Diurnal variation in methane fluxes can cause systematic errors in flux estimates, particularly at places with aquatic vegetation. Closed chamber measurements were made during the ice-free period in central Finland. Methane fluxes were quantified at an interval of a few hours for several boreal lakeshore communities consisting of Phragmites australis (Cav.) Trin. ex Steud. and of wetland sedges, grasses, and herbs. The automated measurement system was operated for 4 months in a P. australis stand and for 2 months in a site with mixed wetland vegetation. Additional manual measurements lasted for a few days. In P. australis stands the CH4 fluxes were typically highest around noon. On the average, efflux of CH4 measured during office hours (0800--1600 local time (LT)) should be corrected using a factor 0.68 to obtain a more reliable estimate of daily CH4 release. The fluctuation magnitude varied between different locations and over the study period, being the greatest in the most productive parts of the stands in August. A harmonic regression model, scaled with temperature and stand biomass, was successfully applied to reconstruct CH4 flux in P. australis stand. In the sedge, grass and herb stands the diurnal fluctuation in CH4 flux was generally small. Commonly, daily maximum in efflux occurred during evening or night and large (threefold) diurnal differences were occasionally found. Daytime measurements alone can result in a slight or moderate underestimate of the total flux for the wetland communities. These results recommend that spatial and seasonal differences in diurnal CH4 flux dynamics should be considered when planning measuring or modeling CH4 fluxes.