• boreal lake;
  • marsh;
  • flooding;
  • littoral zone;
  • methane

1. The annual dynamics of methane (CH4) in a temporarily flooded meadow, mire bank, lacustrine sedge fen, temporarily and continuously inundated sedge (Carex sp.) and reed (Phragmites australis) marshes were studied from June to November in the humic mesoeutrophic Lake Mekrijärvi and in eutrophicated parts of the mesotrophic Lake Heposelkä in the southern part of East Finland. The effects of water level and temperature on littoral CH4 fluxes were determined. Vegetation zonation along the moisture gradient, and associated CH4 fluxes, were evaluated.

2. The CH4 flux increased along the moisture gradient from –0.2 to 14.2 mg CH4 m–2 h–1, and was highest in the permanently inundated marshes. The duration of anoxia in the sediment caused differences in the CH4 flux. Estimated emissions for the period 1 June – 30 September in continuously inundated sparse reed and sedge marshes, drying sedge marsh, and lacustrine sedge fen were 13, 11 and 6 g CH4 m–2, respectively.

3. In continuously inundated vegetation, the fluxes were highest in late July/early August. The seasonal CH4 flux pattern suggested that the fluxes were regulated by the supply of organic matter during the course of the summer and the water level. In the temporarily flooded zone, the seasonal CH4 flux dynamics was greatly affected by changes in the lake water level, the fluxes being highest during the spring flood in early June.