Abstract

The flux of methane from freshwater wetlands is likely to be influenced by the presence of emergent macrophytes. The plants generate an organic loading, which should increase methanogenesis, yet they also aerate the sediment, creating oxic zones inimical to the obligately anaerobic methanogenic bacteria and promoting the oxidation of methane by methanotrophic bacteria. Moreover, emergent plants can act as a conduit for methane to be vented to the atmosphere. Depth profiles of oxygen in beds of B. arthrophylla and T. procerum at Bool Lagoon, South Australia, showed that water at the sediment surface was anoxic during the night but became oxygenated during the latter part of the day. Redox profiles of the sediment gave values that ranged from -50 to +250 mV, indicating a sediment that was not highly reducing. Despite these conditions, the wetlands were sources of significant methane emissions, with total water-atmosphere fluxes ranging from < 0.0 t1o 1.05 mmol m^-2 h^-1. The presence of B. arthrophylla, but not that of T. procerum, affected the bubble concentration of methane in the sediments. Diel variations in ebullitive or total methane fluxes were not detected, but emissions varied according to water depth and time of year. Treatments that removed macrophytes reduced total methane fluxes to rates approximately equivalent to ebullition, indicating that the plants were acting as a major pathway for the release of methane to the atmosphere. The lack of a clear die1 pattern in emissions suggests that diffusion, rather than convective flow, is the mechanism for this flux through the plants.