Date of Award


Degree Type


Degree Name

Doctor of Philosophy


School of Science

First Advisor

Professor Pierre Horwitz

Second Advisor

Dr. Megan Huggett

Third Advisor

Associate Professor Ute Mueller


This biogeographical thesis tests for the presence of taxa-area and distance-decay relationships, which are common among macrobionts, in prokaryotic (bacterial) and micro-eukaryotic (ciliate) communities. Microbial biogeographical patterns may be distinct because of the high abundances, diversity and dispersal capabilities of microbes, in comparison to macrobionts. The Northern Ponds of Lake MacLeod, north-western Australia, provide an ideal location to address this topic, because the ponds are effectively hydrogeomorphologically identical, other than in surface area, and biotic histories can be assumed to differ only according to distance of separation. This means that hypotheses concerning species-sorting and neutral processes on microbial assemblages can be tested in a natural setting.

Characteristics of the physical environment were determined using bathymetric and hydrodynamic surveys in eight ponds. For each pond, evaporative outflow was determined using pan evaporation rates, and the hydrodynamic characteristics of each pond were described by measuring water flowing out of the ponds. Four pond morphotypes were distinguished on the basis of physical characteristics (surface area, volume and mean depth) and hydrodynamic properties (water residence time and percentage of evaporative loss).

For ionic and nutrient variation within and between the ponds, concentrations were expected to vary based on residence time of the brine within the ponds, evapoconcentration and subsequent precipitation of mineral phases. The water chemistry was found to be similar to seawater, with major ionic ratios remaining rather constant throughout each pond. Cygnet Pond differed from the other ponds in that it was enriched in Mg and Ca and depleted in K. Sediment characteristics were also investigated by microscopy. Six sediment types were described based on the particles found in each sample. There were no clear relationships between sedimentology types and water chemistry, and between each of these and the pond morphotypes.

The bacterial and ciliate biofilm communities were analysed using DNA community fingerprinting methods, and constrained to the above environmental parameters using redundancy analyses. Distance-decay relationships were found for the bacterial communities within the ponds, and occurred at relatively short distances (<100m). There were no such relationships for the ciliate communities. Taxa-area relationships were not found in either community. Spatial redundancy analyses suggest that β- diversity across the pond complex manifests itself mainly because of the differentiation of taxa occurrences among the ponds, and could not be explained by the environmental variables. Species co-occurrence models suggest significant segregation in community composition (i.e. not randomly assembled) while none of the communities appear to conform to predictions based on neutral theory.

The results therefore provide evidence that microbial bacteria and ciliate biofilm communities can conform to observed biogeographic patterns for macrobionts, although neither community displayed taxa-area relationships. The communities differed in that a distance-decay relationship was only found in the bacterial community, where ciliate taxa are distributed ‘patchily’, and not as a function of distance. An alternative model is proposed for the bacteria and ciliate communities of Lake MacLeod; each pond, because of their isolation from one another, is influenced by stochastic events which differentiate the ponds via ecological drift. This thesis demonstrates that these microbial communities are capable of having complex biogeographies, and that processes such as ecological drift may be important determinants of their structure.