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

Abstract

Methane fluxes to the troposphere were measured inLake Calado, a dendritic floodplain lake located inthe central Amazon Basin. Methane concentrations inthe surface water of the lake were less than 0.5 µM during a period of high and rising water inApril and May, except when episodic, deep mixingoccurred and surficial concentrations reached as highas 4.8 µM. Diffusive fluxes ranged from ca.2–20 mg CH₄ m⁻²d⁻¹ (mean: 6.6 mg CH₄ m⁻²d⁻¹) normally, but increased up to 220 mg CH₄ m⁻²d⁻¹ during the passageof a rare cold front. During six weeks of risingwater, the volume-weighted methane content of the lakeincreased during periods of stable thermalstratification (accumulation rates ranging from16–1411 mg CH₄ m⁻²d⁻¹) and decreasedduring episodes of partial mixing (rates of decreaseranging from 63–1792 mg CH₄ m⁻²d⁻¹). Surface water methane concentrations during low andfalling water in September varied by more than anorder of magnitude (range, 0.3–9 µM), anddiffusive fluxes ranged from 3–158 mg CH₄m⁻²d⁻¹ (mean, 54 mg CH₄ m⁻²d⁻¹). During falling water in September, total methane flux from the lake to the troposphere measured with a floating chamber rangedfrom 53–328 mg CH₄ m⁻²d⁻¹ (mean, 163 mgCH₄ m⁻²d⁻¹). Ebullitive flux averaged 69% of total flux. Total methane flux was highest when thermal stratification was lost due to decreasing lake depth.