Methane fluxes at the sediment–water interface in some boreal lakes and reservoirs

Methane (CH4) fluxes at the sediment–water interface were studied during summer in six lakes and two reservoirs in Finland. The oxygen penetration in the sediments of these boreal mesotrophic–hypereutrophic freshwater ecosystems varied from 0 to 8 mm, and the CH4 concentrations within the uppermost 10 cm from 17 to 51000 ppm(v). The diffusive methane fluxes to the overlying water column ranged from 0.44 to 105 mg m–2 d–1 and correlated negatively (R2 = 0.68) with the oxygen penetration in the sediment. This demonstrated that the CH4 concentration and the diffusive CH4 fluxes in the lake sediments are closely related to the O2 conditions, which regulate both CH4 production and CH4 oxidation in the sediment. The comparison between the diffusive CH4 fluxes at the sediment–water interface and CH4 fluxes at the water–air interface previously measured in situ in one lake and two reservoirs showed that the sediment CH4 fluxes poorly predict the CH4 release from the water surface to the atmosphere. This indicates that the CH4 oxidation in water column, advective transport of CH4 and CH4 ebullition have importance in the net CH4 release to the atmosphere. The increased CH4 concentration in water column close to the sediment is an indicator of serious oxygen depletion within sediment.