Riverine landscapes are complex. More than just a single channel, they comprise a
shifting mosaic of hydrogeomorphic patches with varying physical and biological
characteristics. These patches are connected by water during flows of varying magnitude
and frequency, at a range of spatial and temporal scales. Combined, landscape
complexity and hydrological connectivity create biological diversity that in turn
maintains the productivity, ecological function, and resilience of these systems. This
thesis investigates the ecological importance of spatial heterogeneity and temporal
hydrological connectivity in a dryland floodplain river landscape. It focuses on
anabranch channels, and uses major carbon sources in these and adjacent landscape
patches as indicators of ecological pattern and process.
A conceptual model was proposed, describing the potential effects upon the distribution
and availability of major carbon sources of: a) a spatial mosaic of hydrogeomorphic
patches in the landscape (e.g. anabranches, river channel, and wider floodplain); and b)
four primary temporal phases of hydrological connection during flow pulses
(disconnection, partial connection, complete connection, and draining). This was then
tested by data collected over a three year period from a 16 km reach of the lower
Macintyre River (NSW/QLD Australia). Results were examined at multiple spatial
scales (patch scale � river channel vs. anabranches vs. floodplain; between individual
anabranches; and within anabranches � entry, middle and exit sites).
The data indicate that spatial heterogeneity in the lower Macintyre River landscape
significantly influences ecological pattern. Carbon quantity was greater in anabranch
channels compared to adjacent river channel patches, but not compared to the floodplain;
while carbon quality was greater in anabranch channels compared to both adjacent river
channel and floodplain patches. Stable isotope analysis indicated that carbon sources that
were predominantly found in anabranch channels supported both anabranch and river
organisms during a winter disconnection phase. Other carbon sources found in the main
river channel and the wider floodplain appeared to play a comparatively minimal role in
the food web.
Different phases of hydrological connection between anabranch channels and the main
river channel were associated with differences in the availability of carbon sources. In the
river channel, draining of water from anabranches (the draining phase) was associated
with relatively high concentrations of dissolved organic carbon (DOC) and low
concentrations of phytoplankton. Conversely, the disconnection phase was associated
with relatively low concentrations of DOC and high concentrations of phytoplankton in
the river channel. In anabranch channels and their waterbodies, the disconnection and
draining phases were associated with high concentrations of both DOC and
phytoplankton. Concentrations of these carbon sources were lowest in anabranches
during the partial and complete connection phases.
Different hydrological connection phases were also associated with changes in trophic
status in the aquatic components of the landscape. On the riverbanks, relatively low rates
of benthic production and respiration during the complete connection phase were
associated with heterotrophy. The remaining phases appeared to be autotrophic. Benthic
production on riverbanks was greatest during the disconnection phase, and respiration
was greatest during the partial connection phase. In the anabranch channels, rates of
production and respiration were similar during the disconnection phase, and were
associated with heterotrophy in the anabranch waterbodies. The remaining phases
appeared to be autotrophic. Respiration was greatest in anabranches during the
disconnection phase, and production was greatest during the draining phase. Both
production and respiration were lowest during complete connection. These differences
and changes varied according to the landscape patch examined.
At a landscape scale, anabranch channels act as both sinks and suppliers of carbon. High
rates of sediment deposition facilitate their role as sinks for sediment-associated carbon
and other particulate, refractory carbon sources. Simultaneously, anabranch channels
supply aquatic carbon sources from their waterbodies, as well as via processes such as
inundation-stimulated release of DOC from surface sediments. Modelled data indicated
that water resource development reduces the frequency and duration of connection
between anabranch channels and the main river channel. This loss of landscape
complexity via loss of connectivity with anabranches has the potential to reduce the total
availability of carbon sources to the ecosystem, as demonstrated by a modelled 13%
reduction in potential dissolved organic carbon release from anabranch sediments.
This thesis has demonstrated the importance of spatial heterogeneity in riverine
landscapes, by documenting its association with variability in the distribution and quality
of primary energy sources for the ecosystem. It has shown that this variability is
augmented by different phases of hydrological connectivity over time. Spatial
heterogeneity and hydrological connectivity interact to increase the diversity and
availability of ecological energy sources across the riverine landscape, at multiple spatial
and temporal scales. This has positive implications for the resilience and sustainability of
the system. Anabranch channels are particularly important facilitators of these effects in
this dryland floodplain river system. Anabranch channels are �intermediate� in terms of
spatial placement, temporal hydrological connection, and availability of carbon sources;
of high value in terms of high-quality carbon sources; and relatively easy to target for
management because of their defined commence-to-flow levels. Further research should
be directed toward evaluating other ecological roles of anabranch channels in dryland
rivers, thereby providing a more complete understanding of the importance of
connectivity between these features and other patches. This knowledge would assist
management of floodplain river landscapes at larger regional scales, including
amelioration of the effects of water resource development.
| Identifer | oai:union.ndltd.org:ADTP/219572 |
| Date | January 2007 |
| Creators | McGinness, Heather M., n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Heather M. McGinness |
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