The ecology of the Microinvertebrate Fauna in a temperate Australian floodplain river

Microinvertebrates play a critical role in riverine food-webs, and recent studies have hypothesised that such animals reproduce and recruit in non-flowing regions associated with the main channel, commonly termed slackwaters. However, little is known regarding the population and community dynamics of microinvertebrates in slackwaters, or regarding the mechanisms driving riverine microinvertebrate dynamics in general. The broad aims of this thesis were to examine the population and community dynamics of microinvertebrates in an Australian floodplain river, along with their response to physico-chemical conditions, food availability and predation. Microinvertebrate communities in the epibenthic and pelagic zones of slackwater and mid-channel regions (i.e. four habitats) were examined in relation to physico-chemical variables and food availability (estimated as chlorophyll-a concentration) between September 2005 and November 2006.
Results from this study indicated that microinvertebrate density and taxon richness were both greater in slackwater regions than in mid-channel regions on most sampling occasions (corresponding with the slower current velocities in the former), and were particularly high in slackwater-epibenthic habitats. Nevertheless, communities in mid-channel habitats were found to be a subset of communities in slackwater habitats overall. Furthermore, the communities in all four habitats were most abundant and diverse in late spring (corresponding with an increase in water temperature at that time of year) and followed the same general seasonal trajectory in terms of density, taxon richness, and community structure and composition. These findings support the view that slackwaters are important for in-channel microinvertebrate production, and suggest that animals frequently disperse (either actively or passively) among slackwater and other main channel habitats year round.
More than 50 % of the taxa in slackwaters were found to produce resting stages at some time between November 2005 and November 2006 while flow was confined to the channel. Most of these taxa were restricted to producing resting stages at a certain time of the year (particularly late spring), whereas only a few produced resting stages on more than one sampling occasion. Furthermore, taxa appeared to fit into one of three categories based on their benthic-pelagic coupling strategy: 1) those that produced their maximum number of resting stages in association with their active population maxima, 2) those that produced their maximum number of resting stages in association with their active population minima, and 3) those that did not demonstrate any relationship between their resting stage production and active densities.
These results indicate that most riverine taxa are capable of persisting through a range of conditions without augmentation from floodplain populations, via their dormant egg banks in slackwaters. To assess the role of predation for microinvertebrate dynamics in slackwaters, an in situ enclosure experiment was undertaken using different biomass levels of the planktivorous fish, carp gudgeon (Hypseleotris spp.) and mosquitofish (Gambusia holbrooki). High biomass levels of Hypseleotris suppressed the density of daphnid microcrustaceans, but enhanced the total density of rotifers, probably via a competitive advantage bestowed by selective predation on microinvertebrate taxa. High biomass levels of Gambusia, on the other hand, suppressed the total density of both microcrustaceans and rotifers. Furthermore, both planktivorous fish influenced the density of ovigerous taxa and resting stage production, and their effects often appeared to reflect those on active representatives.
Results from this study imply that high biomass levels of planktivorous fish can potentially influence microinvertebrate dynamics in riverine slackwaters, similarly to in many lakes and ponds. The influence of planktivorous fish probably impinges upon both active and resting stage communities due to the close coupling between the two communities. This research has been the first of its kind to highlight the value of both active and resting stage microinvertebrate communities in slackwater regions, together with their response to both abiotic and biotic factors. Further consideration of these aspects is needed to advance our knowledge and understanding of riverine microinvertebrate dynamics, and hence, the trophic ecology of rivers.