Spatial and temporal variations of methane flux from subarctic/northern boreal fens

Abstract

Emissions of CH4 were measured by a static chamber technique (five chambers/site) at 23 sites in four fens near Schefferville, subarctic Quebec. At three intensively‐monitored sites along a transect from fen margin to central and flooded sites, mean CH4 fluxes from June to August, 1989 were 65, 125, and 36 mg m−2 d−1, respectively. Pore water CH4 concentrations in the peat profiles to a depth of l m averaged 125 to 200 μM, with lower concentrations (generally <50 μM) at 0.1 m. Total, depth‐integrated storage of CH4 in the peat profiles ranged from 3.5 to 4.3 g m−2. Although CH4 flux was only weakly correlated with either peat temperature at 0.1 m or water table position within each site, there was a strong association of flux and these variables among the three sites, indicating the value of ecological attributes in identifying patterns of CH4 flux. A pulse of CH4 was recorded at two of the three sites in mid‐August, associated with a degassing of the peat profile, based on pore water CH4 concentrations. This pulse appeared to be initiated by the lowering of the water table by between 5 and 10 cm during a 3‐week period of low rainfall and is estimated to have contributed 18 to 65% of the seasonal CH4 emission, depending on the location of the site. An estimate of the regional CH4 flux from 130 km2 of the Schefferville area was based on CH4 flux measurements at 23 sites in the area, stratified by fen type (forested margin, margin, central, flooded, ridge, and pool). June to August regional CH4 flux was 18 mg m−2 d−1, for an area in which fen coverage was 29%. When extrapolated to the global scale, these results indicate that northern fens may contribute about 14 Tg yr−1, somewhat lower than other recent estimates.