“Too many jumbucks in the billabong? Evidence of early post-European settlement impacts on billabongs of the Murray-Darling Basin”
Presented by Dr Michael Reid
Senior Lecturer, Geography and Planning, UNE
Thursday, May 14, 1 – 2 pm
C02 Lecture Theatre
Earth Sciences Building
Palaeoecological records from MDB billabongs show distinct patterns of ecological change following the first evidence of European settlement. Many billabongs appear to undergo rapid change during the early post-European period whereby previously abundant submerged macrophytes disappear and the billabong switches to a phytoplankton-dominated state. Other billabongs retain macrophytes through the post-European period, while still others are unstable, showing no periods of macrophyte or phytoplankton-dominance.
This basic response typology of ‘switching’ ‘stable’ and ‘unstable’ is hypothesised to reflect the interaction between underlying basin morphometry, hydrology and turbidity and the way this interaction influences the light climate of a billabong. Under this model, billabongs with relatively stable depths have the potential to exist in one of two alternative stable states of macrophyte or phytoplankton dominance. Among these billabongs, larger, deeper billabongs are more susceptible to reduced light penetration than smaller shallower billabongs because a reduction of a same magnitude in light penetration can result in the removal of a larger portion of the lake bed from the photic zone.
For billabongs with less stable water depths periods of ‘stable’ macrophyte dominance do not occur because frequent drying and short wet phases mean that the billabongs do not provide stable habitat suitable for submerged macrophyte communities to develop. Similarly, whilst algae are likely to exist in the water column in such billabongs during wet phases, assemblages are dominated by taxa able to persist both in the water column and benthic habitat. Further studies are needed to test this conceptual model using information on turbidity, depth and hydrology to predict the nature of billabong responses to the introduction of agriculture and river regulation.