Anoxygenic phototrophic bacteria and aerobic phototrophs in Normans Lagoon, a 'billabong' adjacent to the Murray River, south-eastern Australia

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research CentreMDFRC item.The development and demise of substantial populations of bacteriochlorophyll (BChl) a and d containing phototrophs are documented within the bottom waters of a billabong (oxbow lake) in south-eastern Australia. The observation of such populations within a freshwater body is unusual, illustrating an unexpected source of organic carbon within the floodplain ecosystem of one of Australia's principal rivers. Values of up to 1086 µg L−1 BChl a and 490 µg L−1 BChl d were recorded during an 11 month study of physico-chemical parameters, algae and bacteria. An unusual density stratification was observed within the billabong, fostered by an increase in bivalent ions at depth, and leading to persistent anoxia below 0.75 m depth over summer. A marked vertical distribution was observed for both algae and bacteria, that for the bacteria being lost with a mixing event in late March. The presence of up to 74 µg L−1 BChl a at the surface of the billabong during the summer months is ascribed to entrainment of bacteria within bubble plumes rising from the sediments.