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

i

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

ii

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.

v

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

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

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

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

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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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.

3

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

4

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.

5

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,

6

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