Temporal and spatial variation in methane emissions from a flooded transgression shore of a boreal lake

Variation of CH4 emissions over a three-year period was studied in a reed-dominated (Phragmites australis) littoral transect of a boreal lake undergoing shoreline displacement due to postglacial rebound. The seasonal variation in plant-mediated CH4 emissions during open-water periods was significantly correlated with sediment temperature. The highest plant-mediated emission rates (up to 2050 mg CH4 m(-2) d(-1)) were found in the outermost reed zone, where culms of the previous growing seasons had accumulated and free-floating plants grew on the decomposing culms. In reed zones closer to the shoreline as well as in mixed stands of reed and cattail, the maximum daily rates were usually >500 mg CH4 m(-2) d(-1). The total plant-mediated CH4 emission during the open-water period was significantly correlated with the seasonal maximum of green shoot biomass. This relationship was strongest in the continuously flooded (water depth >25 cm) outermost zones. In this area, emissions through ebullition were of greatest importance and could exceed plant-mediated emissions. In general, total emissions of the open-water periods varied from ca. 20 to 50 g CH4 m(-2) a(-1), but in the outermost reed zone, the plant-mediated emissions could be as high as 123 g CH4 m(-2) a(-1); ebullition emissions from this zone reached >100 g CH4 m(-2) a(-1). The proportion of CH4 released in winter was usually <10% of annual emissions. Emissions of CH4 were higher in this flooded transgression shore the than those measured in boreal peatlands, but the role of ancient carbon stores as a substrate supply compared with recent anthropogenic eutrophication is unknown.