Methane flux from the Central Amazonian Floodplain

A total of 186 methane measurements from the three primary Amazon floodplain environments of open water lakes, flood forests, and floating grass mats were made over the period 18 July through 2 September 1985. These data indicate that emissions were lowest over open water lakes. Flux from flooded forests and grass mats was significantly higher. At least three transport processes contribute to tropospheric emissions: ebullition from sediments, diffusion along the concentration gradient from sediment to overlaying 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. It was estimated 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 the measurements were applied to the entire Amazonian floodplain, it is calculated that the region could supply up to 12% of the estimated global natural sources of methane

Tropospheric methane from an Amazonian floodplain lake

During July and August 1985, the sources of methane and its flux to the troposphere were measured from a houseboat laboratory anchored in Lago Calado, a stratified, dendritic lake of about 6 km2&nbsp;area, located in the central Amazon basin. Methane concentrations in the mixed layer of the lake were varied (0.0001&ndash;0.0055 mM) and usually less than 0.004 mM CH4, with no consistant temporal trend. Methane concentrations increased with depth across the thermocline as oxygen dropped to less than 0.1 mg O2&nbsp;L&minus;1. Over 6 weeks, methane increased from less than 0.08 to greater than 0.21 mM in the anoxic hypolimnion below 6 m. Methane in the pore water approached saturation, with a pure methane atmosphere within 5 cm of the sediment/water interface. The gradient-supported flux from the sediments to the overlying water could account for the methane increase in the bottom waters plus the surface flux. The measured methane flux from the surface of the open lake to the atmosphere averaged 27 mg CH4&nbsp;m&minus;2&nbsp;d&minus;1. This was consistent with the buildup in ambient methane in the nocturnal surface mixed layer of the troposphere. Ebullition contributed 70% to the average total flux. The diffusive flux measured with a static chamber ranged from 0 to 34 mg CH4&nbsp;m&minus;2&nbsp;d&minus;1, with an average of 8.3 mg m&minus;2&nbsp;d&minus;1. From July 20 to September 2, 1985, average ambient air concentration was 1.89 (&plusmn;0.16) ppm CH4, with a diurnal variation of 0.27 ppm.</span