Methane emissions from an Amazon floodplain lake: Enhanced release during episodic mixing and during falling water

Methane fluxes to the troposphere were measured in Lake Calado, a dendritic floodplain lake located in the central Amazon Basin. Methane concentration s in the surface water of the lake were less than 0.5 mu M during a period of high and rising water in April and May, except when episodic, deep mixin g occurred and surficial concentrations reached as high as 4.8 mu M. Diffus ive fluxes ranged from ca. 2-20 mg CH4 m(-2)d(-1) (mean: 6.6 mg CH4 m(-2)d( -1)) normally, but increased up to 220 mg CH4 m(-2)d(-1) during the passage of a rare cold front. During six weeks of rising water, the volume-weighte d methane content of the lake increased during periods of stable thermal st ratification (accumulation rates ranging from 16-1411 mg CH4 m(-2)d(-1)) an d decreased during episodes of partial mixing (rates of decrease ranging fr om 63-1792 mg CH4 m(-2)d(-1)). Surface water methane concentrations during low and falling water in September varied by more than an order of magnitud e (range, 0.3-9 mu M), and diffusive fluxes ranged from 3-158 mg CH4 m(-2)d (-1) (mean, 54 mg CH4 m(-2)d(-1)). During falling water in September, total methane flux from the lake to the troposphere measured with a floating cha mber ranged from 53-328 mg CH4 m(-2)d(-1) (mean, 163 mg CH4 m(-2)d(-1)). Eb ullitive flux averaged 69% of total flux. Total methane flux was highest wh en thermal stratification was lost due to decreasing lake depth.