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 concentrations 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 mixing occurred and surficial concentrations reached as high as 4.8 mu M. Diffusive 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-weighted methane content of the lake increased during periods of stable thermal stratification (accumulation rates ranging from 16-1411 mg CH4 m(-2)d(-1)) and decreased during episodes of partial mixing (rates of decrease ranging from 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 magnitude (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 chamber ranged from 53-328 mg CH4 m(-2)d(-1) (mean, 163 mg CH4 m(-2)d(-1)). Ebullitive flux averaged 69% of total flux. Total methane flux was highest when thermal stratification was lost due to decreasing lake depth.