[Quantification of Methane Ebullition Flux from Small Ponds Using the Inverted-Funnel Method]

Methane emissions from water bodies are a significant contributor to greenhouse gas fluxes. While diffusion has been studied, ebullition remains less quantified in small ponds. Prior research has shown that ebullition can dominate methane flux in wetlands and lakes. However, no prior work had resolved the proportion of ebullition in subtropical shallow ponds. This gap motivated the use of direct field measurements to assess methane ebullition. The inverted-funnel method allows for precise quantification of gas bubbles escaping the water surface. Wind speed and water depth are known to influence methane release, but their combined effects on small ponds remain unclear. This study aimed to address these uncertainties by measuring methane flux in two small ponds over a two-week period. The results could help refine regional and global carbon cycle models.

The study aimed to quantify methane ebullition flux in subtropical shallow ponds using direct field measurements. Researchers focused on two small ponds in Quanjiao, Anhui Province, to assess the ratio of ebullition to total methane flux. The goal was to determine how much methane is released through bubbling versus diffusion. The study also sought to evaluate the influence of wind speed and water depth on methane ebullition. By comparing measurements from two ponds, the researchers aimed to identify patterns in methane flux variability. The inverted-funnel method was selected for its precision in capturing bubble emissions. This approach allows for high-resolution data on methane release dynamics. The findings could improve estimates of methane emissions from inland water bodies.

The study used the inverted-funnel and water equilibrium methods to measure methane flux in two small ponds. Measurements were taken from July 28 to August 13, 2016, in Quanjiao, Anhui Province. The inverted-funnel method captures gas bubbles at the water-air interface. Water equilibrium measurements tracked methane diffusion. Researchers collected data on ebullition and diffusion fluxes separately. Wind speed and water depth were recorded to assess their influence on methane release. Hourly and daily measurements were analyzed to identify temporal patterns. The study compared methane flux between pond A and pond B to detect variability.

The average methane ebullition flux was 121.78 mg·(m²·d)⁻¹ in pond A and 161.08 mg·(m²·d)⁻¹ in pond B. Diffusion flux averaged 3.38 and 3.79 mg·(m²·d)⁻¹ in pond A and pond B, respectively. Ebullition accounted for 97.5% of total flux in pond A and 96.4% in pond B. Methane ebullition ranged from 0.11 to 446.90 mg·(m²·d)⁻¹ in pond A and 0.05 to 607.51 mg·(m²·d)⁻¹ in pond B. Ebullition rates were higher during the day than at night. Wind speed was a primary driver of methane ebullition at the hourly scale. At daily scale, water depth and wind speed influenced methane flux. Positive correlation was observed between wind speed and methane ebullition.

The study found that methane ebullition dominates total flux in subtropical shallow ponds. The inverted-funnel method provided precise measurements of bubble emissions. Wind speed and water depth were key factors influencing methane release. Ebullition rates varied significantly between the two ponds. The findings suggest that wind speed controls methane ebullition at hourly intervals. At daily intervals, water depth and wind speed interact to affect flux. Methane flux was higher in mid-latitude regions compared to high-latitude regions. These results support the need for direct observations to refine carbon cycle estimates.