Winter methane dynamics beneath ice and in snow in a temperate poor fen

The influence of winter on methane (CH4) stored in pore water and emitted through snow was investigated in a temperate poor fen in New Hampshire over two winters. Methane accumulated beneath ice layers (1 cm) deposited by freezing rain, resulting in snow-pore air mixing ratios as high as 140 ppmv during the first winter and 600 ppmv during the second. An early winter snow crust of 300 kg m(-3) caused no discontinuity in a linear mixing ratio profile and therefore was not observed to retard snowpack emissions. Methane concentration-depth profiles in pore water steepened and concentrations increased by as much as 400 mu M at the 10 and 20 cm depths as the ice cover formed. This suggests that the peat-ice cover plays an important part in CH4 build-up in pore water by limiting the transport of gases between the peat and the atmosphere. Pore water concentrations gradually declined through late winter. The seasonality of dissolved CH4 in pore water over two winters and one summer showed an average annual amplitude of 1.3 g CH4 m(-2) (25-75 cm depth range), with a winter maximum of 4.7 g CH4 m(-2). Emissions during the winter with average snowfall accounted for a larger percentage (9.2% in 1993-1994) of total annual emission than the winter with below-average snowfall and warmer air temperature (2% in 1994-1995). Emissions averaged 56 and 26 mg m(-2) day(-1) during the first and second winter (December, January and February), respectively.