Methane Fluxes from an Australian Floodplain Wetland: The Importance of Emergent Macrophytes

Abstract

Methane emissions were quantified from beds of Eleocharis sphacelata R.Br., Myriophyllum sp., and Vallisneria gigantea Graeb. in a small floodplain lake (Ryans 1 Billabong; 36°07′S, 146°58′E) in south-eastern Australia over a 15-mo period, from March 1993 to May 1994. Gas from sediments of Vallisneria beds had the highest methane concentrations (about 60-90% v/v); methane concentrations in gas from sediments of Myriophyllum beds were the lowest (falling to 17 ± 3% v/v in May 1994), and concentrations in sediments of Eleocharis beds varied from about 25% v/v in winter to about 60-82% v/v in summer-autumn. Total water-atmosphere methane fluxes, measured with 0.8 m³ static chambers, varied from <0.01 to 2.75 mmol m^-2 h^-1 in Eleocharis beds, <0.01 to 0.95 mmol m^-2 h^-1 in Myriophyllum beds, and <0.01 to 1.64 mmol m^-2 h^-1 in areas of open water colonised by Vallisneria. Total fluxes were highly seasonal, being greatest in summer-autumn, when sediment temperatures exceeded 20°C. Strong diurnal and diel variations were also observed, especially in Eleocharis beds during the warmer months. Ebullition rates varied from <0.05 to 1.61 mmol m^-2 h^-1 and, like total water-atmosphere fluxes, fluctuated according to season, being lowest in winter and highest in summer-autumn. Ebullition seemingly accounted for 6-15% of total methane fluxes from Eleocharis beds in the cooler months, and 30-90% in the warmer months. Eleocharis shoots could generate pressures of up to 550 Pa during summer days, and the seasonal pattern to plant pressurization matched closely the seasonality of total methane emissions from Eleocharis beds. Manipulative experiments provided additional evidence that emergent shoots of Eleocharis were a significant pathway for methane transport from the benthos to the atmosphere, because cutting shoots below the water decreased emissions.