Impact of Water Level Draw-Down on Methane Fluxes From a Boreal Fen

Northern wetlands are the biggest natural source of methane into the atmosphere. Large methane emissions from these ecosystems are caused by the prevalence of high water table levels and the resulting anaerobic conditions, which are a prerequisite for methane formation. Global warming is predicted to bring about a water level draw-down in northern peat-forming wetlands, which may have an important effect on their methane emissions. We studied the impact of an experimental water level draw-down on the methane fluxes in a boreal sedge-dominated fen, located in Southern Finland. Measurements with closed chambers were carried out in the control and experimental site during four growing seasons. The first season was a calibration season without a water level draw-down treatment. In the beginning of the second growing season, the experimental site was surrounded with a shallow ditch that lowered the water table level approximately 16 cm. In the control site, annual variability in methane fluxes was considerable, total seasonal emissions ranging from 3.5 to 11 g CH4. The variability was connected to the variation in moisture conditions and to the variation in the leaf area of sedges. Prior to the water level draw-down treatment, methane fluxes did not differ between the control and the experimental sites. Following the treatment, the impact of the water level draw- down was immediate and significant. The daily mean fluxes and the total seasonal emissions were 82 to 95% lower in the experimental site than in the control site during the three water level draw-down years.