Ebullition Controls on CH 4 Emissions in an Urban, Eutrophic River: A Potential Time-Scale Bias in Determining the Aquatic CH 4 Flux.

Rivers and streams contribute significant quantities of methane (CH ₄ ) to the atmosphere. However, there is a lack of CH ₄ flux and ebullitive (bubble) emission data from urban rivers, which might lead to large underestimations of global aquatic CH ₄ 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 CH ₄ emissions in a eutrophic urban river and to evaluate whether the contribution of bubbles is important. We found that ebullition contributed nearly 99% of CH ₄ emissions and varied on hourly to seasonal time scales, ranging from 0.83 to 230 mmol m ^-2 d ^-1 , although diffusive emissions and CH ₄ concentrations in bubbles did not exhibit temporal variability. Ebullitive CH ₄ 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 CH ₄ 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 CH ₄ budgets of eutrophic urban rivers.