Ecological processes for management of wetlands and floodplains - decomposition processes

"July 1996".

Project Number: Ecological processes for management of wetlands and floodplains - decomposition processes - M/03/5100; LWRRDC Project LWMD10.

MDFRC item.

20 pages.

The decomposition of organic matter in the sediments of various wetlands in southeastern Australia was examined. Aerobic decay was estimated on the basis of sediment oxygen demand uptake, with appropriate killed controls. Possible anaerobic decay processes included denitrification, sulfate reduction, Fe3+ and Mn4+ reduction, and methanogenesis. As a range of evidence indicated that methanogenesis was the major route for anaerobic decomposition in the sediments, methane emissions were used as an indicator of anaerobic decay rates. Methanogenesis was at least as important as a pathway for organic matter decay in the sediments as was aerobic decay. Methanogenic archaea, quantified on the basis of their ether-linked lipids, comprised 11-36% of the total bacterial abundance in the sediments, with the methane-producing bacteria being more abundant in permanent than in ephemeral wetlands. Methane-oxidizing bacteria were also detected, on the basis of their distinctive phospholipids and via the use of 32p- and 33P-labelled oligonucleotide probes directed against l6S rRNA. Methane oxidation was detected in both the water column and the surface sediments. It is important because it reincorporates carbon into metazoan foodwebs that would otherwise be lost from the wetland, but methane oxidation complicates the interpretation of foodweb structure via stable-isotope analyses because of the anomalous C signature of biogenic methane. Aquatic plants and hydrological regime were major influences on the rate of benthic decomposition and on the relative importance of various decay pathways. Moreover, 50-95 % of the methane produced in the wetland sediments was vented to the atmosphere as a convective flux through the emergent shoots of the plants.