Using a multi-scale experimental approach, the research undertaken in this thesis investigated the role
of the flood pulse in ephemeral floodplain wetlands of the River Murray, in order to better understand
the impact of river regulation (and permanent inundation) on these wetlands.
An ecosystem-based experiment was conducted on the River Murray floodplain, to compare changes
in nutrient availability and phytoplankton productivity in three ephemeral wetlands (over a drying/reflooding
cycle) with three permanently inundated wetlands. In the ephemeral wetlands, both drying
and re-flooding phases were associated with significant increases in nutrient availability and, in some
cases, phytoplankton productivity. It was demonstrated that the ?flood pulse?, as described by the
Flood Pulse Concept (FPC), can occur in ephemeral wetlands in dryland river-floodplain systems,
although considerable variation in the nature of the pulse existed amongst these wetlands. Results of
this experiment suggest that factors such as the degree of drying and length of isolation during the dry
phase, the rate of re-filling, timing of re-flooding and the number of drying/re-flooding cycles may be
potentially important in producing the variation observed.
Permanent inundation of ephemeral wetlands effectively removed these periods of peak nutrient
availability and phytoplankton productivity, resulting in continuously low levels (of nutrient availability
and phytoplankton productivity). It was concluded that alteration of the natural hydrological cycle in
this way can significantly reduce nutrient availability, primary production and secondary production,
essentially changing the structure and function, the ecology, of these wetlands. Equally, the results of
this experiment indicate that some of the changes resulting from river regulation and permanent
inundation can be somewhat reversed, within a relatively short period of time, given re-instatement of
a more natural hydrological regime.
A mesocosm experiment was used to examine the influence of the dry phase, specifically the effect of
the degree of wetland drying, on patterns of nutrient availability and primary productivity comprising
the flood pulse. Compared to permanent inundation, re-flooding of completely desiccated sediments
increased carbon (C) and nitrogen (N) availability while partial drying generally decreased, or had little
effect on, C and N availability after re-flooding. However, degree of drying had little effect on
phosphorus availability or rates of primary production measured after re-flooding, and it is possible
that these two factors are related. Partial drying reduced rates of community respiration after reflooding,
possibly a reflection of the reduced carbon concentrations measured in these mesocosms in
this phase of the experiment. Degree of drying also influenced the macrophyte community (measured
after three months of flooding), with plant biomass generally decreasing and species diversity
increasing as the degree of drying increased (with the exception of complete sediment desiccation
which had lasting negative effects on both macrophyte biomass and species diversity).
The results of the ecosystem and mesocosm experiments were utilised, in addition to results collected
from the same experiment conducted at two smaller scales (minicosms and microcosms), to assess
whether the effects of hydrological regime on nutrient availability at the ?wetland? scale could be
replicated in smaller-scale experiments. None of the smaller-scaled experiments included in this
investigation were able to replicate the specific response to hydrological regime recorded at the
ecosystem scale, however the mesocosm experiment did produce results that were more similar to
those at the ecosystem scale than those produced by the mini and microcosm experiments. The
results of this study indicated that extrapolation of results from small-scale experiments should be
undertaken with caution, and confirmed that a multi-scale approach to ecological research is wise,
where large-scale field experimentation and/or monitoring provides a check on the accuracy, and
hence relevance, of conclusions reached via mesocosm experiments.
| Identifer | oai:union.ndltd.org:ADTP/219593 |
| Date | January 2005 |
| Creators | Francis, Cathy, n/a |
| Publisher | University of Canberra. Health, Design & Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Cathy Francis |
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