Billabongs are remnants of past river meanders along Australia’s low gradient floodplains. They are similar in morphology and formation to oxbows elsewhere, but their ecology reflects a unique evolutionary time-scale. The Murray-Darling river system, for example, occupied its present drainage basin 60 million years ago in Gondwana. For millions of years, as Australia moved north, the low-gradient rivers meandered back and forth across their floodplains – billabongs formed and re-formed. In this ancient and highly stable river system, extant billabongs may have occupied the same position for thousands of years. They have been subjected to variations of climate, from tropical to arid, but have not been scoured by glaciation. During periods of aridity billabongs may have provided refugia and foci of speciation, and a source of colonizing biota when the climate became wetter.
As part of a long-term study of microfaunal ecology in billabongs, we report here on the species composition and population dynamics of the resident rotifer community in a River Murray billabong pre- and post-flooding in summer 1990. The significance of microfaunal community responses is then considered in relation to the billabong ecosystem, particularly in the context of floodplain food webs.
Daily plankton collections were taken from a billabong of the River Murray for two weeks prior to inundation in March1990 and continued for ten days after flooding. Quantitative responses of the plankton community and the component species were analysed against measured environmental variables and between species. Rotifers and copepod nauplii were the predominant net plankton (> 53 μm). Significant negative or positive responses to inundation were detected for most common taxa of 63 rotifer species recorded. A four-fold dilution from intrusion of river water masked rapid population increases. Opportunistic responses to inundation appear to be a survival strategy in the highly unpredictable billabong environment.
Lor-Wai Tan & Russell J. Shiel
Murray-Darling Freshwater Research Centre P.O. Box 921 Albury, NSW 2640, Australia
Hydrobiologia 255/256: 361-369, 1993.
J. J. Gilbert, E. Lukens & M. R. Miracle (eds), Rotifer Symposium VI.
© 1993 Kluwer Academic Publishers. Printed in Belgium.