DIFFUSIVE FLUX OF METHANE FROM WARM WETLANDS

Diffusion of methane across the air-water interface from several wetland environments in south Florida was estimated from measured surface water concentrations using an empirically derived gas exchange model. The flux from the Everglades sawgrass marsh system varied widely, ranging from 0.18 +/- 0.21 mol CH4/m(2)/yr for densely vegetated regions to 2.01 +/- 0.88 for sparsely vegetated, calcitic mud areas. Despite brackish salinities, a strong methane flux, 1.87 +/- 0.63 mol CH4/m(2)/yr, was estimated for an organic-rich mangrove pond near Florida Bay. The diffusive flux accounted for 23, 36, and 13% of the total amount of CH4 emitted to the atmosphere from these environments, respectively. The average dissolved methane concentration for an organic-rich forested swamp was the highest of any site at 12.6 mu M; however, the calculated diffusive flux from this location, 2.57 +/- 1.88 mol CH4/m(2)/Yr, was diminished by an extensive plant canopy that sheltered the air-water interface from the wind. The mean diffusive flux from four freshwater lakes, 0.77 +/- 0.73 mol CH4/m(2)/yr, demonstrated little temperature dependence. The mean diffusive flux for an urbanised, subtropical estuary was 0.06 0.05 mol CH4/m(2)/yr.