Methane flux from the central Amazonian floodplain

A total of 186 methane measurements from the three primary Amazon floodplain environments of open water lakes, flooded forests, and floating grass mats were made over the period from July 18 through September 2, 1985. These data indicate that emissions were lowest over open water lakes, where flux averaged 27 plus or minus a standard error of 4.7 mg CH 4 m −2 d −1 ( n = 41). Flux from flooded forests and grass mats was significantly higher. Emissions from flooded forests averaged 192±26.8 mg CH 4 m −2 d −1 ( n = 90), while those from floating grass mats averaged 230±72.2 mg CH 4 m −2 d −1 ( n = 55). At least three transport processes contribute to tropospheric emissions: ebullition from sediments, diffusion along the concentration gradient from sediment to overlying water to air, and transport through the roots and stems of aquatic plants. Measurements indicate that the first two of these processes are most significant. Diffusive flux from flooded forests averaged 50.5±11.0 mg CH 4 m −2 d −1 , while that from floating mats averaged 43.7±11.8 mg CH 4 m −2 d −1 . Diffusive flux from open waters averaged 8.3±1.9 mg CH 4 m −2 d −1 . Emissions through bubbling were 168±32.2 (flooded forest), 346±128 mg CH 4 m −2 d −1 (floating grasses), and 17.3±5.2 (open water). We estimate that on the average, bubbling makes up 49% of the flux from open water, 54% of that from flooded forests, and 64% of that from floating mats. If we apply our measurements to the entire Amazonian floodplain, we calculate that the region could supply up to 12% of the estimated global natural sources of methane.