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

Net methane fluxes as well as methane fluxes via gas bubbles and via molecu lar diffusion were measured separately at nine different sampling sites thr oughout a beaver pond located in the low boreal forest region of central On tario, 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 bu bble flux rates were highly variable in space and time, suggesting that low sampling frequencies, common in many methane emission studies, could intro duce large uncertainties to the estimated annual flux rates for different w etland systems. The effects of changes in environmental variables on methan e emissions were found to be strongly dependent on which transport mechanis m dominated the release of methane from the sediments. While the gas bubble flux volume was mainly affected by changes in atmospheric pressure and sed iment temperatures, the diffusive flux component was found to be more sensi tive to changes in the pond water level. Overall, magnitudes of methane flu xes 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.