METHANE FLUX FROM BEDS OF BAUMEA-ARTHROPHYLLA (NEES) BOECKELER AND TRIGLOCHIN-PROCERUM R-BR AT BOOL LAGOON, SOUTH AUSTRALIA

The flux of methane from freshwater wetlands is likely to be influence d by the presence of emergent macrophytes. The plants generate an orga nic loading, which should increase methanogenesis, yet they also aerat e the sediment, creating oxic zones inimical to the obligately anaerob ic methanogenic bacteria and promoting the oxidation of methane by met hanotrophic bacteria. Moreover, emergent plants can act as a conduit f or methane to be vented to the atmosphere. Depth profiles of oxygen in beds of B. arthrophylla and T. procerum at Bool Lagoon, South Austral ia, showed that water at the sediment surface was anoxic during the ni ght but became oxygenated during the latter part of the day. Redox pro files of the sediment gave values that ranged from -50 to +250 mV, ind icating a sediment that was not highly reducing. Despite these conditi ons, the wetlands were sources of significant methane emissions, with total water-atmosphere fluxes ranging from <0.01 to 1.05 mmol m(-2) h( -1). The presence of B. arthrophylla, but not that of T. procerum, aff ected the bubble concentration of methane in the sediments. Diel varia tions in ebullitive or total methane fluxes were not detected, but emi ssions varied according to water depth and time of year. Treatments th at removed macrophytes reduced total methane fluxes to rates approxima tely equivalent to ebullition, indicating that the plants were acting as a major pathway for the release of methane to the atmosphere. The l ack of a clear diel pattern in emissions suggests that diffusion, rath er than convective flow, is the mechanism for this flux through the pl ants.