In situ acoustic quantification of methane ebullition in Xiangxi Bay, Three Gorges Reservoir

Ebullition constitutes a major pathway for methane emissions from reservoirs, particularly in tributary bays with steep depth gradients. However, methane (CH₄) bubble release characteristics remain poorly understood in Three Gorges Reservoir tributaries due to methodological limitations in quantitative monitoring. A novel underwater bubble generation device was employed to establish the target strength (TS) to bubble volume (V) relationship for echo sounder measurements. Sediment bubble release was monitored in situ in the Xiangxi River, a major tributary of the Three Gorges Reservoir (TGR). Site-specific acoustic TS-V relationships effectively enhanced the precision of acoustic bubble volume quantification. Fixed-point monitoring and comprehensive cruise surveys across depth gradients (0-70 m) revealed pronounced spatial heterogeneity in ebullition, with shallow areas (< 20 m) exhibiting fluxes 19-29 times greater than deep regions (> 40 m). A critical depth threshold of 40 m was identified, below which bubble formation is suppressed. Strong coupling between ebullition and dissolved CH₄ distributions (R² = 0.93) indicates bubble release drives dissolved methane concentrations more than sediment organic carbon content. These methodological advances provide essential tools for accurate reservoir methane emission assessments and highlight the importance of depth-dependent spatial variability in global carbon budget evaluations.