Summer methane fluxes from a boreal bog in northern Quebec, Canada, using eddy covariance measurements

Abstract

A boreal bog located in the James Bay lowlands, Canada, was instrumented with an open-path gas analyzer to monitor the turbulent fluxes of methane throughout the summer of 2012. The mostly continuous eddy covariance measurements permitted the study of methane dynamics at the hourly, daily and seasonal scales. To exclude data segments for which the biological methane fluxes were underestimated due to inefficient atmospheric transport under stable stratification, we applied a novel approach based on both the atmospheric stability parameter ζ = z/L and the friction velocity u∗, where z is the measurement height and L the Obukhov length. The field measurements revealed the existence of at least one sustained ebullition event, triggered by low barometric pressures, a declining water table and increasing mechanical turbulence – suggesting that large-scale release of methane bubbles can be an important transport mechanism of methane in boreal bogs. The validity of similarity scaling for atmospheric methane under convective conditions was also assessed and the normalized standard deviations of methane concentrations did not scale well with ζ, highlighting the heterogeneity in natural methane production and release across the bog. Overall the hourly emissions ranged between −2.0 and 32.1 mg CH4 m−2 h−1, with a summertime mean of 2.4 mg CH4 m−2 h−1. At the daily scale, the two main controls on methane emissions were found to be the water table position and the peat temperature at 0.3 m under the surface. Contrary to other studies, seasonal methane emissions peaked when the water table was at its maximum distance from the surface, around mid-August. No clear diurnal pattern could be found in methane emissions, indicating that methane was produced quite deep within the peat. The seasonal emissions were estimated at 4.4 g CH4 m−2, and compared well with other observations over similar landscapes using different measurement techniques. Given that methane releases and transport are greatly affected by local characteristics such as climate and vegetative cover, this study emphasizes the need for further in situ continuous measurements of methane fluxes across northern peatlands.