METHANE FLUX IN A BOREAL FEN - SEASON-LONG MEASUREMENT BY EDDY-CORRELATION

Eddy correlation measurements of methane flux were made at a fen in ce ntral Saskatchewan, as part of the Boreal Ecosystem Atmosphere Study ( BOREAS) in 1994. Data were collected from mid-May to early October. Th e water table was above the average peat surface throughout the measur ement period. Detailed (near continuous) measurements allowed examinat ion of temporal variability at hourly and daily timescales. As compare d with the average nighttime flux, the average daytime methane flux wa s 25-45% higher in July and in August and 5-15% higher earlier and lat er in the season. Distribution of midday (1130-1430 LT) methane emissi on showed varying trends in different parts of the season. From mid-Ma y to early July, methane flux gradually increased from near zero to 4. 1 mg m(-2) h(-1). The water table height (above an average hollow surf ace) varied from 0.09 to 0.18 m, but the trend in methane flux followe d peat temperature (at 0.1-m depth) more closely. The peat warmed from 3.4 degrees to 16.3 degrees C during this time period. Methane flux w as negligible until peat temperature (at 0.1-m depth) was above 12 deg rees C. From early July to early August there was a fivefold increase in methane flux from 4.1 to its seasonal peak of 19.5 mg m(-2) h(-1) o n August 1. The water table ranged from 0.23 m to a brief seasonal pla teau of 0.30 m on July 20-23. Sharp increases in the water table were followed by increasing trends in methane flux by approximately 12 days . Feat temperature reached its seasonal maximum (19.3 degrees C) the s ame time when the methane flux peaked. After early August the methane flux declined steadily and reached a value of 2.4 mg m(-2) h(-1) in ea rly October. The water table and peat temperature showed comparable tr ends and decreased steadily to 0.06 m and 5.7 degrees C, respectively. The seasonally integrated methane emission (mid-May to early October) was estimated at 16.3 g C m(-2). Nonlinear regression analysis of met hane flux against water table (lagged by 12 days) and peat temperature was performed for different periods of the season. Except for a brief period of very high water table (when many hummocks were inundated) t he regression using water table and peat temperature explained between 68 and 94% of the variability in methane flux. The sensitivity of met hane flux to water table (or the slope of the log CH4 flux/water table relationship) obtained from our daily flux values ranged from 3.4 x 1 0(-4) to 5.0 x 10(-4) m(-1).