Methane flux was measured at a Phragmites-dominated marsh in the Sandhills of north-central Nebraska from late July to September 1993. The eddy covariance technique employing a tunable diode laser spectrometer was used to measure spatially integrated fluxes of methane. Rates of methane emission increased rapidly after sunrise and peaked (at up to 50 mg CH 4 m -2 h-1) between midmorning and noon. The emission rates during midday hours were 2 to 5 times higher than the relatively constant rates observed at night. This marked diel variation was attributed to plant-mediated transport of methane by convective throughflow which typically accounted for about 60% of the total methane emission during the daylight hours. Our analysis suggests that the enhanced rate of plant-mediated methane emission during daytime is significantly correlated with changes in photosynthetically active radiation and the temperature difference between the canopy and the ambient air. Data on windy nights showed enhanced methane emission with increasing wind speeds, perhaps indicating an occurrence of Venturi-induced convection and/or enhanced ebullition. When averaged over the study period of 65 days, the methane flux was 25 mg CH4 m-2 h-1 during daytime and 8 mg CH4 m-2 h-1 during nighttime. Changes in the daily averaged methane flux were strongly correlated with sediment temperatures at 0--0.25 m depth. ¿ 1998 American Geophysical Union