Methane emissions from beaver ponds: Rates, patterns, and transport mechanisms

Abstract

Net methane fluxes as well as methane fluxes via gas bubbles and via molecular diffusion were measured separately at nine different sampling sites throughout a beaver pond located in the low boreal forest region of central Ontario, Canada. In 1990, the average daily methane emission rate was 37.2 ± 30.4 mg CH4 m-2 d-1, yielding a total annual methane efflux of 5.8 g CH4 m-2 of which 1.1 g CH4 m-2 were produced in the wintertime when the pond was ice covered. On average, 65% of the net methane flux was released via gas bubble emission, the remaining 35% via molecular diffusion. Gas bubble flux rates were highly variable in space and time, suggesting that low sampling frequences, common in many methane emission studies, could introduce large uncertainties to the estimated annual flux rates for different wetland systems. The effects of changes in environmental variables on methane emissions were found to be strongly dependent on which transport mechanism dominated the release of methane from the sediments. While the gas bubble flux volume was mainly affected by changes in atmospheric pressure and sediment temperatures, the diffusive flux component was found to be more sensitive to changes in the pond water level. Overall, magnitudes of methane fluxes from this pond were similar to other beaver ponds and moderate to high when compared to other wetland types in the low boreal forest region.