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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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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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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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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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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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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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(‘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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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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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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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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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.
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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