-
Floodplain river function in Australia’s wet/dry tropics,
with specific reference to aquatic macroinvertebrates
and the Gulf of Carpentaria
Catherine Leigh
Bachelor of Science (Hons)
Australian Rivers Institute
Griffith School of Environment
Science, Environment, Engineering and Technology
Griffith University, Nathan, Queensland, Australia
Submitted in fulfilment of the requirements of the degree of
Doctor of Philosophy
July 2008
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Abstract
This thesis provides significant insight into our understanding
of river function in highly
seasonal systems. In north Australia’s vast wet/dry tropics,
large rivers and associated
wetlands are regarded among the continent’s most biologically
diverse and ecologically
healthy. Until recently however, research on the hydrology,
biodiversity and function of
Australian rivers has focussed on the south. My thesis
investigates floodplain river
function in Australia’s wet/dry tropics, more specifically in
the Gulf of Carpentaria
drainage division, and is the first to present a dynamic
conceptual model of river
function for these systems.
Three major themes reside within riverine ecology: flow, pattern
and process. These
themes feature within existing conceptual models of large river
function, for example,
the River Continuum Concept, the Flood Pulse Concept and the
Riverine Productivity
Model. These themes and models were used as a template to
explore river function in
the study region: flow, as broad-scale hydrology and more
localised hydrological
connectivity; patterns, as spatiotemporal variation in aquatic
macroinvertebrate
biodiversity; and processes, as organic carbon flow through
aquatic macroinvertebrate
food webs.
The flow regime is major driver of river function, and as such,
a multivariate analysis of
daily flow data from large, Gulf of Carpentaria rivers was
conducted. Two major classes
of river were found, each with a distinct flow regime type:
‘tropical’ rivers were
characterised by flow regularity and permanent hydrological
connection, ‘dryland’
rivers by high levels of flow variability and ephemerality,
similar to rivers in Australia’s
central and semi-arid zones. However, both river types
experienced seasonal change,
associated with higher flow magnitudes in the wet and lower flow
magnitudes in the
dry, with ‘dryland’ rivers typified by greater numbers of zero
flow days. These
features—flow regularity and permanence for ‘tropical’ rivers,
flow variability and
absence for ‘dryland’ rivers, and wet/dry seasonality for both
river types—were
proposed as the broad-scale hydrological drivers of river
function in the Gulf region and
are expected to be found as important drivers throughout the
wet/dry tropics.
Along with the flow regime, spatiotemporal patterns of variation
in biotic assemblages,
and in biophysical and chemical characteristics, are an
important aspect of river
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function and its conceptual description. To this end, a spatial
study of main channel and
floodplain waterbodies in the lower catchments of the ‘tropical’
Gregory and ‘dryland’
Flinders Rivers (southern Gulf of Carpentaria) was conducted
during the 2005 dry
season and repeated on a smaller scale the following year.
Waterbodies were either lotic
or lentic at the time of sampling, representing their
hydrological state of connection
(lotic) or disconnection (lentic). In addition, wet season
characteristics and temporal
change between wet and dry seasons were explored for the Gregory
River during the
2007 wet season.
Spatiotemporal patterns were investigated using univariate and
multivariate analyses,
with emphasis on macroinvertebrate structure (taxonomic
abundances), function
(functional feeding group proportions), and diversity
(calculated metrics). A diverse
fauna was found: forty-five samples were represented by 124
morphotaxa, over 45 000
individuals, and dominated by gatherers and the Insecta. In
particular, the analyses
demonstrated a robust association between hydrological
connectivity and the
macroinvertebrate biota. Specifically, assemblages from
waterbodies with similar
hydrological connection histories and states of flow were most
alike, in both structure
and function, the effect of hydrological connectivity
outweighing effects directly
associated with catchment. In addition, beta-diversity was
maximal between lotic and
lentic waterbodies, and tended to increase with increasing
spatial separation. At smaller
spatial scales, a number of environmental factors like
biophysical habitat and water
physicochemistry were also important for explaining variation in
assemblage structure.
Characteristics associated with primary productivity potential
and habitat diversity were
important for explaining variation in assemblage function.
However, much of the small-
scale environmental variation across the study region was
related to broad-scale
variation in hydrological connectivity, which thus had both
direct and indirect effects on
the macroinvertebrate assemblages.
Food webs describe the movement of energy through ecosystems,
and this process, like
patterns of variation in biotic assemblages, is a key component
of river function.
However, debate exists about the relative importance of
different sources of organic
carbon fuelling aquatic food webs in floodplain rivers.
Therefore, the major basal
sources of organic carbon fuelling macroinvertebrate food webs
in the study region
were explored, via the analysis of stable carbon and nitrogen
isotopes. Potential subsidy
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from the aquatic food webs to the terrestrial zone was also
investigated by analysing the
dietary guilds of terrestrial consumers observed at study
sites.
Algae, associated with phytoplankton and biofilm, were the
primary source of organic
carbon in the macroinvertebrate food webs, commonly contributing
over 55% of
organic carbon to the consumer biomass. Consumers were also
shown to rely on
additional contribution from other sources of organic carbon,
e.g. terrestrial detritus
derived from local C3 riparian vegetation. In addition, food
webs were characterised by
substantial flexibility in source importance (generalism) and
the assimilation of organic
carbon across trophic levels (omnivory). These key
characteristics may impart a degree
of resilience against natural disturbances like flow regime
seasonality, flow variability
and variation in hydrological connectivity, such that the
aquatic food webs display
dynamic stability through space and time. Furthermore, the
majority of vertebrate taxa
identified in and around riparian zones were known consumers of
aquatic fauna
(invertebrates and fish). The aquatic food webs therefore
represented a potentially large
source of organic carbon for these terrestrial-zone
consumers.
Together, the analyses of flow, patterns and processes were used
to develop a new and
dynamic conceptual model of function specific to floodplain
rivers in the study region,
and more broadly to similar systems across Australia’s wet/dry
tropics. The new model
highlighted three key aspects:
1. Large-scale hydrological drivers—‘tropical’ rivers: flow
permanence and
regularity; ‘dryland’ rivers: flow variability and absence; all
rivers: wet-dry
seasonality—are important for overall river function in the
region
2. Multi-scale spatiotemporal variation in macroinvertebrate
assemblage
composition and diversity is driven both directly and indirectly
by hydrological
connectivity, connectivity potential and connection history
3. Links between organic carbon sources and macroinvertebrate
consumers, and
the factors that influence them—specifically, algal production,
local riparian
litterfall, food web flexibility and omnivory—support aquatic
food webs that
show resilience against natural hydrological disturbance, and
represent a large
potential subsidy to the terrestrial environment.
Using this conceptual model, Bayesian Belief Network scenarios
provided a novel way
of exploring potential impacts of two water resource development
options (flow
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regulation and water abstraction) on the composition and
diversity of macroinvertebrate
assemblages and on macroinvertebrate food web dynamics in the
study region.
Scenarios clearly showed that unmitigated flow regulation, via
damming of rivers or
other control methods, has the potential to alter and adversely
impact upon the
ecosystem function of these floodplain river systems, perhaps
most significantly
affecting their biodiversity. Consequently, flow regulation must
be considered with
great caution as a broad-scale water resource development option
for rivers in
Australia’s wet/dry tropics.
In summary, this thesis adds greater depth to our understanding
of river function in
Australia’s wet/dry tropics and offers potential insight into
the function of highly
seasonal systems elsewhere. Ultimately, we must continue to
improve our knowledge
and understanding of river function in these important riverine
ecosystems.
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Declaration
This work has not previously been submitted for a degree or
diploma in any university.
To the best of my knowledge and belief, the thesis contains no
material previously
published or written by another person except where due
reference is made in the thesis
itself.
Catherine Leigh July 2008
Gregory River, September 2006. Photograph by Terry Reis.
Cloncurry River, September 2006. Photograph by Terry Reis.
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Table of Contents
Abstract..............................................................................................................................
i Declaration
.......................................................................................................................
v
Table of Contents
...........................................................................................................
vii
List of Tables
...................................................................................................................
xi
List of
Figures................................................................................................................
xiv
List of
Appendices.........................................................................................................
xix
Acknowledgements
.......................................................................................................
xxi
Chapter 1. General
introduction.......................................................................................
1
1.1 Introduction
.....................................................................................................................
1 1.2 Large rivers, floodplains and
function.............................................................................
2 1.3 The River Continuum
Concept........................................................................................
3 1.4 The Flood Pulse
Concept.................................................................................................
5 1.5 The Riverine Productivity Model
....................................................................................
6 1.6 Australia: an outlier in conceptual model development
.................................................. 7 1.7 Conceptual
models united? Key aspects for understanding floodplain river
function in
Australia’s wet/dry
tropics...............................................................................................
8 1.8 Thesis aims
....................................................................................................................
10 1.9 Implications for future management and
protection...................................................... 11
1.10 Thesis
outline.................................................................................................................
11
Chapter 2. Study region and design
...............................................................................
13 2.1 Introduction to northern Australia’s wet/dry tropics
..................................................... 13
2.1.1 Gulf of Carpentaria drainage
division....................................................................
14 2.1.1.1 Southern Gulf of Carpentaria
..........................................................................
14
2.2 Study design and sampling regime
................................................................................
15 2.2.1 Overall study
design...............................................................................................
15 2.2.2 Site
location............................................................................................................
17 2.2.3 Temporal
sampling.................................................................................................
19 2.2.4 Notes on design, sampling and aims of
thesis........................................................
20
Chapter 3. Hydrological drivers of river function in the Gulf of
Carpentaria drainage division and potential impacts of water
resource development...................................... 21
3.1 Preamble
........................................................................................................................
21 3.2 Classification of flow regimes and hydrological drivers of
river function .................... 22
3.2.1 Introduction
............................................................................................................
22 3.2.2
Methods..................................................................................................................
24
3.2.2.1 Study region
....................................................................................................
24 3.2.2.2 Classification of flow
regimes.........................................................................
24
3.2.3 Results
....................................................................................................................
27 3.2.3.1 Set aspects and variability of magnitude
......................................................... 27
3.2.3.2 Set aspects of
duration.....................................................................................
29 3.2.3.3 Comparison with other Australian
rivers......................................................... 29
3.2.3.4 Multivariate analysis: separation among river types
....................................... 30
3.2.4 Discussion
..............................................................................................................
34 3.2.4.1 Flow regime classifications
.............................................................................
34 3.2.4.2 Proposed hydrological drivers of large river
function..................................... 35
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3.2.4.3 Conceptual model applicability
.......................................................................
36 3.2.4.4 Applied
issues..................................................................................................
37
3.3 Hydrological changes and ecological impacts potentially
associated with water resource
development...................................................................................................................
38
3.3.1 Introduction
............................................................................................................
38 3.3.2
Methods..................................................................................................................
39
3.3.2.1 Study region
....................................................................................................
39 3.3.2.2 Assessment of post-WRD impacts
..................................................................
40
3.3.3 Results
....................................................................................................................
42 3.3.3.1 Pre-WRD flow metrics
....................................................................................
42 3.3.3.2 Pre- to post-WRD changes
..............................................................................
43 3.3.3.3 Variation among rivers and directions of change due to
flow modification ... 44
3.3.4 Discussion
..............................................................................................................
45 3.4 Conclusion
.....................................................................................................................
51
Chapter 4. Spatiotemporal variation in hydrological connectivity
and the biophysical and chemical characteristics within and among
waterbodies in the lower Gregory and Flinders River systems
...................................................................................................
53
4.1 Introduction
...................................................................................................................
53 4.2 Methods
.........................................................................................................................
55
4.2.1 Study area and sampling regime
............................................................................
55 4.2.2 Waterbody-scale morphology
................................................................................
56 4.2.3 Within-waterbody-scale
morphology.....................................................................
57 4.2.4 Dry season sample collection and laboratory analyses
.......................................... 57
4.2.4.1 Water physicochemistry
..................................................................................
57 4.2.4.2 Chlorophyll a concentration and suspended solids
......................................... 60 4.2.4.3 Benthic
organic
material..................................................................................
62
4.2.5 Wet season sample collection and laboratory
analyses.......................................... 63 4.2.6 Data
analysis
..........................................................................................................
63
4.3 Results
...........................................................................................................................
66 4.3.1 Waterbody-scale morphology
................................................................................
66 4.3.2 Within-waterbody scale morphology
.....................................................................
70 4.3.3 Physicochemical parameters, chlorophyll a concentration
and benthic organic
material...................................................................................................................
71 4.3.3.1 Dry season, 2005
.............................................................................................
71 4.3.3.2 Dry season, 2006
.............................................................................................
78 4.3.3.3 Wet season,
2007.............................................................................................
84
4.4
Discussion......................................................................................................................
85 4.4.1 Spatiotemporal variation and hydrological
connectivity........................................ 85 4.4.2
Conceptual model
applicability..............................................................................
89 4.4.3 Extent and effects of human-induced disturbance
................................................. 92
4.5 Conclusion
.....................................................................................................................
94 Chapter 5. Spatiotemporal variation in the structure, function
and diversity of macroinvertebrate assemblages within and among
waterbodies in the lower Gregory and Flinders River systems
...................................................................................................
97
5.1 Introduction
...................................................................................................................
97 5.2 Methods
.......................................................................................................................
101
5.2.1 Study area and sampling regime
..........................................................................
101 5.2.2 Macroinvertebrate samples
..................................................................................
101 5.2.3 Environmental characteristics
..............................................................................
101 5.2.4 Data analysis
........................................................................................................
102
5.3 Results
.........................................................................................................................
108 5.3.1 Environmental characteristics
..............................................................................
108
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5.3.2 Macroinvertebrate assemblages
...........................................................................
110 5.3.3 Spatial variation in assemblage structure, 2005 dry
season ................................. 112 5.3.4 Spatial
variation in assemblage function, 2005 dry
season.................................. 122 5.3.5 Spatial
variation in assemblage diversity, 2005 dry season
................................. 127 5.3.6 Temporal variation in
assemblage
composition...................................................
130
5.4
Discussion....................................................................................................................
134 5.4.1 Assemblage biodiversity
......................................................................................
134 5.4.2 Spatial variation and hydrological
connectivity................................................... 134
5.4.3 Temporal variation
...............................................................................................
139 5.4.4 Conceptual model
applicability............................................................................
140 5.4.5 Conclusion and recommendations
.......................................................................
142
Chapter 6. Sources of organic carbon fuelling macroinvertebrate
food webs in waterbodies within the lower Gregory and Flinders
River systems and potential subsidy to terrestrial-zone
consumers........................................................................................
145
6.1 Introduction
.................................................................................................................
145 6.1.1 Stable isotopes analysis
(SIA)..............................................................................
146 6.1.2 Chapter objectives, questions and hypotheses
..................................................... 149
6.2 Methods
.......................................................................................................................
152 6.2.1 Study area and sampling regime
..........................................................................
152 6.2.2 Stable isotopes: sample collection, preparation and
analysis............................... 152
6.2.2.1 Basal sources
.................................................................................................
152 6.2.2.2 Aquatic-zone consumers
...............................................................................
154 6.2.2.3 Terrestrial-zone consumers
...........................................................................
155 6.2.2.4 Preparation and analysis
................................................................................
155 6.2.2.5 Supplementary analyses
................................................................................
157
6.2.3 Data analysis
........................................................................................................
158 6.2.3.1 Variation within and among basal sources and consumers
........................... 158 6.2.3.2 Trophic enrichment
estimation......................................................................
159 6.2.3.3 Trophic levels (TL) and omnivory
................................................................
160 6.2.3.4 Mixing models and basal source contribution to consumer
biomass ............ 161 6.2.3.5 Aquatic-terrestrial subsidies
..........................................................................
163 6.2.3.6 Conceptual model applicability
.....................................................................
163
6.3 Results
.........................................................................................................................
164 6.3.1 Basal source origins and variation among sources and
consumers...................... 164
6.3.1.1 Stable carbon isotope ratios (δ13C)
................................................................
164 6.3.1.2 Stable nitrogen isotope ratios (δ15N)
............................................................. 170
6.3.1.3 Origins and quality of basal
sources..............................................................
171 6.3.1.4 Temporal variation
........................................................................................
173
6.3.2 Food webs
............................................................................................................
175 6.3.2.1 Consumer trophic levels and
omnivory.........................................................
175 6.3.2.2 Basal source contribution to macroinvertebrate food
webs: mixing model
solutions.........................................................................................................
176 6.3.2.3 Aquatic subsidy to the terrestrial food web
................................................... 179
6.4
Discussion....................................................................................................................
180 6.4.1 Basal source origins and conceptual model applicability
.................................... 180 6.4.2 Sources of organic
carbon fuelling macroinvertebrate consumers.......................
183
6.4.2.1 Dry season food
webs....................................................................................
184 6.4.2.2 Temporal variation
........................................................................................
188
6.4.3 Aquatic subsidies to the terrestrial environment and
implications for aquatic food webs
.....................................................................................................................
189
6.4.4 SIA: issues, assumptions and considerations
....................................................... 190 6.4.5
Conclusion
...........................................................................................................
194
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Chapter 7. Floodplain river function in Australia’s wet/dry
tropics: a new and scenario-driven conceptual
model...............................................................................................
197
7.1 Introduction
.................................................................................................................
197 7.1.1 Floodplain river function in the study region and
Australia’s wet/dry topics: a
thesis
summary.....................................................................................................
197 7.1.1.1
Flow...............................................................................................................
197 7.1.1.2 Patterns: biophysical and chemical
characteristics........................................ 198 7.1.1.3
Patterns: macroinvertebrate assemblages
...................................................... 200 7.1.1.4
Processes
.......................................................................................................
201
7.1.2 River function in the study
region........................................................................
203 7.2 Perspective: a new conceptual model of floodplain river
function in Australia’s wet/dry
tropics
..........................................................................................................................
207 7.2.1 Introduction
..........................................................................................................
208 7.2.2 The conceptual model as diagrammatic
............................................................... 209
7.2.3 The conceptual model as dynamic and
probabilistic............................................ 217 7.2.4
The conceptual model as scenario-driven
............................................................
222
7.2.4.1 Flow regimes
.................................................................................................
222 7.2.4.2 Macroinvertebrate
assemblages.....................................................................
223 7.2.4.3 Macroinvertebrate food webs
........................................................................
229
7.2.5 Summary, caveats and limitations of the conceptual model
................................ 233 7.3 Recommendations and future
research directions
....................................................... 235
Appendices
...................................................................................................................
239 References
....................................................................................................................
289
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List of Tables
Table 2.1: Waterbodies sampled in the study region, with site
codes used throughout this thesis, and detail on catchment, river
section, lateral position in relation to the main channel, flow
status at the time of sampling
..................................................................
19
Table 3.1: Continuous daily flow records from gauging stations
in the Gulf of Carpentaria drainage division used to classify the
flow regimes of large rivers. ........... 25
Table 3.2: Flow metrics used to classify flow regimes of rivers
in the Gulf of Carpentaria and categories used in multivariate
analysis, including the ecologically relevant description of a
river’s flow regime (facet); aspect of these facets described by
the metric; and the relevant period of record described by the
metric. .......................... 26
Table 3.3: Calculated flow metrics, standardised per km2
upstream catchment area, used to classify flow regimes of rivers in
the Gulf of Carpentaria. ........................................
28
Table 3.4: Comparison of flow variability metrics among large
rivers in the Gulf of Carpentaria drainage division and other
previously studied Australian rivers............... 30
Table 3.5: Characteristics of Murray-Darling Basin (MDB) gauging
stations and flow data used to assess potential post-water resource
development impacts on selected Gulf of Carpentaria (GC)
rivers..............................................................................................
41
Table 3.6: Ecologically relevant hydrological measures used in
the assessment of post-water resource development impacts on
southern Gulf of Carpentaria rivers, calculated from mean annual
flow data standardised by upstream catchment area.
....................... 41
Table 3.7: Eigenvectors for the PCA on pre- and post-water
resource development flow metrics, as described by Figure 3.7, with
the variance explained by the first two principal component axes,
PC1 and PC2, given in
parentheses..................................... 44
Table 3.8: Potential ecological impacts of predicted
hydrological changes associated with water resource development in
large floodplain rivers of Australia’s wet/dry tropics, adapted to
different flow regimes as represented by three key hydrological
drivers of ecosystem
function.........................................................................................
48
Table 4.1: Waterbody-scale morphology (biophysical features) of
sites sampled in the study region during the 2005 and 2006 dry
seasons.......................................................
68
Table 4.2: Conductivity, salinity, pH and Secchi depths (ZSD) of
water sampled from a mid-channel location for eleven sites in the
study region during the 2005 dry season.. 72
Table 4.3: Eigenvectors for the PCA on the physicochemical
characteristics of sites sampled during the 2005 dry season, with
variation explained by each of the first two principal components
axes (PC1 and PC2) given in
parentheses................................... 75
Table 4.4: Conductivity, salinity, pH, turbidity and Secchi
depths (ZM) of waterbodies sampled during the 2006 dry season,
measured at mid-channel and littoral zone locations, compared with
2005 dry season data (given in parentheses) when
appropriate.........................................................................................................................................
78
Table 4.5: Diel (24 h) minima and maxima for temperature (°C)
and dissolved oxygen (% saturation) in waterbodies sampled during
the 2006 dry season, measured at the mid-channel and littoral zone
locations.
................................................................................
80
Table 4.6: Physicochemical characteristics of water collected
from GWm in the 2007 wet season (spot measures and medians;
inter-quartile ranges in parentheses). ............ 84
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Table 5.1: Abundance (N) and richness (S) data (absolute and
relative) for the major taxonomic groups (plus Orders within
Insecta) and functional feeding groups (FFGs) of macroinvertebrates
collected from the study region in the dry seasons of 2005 and
2006.......................................................................................................................................
110
Table 5.2: Results of ANOSIM on assemblage structure (based on
Bray-Curtis dissimilarities using log-transformed abundance data)
between groups within apriori-defined factors. Results are
presented with taxa identified by SIMPER as contributing to more
than 50% of the difference between statistically different groups.
..................... 113
Table 5.3: Correlations between assemblage composition of
macroinvertebrate samples (based on taxonomic abundances,
‘structure’; and functional feeding group proportions, ‘function’)
and combinations of environmental variables (BIOENV results) for
the study region during the 2005 dry season.
.....................................................................
120
Table 5.4: Results of ANOSIM based on assemblage function (based
on Bray-Curtis dissimilarities using log-transformed FFG
proportions) between groups within apriori-defined factors. Results
are presented with FFGs identified by SIMPER as contributing to
more than 50% of the significant difference between groups within
factors. .......... 124
Table 5.5: Mean values of diversity measures for groups within
apriori-defined factors with significant differences (ANOVA), based
on macroinvertebrate abundance data for samples collected in the
study region during the 2005 dry
season............................... 129
Table 6.1: Differences in δ13C values between catchments and
states of flow for the major basal sources (FBOM, CBOM, seston and
biofilm) collected from the study region during the 2005 dry season
(results of Mann-Whitney U tests of difference). . 167
Table 6.2: Mean trophic levels (TLs) calculated for the major
groups of secondary consumers (with TLs > 1) collected from the
study region, based on a 1.0‰ enrichment in δ15N per trophic step
above basal sources.
...............................................................
175
Table 6.3: Ranked importance of basal sources to
macroinvertebrate consumers within the study region based on the
frequency that each source made high max (> 55%), high min (>
40%) or low max (< 35%) contributions to consumer diets.
............................ 178
Table 6.4: Significant differences in min and max source
contributions to consumer diets between groups of waterbodies in
the study region (Mann-Whitney U test
results).......................................................................................................................................
179
Table 6.5: Vertebrate fauna* observed in and around the riparian
zones of waterbodies sampled in the 2006 dry season, with
information on their feeding habits† and potential proportional
reliance on aquatic fauna as a food source.
............................................. 180
Table 6.6: Summary of likely origins of basal sources sampled
from within waterbodies in the study region during the 2005 and
2006 dry seasons........................................... 181
Table 7.1: Floodplain river function (drivers, patterns and
processes) in the study region (in Australia’s wet/dry tropics),
based on analyses of the flow regimes of large rivers in the Gulf
of Carpentaria and macroinvertebrate assemblages of waterbodies in
the lower Gregory and Flinders River systems (Chapters 3-6),
presented with relevant aspects of existing concepts of large river
function.
.....................................................................
205
Table 7.2: Key conceptual features of two major, but
contrasting, types of undisturbed, floodplain river systems (in
terms of flow, biodiversity patterns and food web processes) in
comparison with the studied river systems in Australia’s wet/dry
tropics.......................................................................................................................................
207
Table 7.3: States within nodes that represent the key drivers,
patterns and processes depicted in Figure 7.2, under current
conditions (states are relative to each other within
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the range of these conditions), and the potential factors of
concern with respect to land and water resource development options
or climate change. ....................................... 213
Table 7.4: Sensitivity of response nodes to input nodes’ states
and probabilities within the BBN as depicted in Figure 7.2 and
Table 7.3, when ‘Season’ and ‘Flow regime’ nodes are unselected
(all season and flow regime states equally
likely)...................... 218
Table 7.5: Posterior probabilities for states within response
nodes of the BBN, modelled on the conceptual diagram of river
function in the study region (see Figure 7.2), for different
seasons (dry or wet) and flow regime types (‘tropical’ or
‘dryland’)............ 220
Table 7.6: Posterior probabilities for states within response
nodes of the BBN modelled on the conceptual diagram of
macroinvertebrate assemblages (structure, function and diversity)
in the study region (Figures 7.2 and 7.3), for a ‘dryland’ or
‘tropical’ flow regime in the dry or wet season, given current
conditions, under water abstraction or flow regulation
scenarios..............................................................................................
226
Table 7.7: Posterior probabilities for states within response
nodes of the BBN modelled on the conceptual diagram of
macroinvertebrate food web dynamics in the study region (Figures
7.2 and 7.4), for a ‘dryland’ or ‘tropical’ flow regime in the dry
or wet season, given current conditions, under water abstraction or
flow regulation scenarios. ......... 232
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List of Figures
Figure 2.1: The Australian tropics (north of the Tropic of
Capricorn) with detail on the Gulf of Carpentaria drainage
division, southern Gulf of Carpentaria sub-catchments as referred
to in the text, and in relation to Griffith University in southeast
Queensland.. 13
Figure 2.2: Study area in the southern Gulf of Carpentaria,
showing location of sites sampled in the lower Gregory and Flinders
River systems during the 2005 and 2006 dry seasons, and the 2007
wet season, as described in the text (see also Appendices A-B).
18
Figure 3.1: Map of Australia showing major drainage divisions
(Gulf of Carpentaria, Timor Sea, Lake Eyre Basin and Murray-Darling
Basin) and sub-catchments of interest to this
study.....................................................................................................................
24
Figure 3.2: Group average dendrogram, using a normalised
Euclidean distance similarity matrix, of flow metrics calculated
for 15 large rivers in the Gulf of Carpentaria, indicating
differentiation (dashed line) between Type 1 rivers (higher flow
magnitudes and less skew) and Type 2 rivers (higher variability and
zero flow days).. 31
Figure 3.3: MDS plots of two-dimensional solutions for 15 large
rivers in the Gulf of Carpentaria, based on normalised Euclidean
distance similarity matrices of: a) set aspects of flow magnitude
with ‘bubble-plot’ of the median dry season flow (lower flow
magnitudes top right); b) variability of flow magnitude with
‘bubble-plot’ of the coefficient of variation of annual flows
(higher variability to the right); and c) set aspects of zero flow
duration with ‘bubble-plot’ of the median number of annual zero
flow days (higher numbers of zero flow days to the right)
............................................ 31
Figure 3.4: Twenty year hydrographs of annual discharges (ML)
standardised per km2 catchment area for 15 large rivers in the Gulf
of Carpentaria drainage division ........... 32
Figure 3.5: Group average dendrogram, using a normalised
Euclidean distance similarity matrix, of dry and wet season flow
metrics calculated for 15 large rivers in the Gulf of Carpentaria
drainage division. Groups (indicated by dashed lines) separate
rivers with dry-wet seasonality based on changes in flow magnitude
(Group 1) or changes in zero flow days (Group 3). Group 2
represents rivers either with flow metrics in between the extremes
of Group 1 and 3 or a combination of
both............................. 34
Figure 3.6: Flow metrics for two Murray-Darling Basin (MDB) and
five southern Gulf of Carpentaria (SGC) rivers, based on 20 years
of mean annual discharges (ML d-1) standardised by upstream
catchment area (km2).
........................................................... 43
Figure 3.7: PCA bi-plot of the first two principal component
axes (PC1 versus PC2) for pre- and post-WRD flow metrics calculated
for five southern Gulf of Carpentaria (G, C, FG, FR and J) and two
Murray-Darling Basin (DB and DW) rivers. Solid arrows indicate
gradients of change in flow metrics that have dominant eigenvector
loadings on PC1 and PC2. Broken arrows indicate direction of change
defined in two-dimensional PCA space between pre- and post-WRD
conditions ......................................................
45
Figure 4.1: Number and diversity of aquatic macroinvertebrate
habitat types within the study region: a) relative proportions of
macroinvertebrate habitat types present within each site sampled
during the 2005 and 2006 dry seasons; b) relative proportions of
woody debris size classes present in sites sampled during the 2006
dry season............ 70
Figure 4.2: Depth of waterbody (m) compared with calculated
euphotic (ZEU) and surface mixed layers (ZM), for sites sampled
from a mid-channel location in the 2005 dry season
.......................................................................................................................
72
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xv
Figure 4.3: Median nutrient concentrations (mg L-1) and
dissolved molar N:P ratios of water sampled from a mid-channel
location for waterbodies in the study region during the 2005 dry
season
........................................................................................................
73
Figure 4.4: PCA bi-plot for physicochemical characteristics of
sites sampled during the 2005 dry season, presented as site
centroids (mean ± 1 standard error bars) and with eigenvectors for
each physicochemical variable included in the
analysis...................... 75
Figure 4.5: Median chlorophyll a (Chl) and total suspended
solids (TSS) concentrations and organic TSS to chlorophyll a ratios
(OTSS:Chl) of sites sampled during the 2005 dry season
.......................................................................................................................
76
Figure 4.6: Comparison of coarse (CBOM) and fine (FBOM)
fractions within benthic organic material (BOM; mean dry weights
and relative proportions, n = 3) collected from sites sampled
during the 2005 dry
season..............................................................
78
Figure 4.7: Mean depth (n=3, standard error ≤ 0.1 m) of
waterbodies sampled during the 2006 dry season, at mid-channel (MC)
and littoral zone (LZ) locations, compared with calculated euphotic
depths (ZEU) and surface mixed layers
(ZM)................................... 79
Figure 4.8: Median nutrient concentrations (mg L-1) and
dissolved molar N:P ratios of water sampled from a mid-channel
location for four sites in the study region during the 2006 dry
season, presented with inter-quartile ranges as bars (n = 3) and
compared with 2005 dry season data where available
............................................................................
81
Figure 4.9: Median concentrations (mg L-1) of organic and
inorganic fractions of particulate (< 75 µm) carbon (C) and
dissolved (< 0.45 µm) carbon (C), nitrogen (N) and phosphorus
(P) in the water column of sites sampled from a mid-channel
location during the 2006 dry
season.............................................................................................
82
Figure 4.10: Median chlorophyll a (Chl) and total suspended
solids (TSS) concentrations and organic TSS to chlorophyll a ratios
(OTSS:Chl) of sites sampled during the 2006 dry season, compared
with 2005..........................................................
83
Figure 4.11: Median concentrations (mg L-1) of total particulate
nitrogen (TN) and phosphorus (TP) in the Gregory River between dry
seasons (2005 and 2006) and within a wet season (2007)
........................................................................................................
84
Figure 5.1: Historical hydrographs of mean daily flow
standardised by upstream catchment area (ML d-1 km-2) at gauging
stations (open squares) near waterbodies (closed circles) sampled
in the Flinders and Gregory study
regions............................ 109
Figure 5.2: a) Agglomerative dendrogram with group-average
linking and b) MDS ordination with sites as centroids (mean
ordination co-ordinates for n = 3 samples with ± 1 standard error
bars), based on Bray-Curtis sample dissimilarities from
log-transformed abundance data of 33 samples of macroinvertebrates
collected from 11 waterbodies in the study region during the 2005
dry season........................................ 115
Figure 5.3: TWINSPAN dendrogram of 33 samples collected from the
study region in the dry season of 2005, with two-way table of
species group fidelities (F) to sample groups
...........................................................................................................................
116
Figure 5.4: Spatial variation among assemblages at different
scales of resolution and as measured by pair-wise Bray-Curtis
dissimilarities within and between waterbodies for the 11 sites
sampled during the 2005 dry season, based on log-transformed a)
abundance data or b) FFG proportion
data...................................................................
118
Figure 5.5: ‘Bubble plots’ of important variables identified by
BIOENV in explaining patterns of variation in macroinvertebrate
assemblages of waterbodies sampled in the
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xvi
2005 dry season, overlain on the MDS ordination plot (lower
left) of sample assemblages
..................................................................................................................
121
Figure 5.6: Mean relative abundances of taxa within functional
feeding groups for waterbodies sampled in the 2005 dry
season................................................................
122
Figure 5.7: MDS ordination with sites as centroids (mean
ordination co-ordinates for n = 3 samples) with ± 1 standard error
bars, based on Bray-Curtis sample dissimilarities from
log-transformed FFG proportion data of 33 samples of
macroinvertebrates collected from 11 waterbodies in the study
region during the 2005 dry season. ......... 123
Figure 5.8: ‘Bubble plots’ of important variables identified by
BIOENV in explaining patterns of variation in functional
organisation of macroinvertebrate assemblages sampled in the 2005
dry season, overlain on the MDS ordination plot (lower right) of
sample
assemblages......................................................................................................
126
Figure 5.9: Diversity measures (mean +1 standard error bars),
based on macroinvertebrate abundance data and habitat types for
waterbodies sampled in the study region during the 2005 dry season
......................................................................
127
Figure 5.10: (a) Agglomerative dendrogram with group-average
linking and (b-c) MDS ordination with sites as centroids (mean
ordination co-ordinates for n = 3 samples with ± 1 standard error
bars), based on Bray-Curtis sample dissimilarities from
log-transformed abundance data (a and b) and FFG proportions (c) of
macroinvertebrate samples collected from the study region during
the 2005 and 2005 dry seasons......... 131
Figure 5.11: Spatial variation among assemblages at different
scales of resolution, measured by pair-wise Bray-Curtis
dissimilarities (based on log-transformed abundance data in the
upper figure, and FFG proportion data in the lower figure) within
and between waterbodies, and between years, for the 4 sites sampled
in both the 2005 and dry
seasons....................................................................................................................
133
Figure 5.12: Conceptual diagram of beta-diversity between
macroinvertebrate assemblages of sites in the study region, shown
in relationship with the hydrological connectivity potential
between any two waterbodies
................................................... 136
Figure 6.1: Box-plots of δ13C values for basal sources and all
consumer groups (as listed in Appendix Q) sampled from the study
region during the 2005 dry season............... 165
Figure 6.2: Comparison of mean site δ13C values between
potential end-member basal sources and other sources collected
during the 2005 dry season, showing linear correlation trendlines
and R2 values: C3 riparian vegetation versus CBOM, FBOM, seston and
biofilm; biofilm versus CBOM, FBOM and seston; and CBOM versus
FBOM...........................................................................................................................
166
Figure 6.3: Correlation in site mean δ13C values (‰) between
basal sources and macroinvertebrate consumers in the study region
during the 2005 and 2006 dry seasons, showing linear correlation
trendlines and R2 values: a) seston, b) biofilm, and c) FBOM versus
primary and secondary consumers
....................................................................
169
Figure 6.4: Box-plots of δ15N values for basal sources and all
consumer groups (as listed in Appendix S) sampled from the study
region during the 2005 dry season...... 170
Figure 6.5: Bi-plot of mean site δ13C and molar C:N ratios of
sources collected from the study region during the 2005 dry season,
with the two most distinct sources encircled (biofilm representing
aquatic sources, and CBOM representing terrestrial sources)...
171
Figure 6.6: Mean C:N molar ratios (with ± 1 standard error bars)
of CBOM, FBOM and seston (sources with the potential to originate
and be transported from elsewhere), in
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xvii
comparison with local riparian vegetation, for sites that follow
a downstream continuum and were linked by flow at the time of
sampling (during the 2005 dry season). ......... 172
Figure 6.7: Comparison of mean δ13C values (presented with ± 1
standard error bars) of basal sources with their potential
end-member sources and among sites sampled during the 2005 and 2006
dry seasons: a) CBOM versus C3 riparian vegetation, b) FBOM versus
CBOM and biofilm, c) seston.
..........................................................................
174
Figure 6.8: Comparison among mean δ13C values of seston sampled
from the Gregory River during the 2005 and 2006 dry seasons (at
site GUm) and during the 2007 wet season (at site
GWm)....................................................................................................
175
Figure 6.9: Minimum and maximum feasible contributions from
basal sources (excluding FBOM) to consumer diets within waterbodies
(11 sampled in 2005, 4 re-sampled in 2006) in the study region
during the dry season, calculated with IsoSource mixing models on
δ13C data
.........................................................................................
177
Figure 7.1: Initial conceptual model (influence diagram) of
important components of floodplain river function in the study
region (in Australia’s wet/dry tropics), across large and small
spatiotemporal scales and with particular reference to hydrology
and aquatic biota
..............................................................................................................................
210
Figure 7.2: Simplified influence diagram (conceptual model) of
the key drivers, patterns and processes operating within
macroinvertebrate assemblages and food webs within floodplain
rivers in the study region (in Australia’s wet/dry tropics) at
various spatiotemporal scales (modified from Figure 7.1)
....................................................... 212
Figure 7.3: Influence diagram (conceptual model) of the links
between the most important drivers of river function for
macroinvertebrate assemblage composition and diversity within
floodplain rivers in the study region (in Australia’s wet/dry
tropics) under current dry season conditions.
............................................................................
215
Figure 7.4: Influence diagram (conceptual model) of the
important links between common organic carbon sources and
macroinvertebrate consumers within floodplain rivers in the study
region (in Australia’s wet/dry tropics), along with other important
drivers of river function for macroinvertebrate food webs, under
current conditions . 216
Figure 7.5: Example BBN and posterior probabilities given a
‘dryland’ river flow regime during the dry season (cf. Table 7.4)
for the key drivers, patterns and processes operating within
macroinvertebrate assemblages and food webs within floodplain
rivers in the study region (in Australia’s wet/dry tropics) at
various spatiotemporal scales.. 221
Figure 7.6: Example BBN, given a flow regulation scenario in a
‘dryland’ river during the dry season, showing the potential effect
(represented by posterior probabilities) of water resource
development options on the composition (structural and functional)
and diversity of macroinvertebrate assemblages within floodplain
rivers in the study region (in Australia’s wet/dry
tropics).....................................................................................
225
Figure 7.7: Two-dimensional MDS ordination of current and
modified macroinvertebrate composition and diversity within
floodplain rivers in the study region (in Australia’s wet/dry
tropics) given a water abstraction or flow regulation scenario,
based on a normalised Euclidean distance matrix of posterior
probabilities from the BBN as described in the text (see Table
7.5).
..............................................................
228
Figure 7.8: Example BBN, given a flow regulation scenario in a
‘dryland’ river during the dry season, showing the potential effect
(represented by posterior probabilities) of water resource
development options on macroinvertebrate food web dynamics within
floodplain rivers in the study region (in Australia’s wet/dry
tropics) .......................... 230
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xviii
Figure 7.9: Two-dimensional MDS ordination of current and
modified macroinvertebrate food web dynamics within floodplain
rivers in the study region (in Australia’s wet/dry tropics) given a
water abstraction or flow regulation scenario, based on a
normalised Euclidean distance matrix of posterior probabilities
from the BBN as described in the text (see Table 7.6)
.............................................................................
233
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xix
List of Appendices
Appendix A: Streamflow gauging stations in the lower reaches of
the Nicholson and Flinders sub-catchments and study region.
..................................................................
240
Appendix B: Catchment, river section and waterbody-scale
description of sites sampled in the study region during the 2005
dry season.
........................................................... 241
Appendix C: Conditions observed at GWm during the 2007 wet
season for the January, March and April sampling periods.
..............................................................................
242
Appendix D: Aquatic macrophytes and macroalgae (aquatic
vegetation) and most common or dominant terrestrial plants
(riparian vegetation) present (p) at sites sampled in the study
region during the 2005 and 2006 dry
seasons........................................... 243
Appendix E: TRARC scores for sites sampled in 2006, assessed
following methods outlined in Dixon et al. (2006).
....................................................................................
244
Appendix F: Spot measures for various physicochemical properties
of waterbodies sampled in the study region during the 2005 and 2006
dry seasons and the 2007 wet season.
..........................................................................................................................
245
Appendix G: Median chlorophyll a concentration and
physicochemical characteristics of waterbodies sampled in the study
region during the 2005 and 2006 dry seasons and the 2007 wet
season......................................................................................................
246
Appendix H: Mean chlorophyll a concentration and physicochemical
characteristics of waterbodies sampled in the study region during
the 2005 and 2006 dry seasons and the 2007 wet
season............................................................................................................
248
Appendix I: Keys and guides used for taxonomic and functional
feeding group identification of macroinvertebrates collected from
the study region.......................... 250
Appendix J: Datasets used to explore relationships between
patterns of variation in macroinvertebrate assemblages and their
biophysical and chemical environment, at different scales of
resolution.
.......................................................................................
251
Appendix K: Biophysical and chemical characteristics of
waterbodies in the dry season of 2005, described by variables used
in the correlation analyses with macroinvertebrate assemblage data
(BIOENV), with data for waterbodies re-sampled* in the 2006 dry
season.
..........................................................................................................................
252
Appendix L: Taxa identified from samples collected from
waterbodies in the Gregory and Flinders study region during the
2005 and 2006 dry seasons, associated functional feeding groups
(FFG) and species groups identified by TWINSPAN (for samples
collected in 2005 only)
.................................................................................................
253
Appendix M: Box-plots of δ13C and δ15N values, and organic
carbon to chlorophyll a (mass C:Chl) and to nitrogen (molar C:N)
concentration ratios for seston collected from littoral zones and
mid-channel (pelagic-zone) locations of waterbodies during the 2006
dry season.
....................................................................................................................
256
Appendix N: Live versus detrital fractions within seston and
biofilm......................... 257
Appendix O: Trophic enrichment within the study
region........................................... 260
Appendix P: Vertebrate species (mammals, birds, terrestrial
reptiles and amphibians) observed (‘o’) at sites sampled in the
study region during the 2006 dry season and their main dietary
guild classification, presented with detail on their potential
consumption
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xx
(‘yes’) of aquatic food sources (fish, crustaceans,
invertebrates, and emergent adult insects)
..........................................................................................................................
263
Appendix Q: Mean δ13C (‰) of basal sources and consumers for
samples collected from the study region during the 2005 dry season.
...................................................... 266
Appendix R: Common or dominant C3 plants found in the riparian
zones of sites sampled during the 2005 dry season and those C3
riparian plants used in SIA as well as taxa identified from CBOM
samples............................................................................
268
Appendix S: Mean δ15N (‰) of basal sources and consumers for
samples collected from the study region during the 2005 dry season.
...............................................................
269
Appendix T: Mean molar C:N ratios of basal sources and consumers
for samples collected from the study region during the 2005 dry
season........................................ 271
Appendix U: Mean δ13C and δ15N values for zooplankton and
primary consumers collected from the study region during the 2005
and 2006 dry seasons....................... 273
Appendix V: Mean δ13C and δ15N values for secondary consumers
collected from the study region during the 2005 and 2006 dry
seasons.....................................................
274
Appendix W: Mean δ13C (‰) of basal sources collected from the
study region during the 2005 and 2006 dry seasons
.....................................................................................
275
Appendix X: Mean molar C:N ratios of basal sources collected
from the study region during the 2005 and 2006 dry seasons
.........................................................................
276
Appendix Y: Mean δ15N (‰) of basal sources collected from the
study region during the 2005 and 2006 dry seasons.
....................................................................................
277
Appendix Z: Site bi-plots of mean δ13C and δ15N values (‰) for
basal sources and consumers collected from the Gregory River study
region during the 2005 dry
season.......................................................................................................................................
278
Appendix AA: Site bi-plots of mean δ13C and δ15N values (‰) for
basal sources and consumers collected from the Flinders River study
region during the 2005 dry
season......................................................................................................................................
279
Appendix BB: Site bi-plots of mean δ13C and δ15N values (‰) for
basal sources and consumers collected from the Gregory and Flinders
Rivers study regions during the 2006 dry season
............................................................................................................
280
Appendix CC: 1st-99th percentile ranges of the contribution (%)
of basal sources (excluding FBOM) to primary consumer diets in the
study region during the 2005 and 2006 dry seasons, produced using
IsoSource mixing models based on δ13C data ....... 281
Appendix DD: 1st-99th percentile ranges of the contribution (%)
of basal sources (excluding FBOM) to secondary consumer diets in
the study region during the 2005 and 2006 dry seasons, produced
using IsoSource mixing models based on δ13C data ....... 282
Appendix EE: Conditional probability tables (priors) for
Bayesian Belief Networks (BBNs) formulated in Chapter 7 (Tables
EE.1-3)........................................................
283
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xxi
Acknowledgements
Throughout my PhD candidature, I have received help and support
from many people
and organisations, for which I am very grateful. I apologise to
anyone I may have
missed: all who have contributed are greatly appreciated. In
particular, I thank my
supervisors, Drs Fran Sheldon and Michele Burford and Prof.
Stuart Bunn, for their
encouragement, guidance and supervision, without which the work
presented in this
thesis would not have been possible.
The PhD project was funded by a Land & Water Australia
Postgraduate Research
Scholarship (GRU35), administrated by Griffith University, and a
Griffith School of
Environment Completion Assistance Postgraduate Research
Scholarship. I also received
funding and in-kind support for conference attendance, travel,
field work, sample
analysis and mentorship from: the Australian Rivers Institute /
Centre for Riverine
Landscapes at Griffith University; the Australian Society for
Limnology; the Griffith
School of Environment / Australian School of Environmental
Studies at Griffith
University; Southern Gulf Catchments (SGC) in Mt Isa; and the
Wentworth Group of
Concerned Scientists through a 2007 Wentworth Group Science
Program Scholarship
award.
The Australian wet/dry tropics and the Gregory and Flinders
River systems are stunning
places to have studied. I am very grateful for the opportunity
to visit and study them,
and, I hope, to assist in their future protection. My warmest
regards and thanks extend
to the traditional owners of this country, as represented
through Moungibi and the
Carpentaria Land Council, and to the pastoral leaseholders and
station managers in the
region, all for granting permission to access and study these
river systems and for the
on-site assistance they all provided. In addition, SGC, Shire
Councils and a number of
colleagues with experience in the region (especially James
Fawcett and Joel Huey) were
instrumental in helping me to establish these local
contacts.
Many people assisted me with field trip preparation, equipment
and analytical methods.
I thank them all, including: Jeff Argo, Andrew Brooks, Peter
Brunner, Stuart Bunn,
Michele Burford, Scott Byrnes, Chengrong Chen, Andrew Cook, Rene
Diocares,
Noreen Dejoras, Michael Douglas, James Fawcett, Christy Fellows,
Vanessa Fry, Jane
Gifkins, Susie Green, Wade Hadwen, Stephen Hamilton, Courtney
Henderson, Mark
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xxii
Kennard, Jason Kerr, Priyanesh Muhid, Kylie Pitt, Carolyn
Polson, Amanda Posselt,
Jim Puckridge, Fran Sheldon, Terry Reis, David Roberts, Kate
Smolders, John Spencer,
and Loretta Young. Everyone at the Australian Rivers Institute
and the Griffith School
of Environment, especially Fran Sheldon, Lacey Shaw, Deslie
Smith, Heidi Millington,
Michele Burford and her RnR group, have been very helpful
throughout my
candidature. They have offered support, advice, and constructive
criticism.
Additionally, I was assisted in the field by wonderful
volunteers. These people gave
their time and worked amazingly hard. I cannot thank them
enough: Erika Alacs, Jim
McGuire, Ben Cook, Tim Page, Joel Huey and James Fawcett for
their help during my
first trip to Australia’s great north in 2005, Terry Reis and
Brett Taylor for their
fantastic help and company during the 2006 field trip. Wet
season sampling in 2007
would not have been possible without volunteers: Jo from the
Gregory Downs Hotel,
Murray from the Gregory Downs general store, and especially
Megan Munchenberg
from Gregory Downs, along with the assistance I received from
SCG, in particular from
Matthew Vickers and Mark van Ryt. Two volunteers also helped
sort the seemingly
endless detritus and sediment from my bug samples: thank you
Jane Ogilvie and
Jennifer Sanger.
Flow data for Gulf of Carpentaria rivers were provided in 2005
by the Queensland
Department of Natural Resources and Mines, which gives no
warranty in relation to the
data (including accuracy, reliability, completeness or
suitability) and accepts no liability
(including without limitation, liability in negligence) for any
loss, damage or costs
(including consequential damage) relating to any use of the
data. Integrated Quantity
and Quality Model (IQQM) flow data for the Darling River gauges
were provided to
Fran Sheldon, my principal supervisor, from the New South Wales
Department of Land
and Water Conservation (1995). The project was given ethical
clearance by Griffith
University’s Animal Ethics Committee and research was conducted
in accordance with
the requirements of this Committee.
I attended and presented components of research detailed in this
thesis at national and
international conferences. I learned much from these
experiences, from other
presentations and from discussions with other attendees, for
which I am very
appreciative. These conferences are the Australian Society for
Limnology (ASL)
conference in Hobart, Tasmania, 2005; the Third International
Symposium on Riverine
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xxiii
Landscapes (TISORL) on South Stradbroke Island, Queensland,
2007; the 10th
International RiverSymposium and Environmental Flows Conference
in Brisbane,
Queensland, 2007; and the joint ASL and New Zealand Freshwater
Sciences Society
(NZFSS) conference in Queenstown, New Zealand, 2007.
In addition, I have published work arising from this thesis as
principal author. The main
body of Chapter 3 forms the basis of an original research paper,
published as:
Leigh C, Sheldon F. 2008. Hydrological changes and ecological
impacts associated with water resource development in large
floodplain rivers in the Australian tropics. River Research and
Applications. DOI: 10.1002/rra.1125.
The main body of Chapter 5 forms the basis of the following
journal manuscript:
Leigh C, Sheldon F. In review. Hydrological connectivity drives
patterns of macroinvertebrate biodiversity in floodplain rivers of
the Australian wet/dry tropics. Freshwater Biology.
I conducted and produced the work outlined in these articles
under supervision from Dr
Fran Sheldon during my PhD candidature.
Overall, the support of friends and family throughout my
candidature has been
paramount. These people have seen me through the low troughs.
They have got me out
and about and enjoying life. Thank you! Special thanks to my
gorgeous friends Meg,
Angie and Lynette, and my dear sister, Rachel.
I dedicate this thesis to my parents, Eileen (1933 – 1982) and
Robert Leigh (1924 –
2000).
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xxiv
-
1
Chapter 1. General introduction
1.1 Introduction
Large river ecosystems are energetically dynamic and bio-complex
(Robinson et al.,
2002; Thorp et al., 2006). Understanding the key drivers (e.g.
flow), patterns (e.g.
biodiversity) and processes (e.g. food webs) involved in this
complexity, otherwise
known as river function, is a major aim of limnological
research. To this end, there have
been numerous attempts to condense the key aspects of large
river function into simple
conceptual models. The various merits of these models and their
ability to apply broad-
scale across all river systems, climates and biomes continues to
be argued (e.g. Thorp et
al., 1998; Dettmers et al., 2001; Junk and Wantzen, 2003; Thorp
et al., 2006; Gawne et
al., 2007). However, in river ecosystems that are poorly
understood, these models
provide a solid and comparative platform from which to begin
investigating the key
aspects that define their function.
In this manner, the present study explores floodplain river
function in northern
Australia’s wet/dry tropics. River systems here are currently
among the continent’s
most biologically diverse and ecologically intact (ATRG, 2004;
Woinarski et al., 2007).
However, until recently they have received scant research
attention. Consequently, their
ecology and river function is little understood and a directive
to narrow this knowledge
gap has been issued (Hamilton and Gehrke, 2005). My thesis
addresses this call by
investigating river function—comparatively with established
concepts—of floodplain
rivers in the Gulf of Carpentaria, in Australia’s northeast
wet/dry tropics.
This general introductory chapter provides an overview for the
thesis proper, placing it
in the context of past research, and identifying its major aims,
structure and
significance. Accordingly, major conceptual models of large
river function will be
reviewed and their application to different river systems, as
demonstrated in the
literature, will be discussed; literature specific to the
chapters following will be
discussed more thoroughly within each. In addition, the gaps in
our knowledge will be
identified, specifically as they relate to Australia’s wet/dry
tropics.
-
2
1.2 Large rivers, floodplains and function
Large rivers and floodplains can be defined in many ways. For
example, large rivers
have been defined by stream order (> 6th order, Vannote et
al., 1980) and upstream
catchment area ( > 1000 km2, Finlayson and McMahon, 1988).
Floodplains tend to be
defined as regions that become periodically inundated and thus
alternate between
terrestrial and aquatic states via the lateral overflow of main
channels or lakes, or by
direct rain or groundwater input (Junk et al., 1989; Junk and
Wantzen, 2003). For the
purposes of this thesis, large rivers are defined as having
upstream catchment areas
greater than 1000 km2. Natural floodplain rivers are defined as
unregulated systems that
consist of these large rivers (main channels, major tributaries
and distributaries) and
their floodplain area (including minor anabranches, backwaters,
oxbow lakes or
billabongs, and other inundation zones). Thus, large floodplain
river systems naturally
comprise both permanent and temporary, lotic and lentic habitats
(cf. Junk et al., 1989;
Thorp et al., 2006).
River function is rarely defined explicitly. However, it
describes the input, production,
movement, storage, use, interactions and output of energy (e.g.
organic matter,
temperature, light) within river systems (from headwaters to
mouth, from main channels
to the extremities of the floodplain, vertically from surface to
ground waters, and
through time) and the drivers behind these patterns and
processes (Vannote et al., 1980;
Junk et al., 1989; Ward, 1989). One of the first and most
influential conceptual models
that integrated rivers with function, as defined above, was the
River Continuum
Concept (RCC, Vannote et al., 1980). This model viewed streams
and rivers as
ecosystems, and stimulated much discussion and hypotheses about
the function of large
and floodplain rivers, as encapsulated by new models. The three
most well-known
conceptual models of river function are the RCC itself, the
Flood Pulse Concept (FPC,
Junk et al., 1989) and the Riverine Productivity Model (RPM,
Thorp and Delong,
1994). These models provide a useful means of investigating and
comparing amongst
real river systems. To this end, they will be described
below.
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1.3 The River Continuum Concept
The River Continuum Concept (RCC) was developed for natural
(anthropogenically
undisturbed) river systems with predictable physical
characteristics, including the
hydrological cycle, and focuses on macroinvertebrates (Vannote
et al., 1980). It views
river systems as longitudinal gradients of physical conditions
that create a series of
biotic responses that match patterns of organic matter
processing from headwaters to
river mouth. This matched response occurs because riverine biota
have developed
“processing strategies” that enable minimum loss of energy
(Vannote et al., 1980: 130).
Any energy lost by one set of biota will be exploited by a
different set along the river
continuum.
In particular, the RCC places importance on headwater regions
for downstream
production: inefficiencies (leakages) in upstream processing of
dead leaves and woody
debris are exploited by downstream biota. In this way, the model
connects stream size
with structure and function. At headwaters (stream orders 1 –
3), riparian vegetation
provides input of coarse particulate organic matter (CPOM), as
well as shade, which
then limits in-stream primary production. In medium-sized
streams (orders 4 – 6),
shading from the riparian zone is reduced and leads to increased
in-stream primary
production. In large rivers (orders > 6), much fine
particulate organic matter (FPOM)
arrives from the ‘leaked’ upstream sources and forms a
substantial component of the
total organic carbon pool in comparison with local riparian
litterfall. In addition, the
turbidity and depth of large rivers limits light infiltration
and in-stream primary
production. The three river sections (headwaters, medium-sized
streams and large
rivers) thus create a predictable pattern of food resources
along a continuum, with which
functional groups of aquatic macroinvertebrates predictably
correspond.
Differentiating aquatic macroinvertebrates on the basis of
functional feeding groups
(FFGs) is a technique initially described by Cummins (1973) and
modified to some
extent over the years (Cummins and Klug, 1979; Merritt and
Cummins, 1996a; Boulton
and Brock, 1999). The method groups aquatic invertebrates
according to morphological
and behavioural adaptations that correspond with different
feeding mechanisms and
access to different nutritional resources (Merritt and Cummins,
1996b). The main
categories of food resources available to aquatic invertebrates
are CPOM (processed by
shredders), FPOM (fed on by collectors, with filterers
collecting FPOM in the water
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column and gatherers collecting FPOM in the benthic zone),
biofilm and epiphytes (fed
on by grazers, also known as scrapers), and prey (fed on by
predators). Additionally,
invertebrates may be classed as omnivores or generalists,
indicating they utilise more
than one type of food resource. Ultimately, the presence and
proportions of different
resources within a habitat should correspond with the presence
and proportions of
different FFGs.
This concept was applied in the development of the RCC, which
predicts that the
relative dominance (as biomass) of macroinvertebrate FFGs
reflects changes in food
resource type and location along the continuum. According to the
RCC, shredders and
collectors co-dominate in headwaters due to the high
availability of CPOM and FPOM
derived mainly from riparian inputs. Grazers dominate in
medium-sized streams where
algal production is high. Collectors dominate in large rivers
due to the large amounts of
transported FPOM available. Predator biomass changes little
along the river continuum.
Overall, the RCC describes river biota as longitudinally linked
assemblages, predictably
associated with the physical upstream-downstream gradient and
determinably
influenced by hydro-geomorphic processes.
The RCC may apply most readily to the small temperate headwaters
and streams,
particularly those in forested catchments, from which its
hypotheses stemmed, rather
than to the large rivers for which its hypotheses were extended
(Johnson et al., 1995;
Junk and Wantzen, 2003). However, there is a certain amount of
adaptability inherent in
the RCC because it ultimately focuses on the connection between
a river’s physical
setting and the responses of its biotic community. It suggests
that community structure
and function change in response to variation in geomorphology
and the biophysical
characteristics of their environment (Vannote et al., 1980).
Thus, if longitudinal changes
in, for example, stream flow, channel morphology, CPOM/FPOM
ratios, or in-stream
primary production can be matched with community structure and
function, the RCC
should have application. Even so, the model is generally
considered to have less
application to ephemeral or unconstricted large rivers because
it does not account for
aquatic-floodplain-terrestrial interactions (Sedell et al.,
1989). Hence, conceptual
models of river function continued to be developed.
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1.4 The Flood Pulse Concept
The Flood Pulse Concept (FPC) was developed in response to the
RCC’s restricted view
of large rivers as permanent and longitudinally connected main
channels rather than as
dynamic floodplain rivers with lotic and lentic habitats
connected longitudinally and
laterally (Junk et al., 1989). In particular, the FPC was
developed with large tropical
floodplain rivers in mind, identifying floods as beneficial
disturbance events. The
concept’s founders refer to large river floodplains as
aquatic/terrestrial transition zones
(ATTZ), with boundaries that vary in space and time due the
lateral movement of long
and predictable flood pulses. These flood pulses of river
discharge, and the influence of
associated hydrological processes, are deemed as the major
determinant of biota in
river-floodplain systems. In this way, the model places emphasis
on lateral connections
between a river and its floodplain, rather than on longitudinal
connections as
emphasised by the RCC. Grandly, it asserts that production
within floodplains provides
the bulk of riverine animal biomass in large, unmodified
river-floodplain systems in
subtropical, tropical and temperate zones.
The long and predictable movement of the flood pulse across the
floodplain produces an
edge environment, described as a ‘moving littoral’, and creates
increased habitat
diversity (allowing for high species diversity) and a large area
in which primary
production can occur. This production, particularly from
terrestrial and aquatic vascular
plants, is thought to supply the main source of energy for
in-channel and floodplain
biota in floodplain river systems. In fact, primary production
within main channels is
considered light limited as a result of the depth and turbidity
associated with large
rivers, and is unfavoured due to high turbulence and current
velocity. Thus, floodplain
production is seen as providing biota with large and direct
benefits.
However, the FPC asserts that the timing, duration and rates of
rise and fall of the flood
pulse influences production and biotic communities in both
channel and floodplain
habitats. This aspect of the FPC is summarised by Bayley (1995).
In general, nutrients
mineralised on the floodplain during dry times become dissolved
or are adsorbed onto
suspended sediments transported from the main channel during
inundation. Vascular
plant production (terrestrial and aquatic) increases, as does
decomposition; however,
primary production dominates. As floodwaters stabilise,
decomposition rates increase.
During drawdown, nutrients become concentrated in receding
floodplain waterbodies,
phytoplankton production can increase, and flood banks
restabilise. However,
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characteristics of the flood pulse alter these cycles. For
example, fast rates of inundation
and drawdown may limit opportunities for riverine biota to
benefit from floodplain
productivity, whereas slow rates may reduce total floodplain
inundation and therefore
production potential (Bayley, 1995). Despite this potential
variation, the flood pulse is
still seen as the key driver of floodplain river function and
remains the overarching
theme of the FPC.
1.5 The Riverine Productivity Model
In contrast to the RCC and FPC, the Riverine Productivity Model
(RPM) was developed
with an emphasise on local in-channel processes as the key
driver of large river function
(Thorp and Delong, 1994). It contends that the RCC and FPC, as
models outlining the
structure and function of large river systems that focus
respectively on headwaters and
seasonal flood pulses as sources of nutrients for riverine food
webs, neglect the
importance of local in-stream primary production and riparian
litterfall in large rivers.
However, the model was developed with particular application to
large and deep rivers
with firm beds and constricted (non-floodplain) channels.
The RPM suggests that local in-stream primary production
(phytoplankton in particular
but also benthic algae, aquatic vascular plants and mosses),
along with direct inputs
from the riparian zone (leaf litter, particulate and dissolved
organic carbon), are the
primary sources of organic carbon assimilated by metazoan
consumers in large rivers.
This is because the majority of consumers occur in benthic
littoral zones where flow is
usually slower and organic matter accumulates. This is in
opposition to the RCC, which
purports that local riparian inputs (CPOM) in large rivers are
insignificant because the
riparian zone is small relative to the size of the river channel
(Vannote et al., 1980). The
RPM also states that local riparian inputs, along with local
in-stream primary producers,
are more labile than refractive inputs of organic matter derived
from upstream or
floodplain areas. Even though substantial volumes of upstream-
or floodplain-derived
organic material may enter large rivers, this material does not
have great importance for
food webs and overall productivity of the river system. In
general, the model predicts
that local habitat characteristics combined with the quality of
organic carbon sources
available will determine community structure and function in
large rivers.
The above hypotheses of the RPM were subsequently refined, based
on the results of
numerous stable isotope studies of aquatic food webs