Effects of wetting and drying on methane emissions from ephemeral floodplain wetlands in south-eastern Australia

Abstract

Four approaches were used to assess the effect ofinundation on methane emissions from floodplainwetlands in Victoria, Australia: (i) fieldobservations following natural rainfall events; (ii)experimental manipulation of water levels in smallfloodplain depressions; (iii) experimentalmanipulation of water levels in replicated mesocosms;and (iv) in vitro incubation of floodplainsediment under laboratory conditions. Raftery'sSwamp, a large (150 ha) wetland on the floodplain ofthe Goulburn River, became inundated in June 1993following autumn-winter rainfall. Methane emissionspeaked (1.7 ± 0.05 mmol m^-2 h^-1) somesix months later, and the methane content of sedimentgas bubbles reached 59% v/v, even though the positivesediment redox potentials (176 to 243 mV) indicatedthat sediments were only moderately reducing. Threesmall (< 1 ha) depressions on the floodplains of theRiver Murray and Kiewa River were inundated eithernaturally (by rain and/or overflow from nearby rivers)or artificially by flooding at specific times of year;emissions from these sites were usually negligibleafter flooding in autumn or winter. In contrast, theonset of methane emission was very rapid (within 3 to6 days) after the depressions had been flooded insummer, and the methane content of sediment gasbubbles could then reach 36% v/v. At their peak,emissions from the ephemeral wetlands were similar topeak emissions from permanent wetlands insouth-eastern Australia. Emissions from replicatedwetland mesocosms (4.5 m diameter, 0.9 m deep) werealways very small (<0.2 mmol m^-2 h^-1),regardless of time of flooding, water depths, orseason. In vitro incubation of wetland sedimentunder anaerobic conditions indicated a progressivedecrease in benthic methanogenesis with sedimentdesiccation and exposure to air. Ephemerallyinundated floodplain wetlands may be sites ofsignificant methane emission, especially over thesummer months. Moreover, the survival and rapidreactivation of methanogenic archaea after prolongeddrying of wetland sediments suggests thatmethanogenesis is possible even from re-wettedfloodplain environments that had earlier experiencedan extended dry phase.