Bubbles dominated the significant spatiotemporal variability and accumulation of methane concentrations in an...

Climate-sensitive ice-covered reservoirs are critical components of methane (CH₄) release. However, the mechanisms that influence CH₄ dynamics during ice-covered periods remain poorly studied. To investigate the effects of bubbles on CH₄ dynamics, we conducted intensive field and incubation experiments in an ice-covered reservoir (ice growth, stability, and melt period) in Northeast China. We found that the mean dissolved CH₄ concentrations in the ice (625.9 ± 2419.7 nmol L^-1) and underlying water (1218.9 ± 2678.9 nmol L^-1) were high, making them atmosphere CH₄ sources. The visible bubble bands (bubble area) in the riverine zone and the vertical profile of the CH₄ concentration in the ice reflect the distribution of trapped bubbles. The mean CH₄ concentration in the ice of the bubble area (1674.8 ± 3926.8 nmol L^-1) was 2 orders of magnitude higher than that of no-bubble area (53.7 ± 9.2 nmol L^-1). Moreover, a large amount of CH₄ accumulated under the ice in the bubble area. These findings suggest that bubbles determine the CH₄ storage in ice and CH₄ accumulation in the underlying water. Ice growth increases CH₄ storage in ice and the underlying water because of the entrapment and re-dissolution of CH₄ bubbles. However, ice melting releases the CH₄ accumulated in the ice and underlying water. A comparison of the field and incubation experiments indicated that the deep-water environment of the reservoir had a CH₄ burial effect. Stepwise regression analysis revealed that higher sediment organic matter content, median particle size, and porosity increased the production and release of CH₄ bubbles, trapping more CH₄ bubbles in ice. Overall, this study improves the mechanistic understanding of CH₄ dynamics and predictability of CH₄ emissions during ice-covered periods.