban rivers.