Short-term methane and carbon dioxide flux rates were measured in two created, experimental marshes in the Midwestern U.S. over a two-year period (2004–2005) in which hydrologic conditions were manipulated to simulate flood-pulse and steady-flow conditions. Gas flux rates were measured in three distinct wetland zones: continuously inundated areas; edge zones with emergent macrophytes; and edge zones in which emergent macrophytes were removed. Methane fluxes between years were not significantly different in edge zones with and without emergent vegetation, but were twice as high in continuously inundated zones during the steady-flow year compared to the flood pulsed year. There was no apparent relationship between emergent vegetation and methane flux, as mean flux rates were not significantly different in either year in edge zones where emergent vegetation was removed, compared to edge zones containing emergent vegetation. Continuously inundated wetland zones emitted methane from summer through fall, while in edge zones methane fluxes were only substantial in spring and summer. Neither daytime rates of carbon dioxide uptake or nighttime rates of respiration were significantly different between the years for any wetland zone. When CO2 flux rates (daytime uptake plus nighttime respiration) were normalized for solar radiation and day length, solar efficiency was found to be comparable between the steady flow and pulsed years. Methane fluxes were more strongly affected than carbon dioxide fluxes by the differences in hydrology, but only in the deeper areas of the wetlands.
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Vol. 28 • No. 2