Academic literature on the topic 'Australian lowland rivers'

In freshwater systems, phosphorus is adsorbed predominantly to clay within the sediments. Assuming a linear adsorption/desorption isotherm, rapid equilibrium adsorption, and transport by molecular diffusion, estimates are derived for (a) the rates of exchange between the adsorbed phosphorus pool in the sediments and the dissolved pool in the water column and (b) time scales to re-establish equilibrium after a step change in the water column phosphorus concentration. For oxic sediments, the time scale is of the order of tens of days. Anoxic release is much faster;the time scale is tens of minutes. The release of treated sewage at Narrabri abruptly raises the phosphorus concentration in the Namoi River. The concentration only returns to its original level 10–20 km downstream. A sediment adsorptive-uptake model underestimates the downstream phosphorus uptake rates. An alternative model, based on biotic uptake by Cladophora , describes reality better. It treats phosphorus transfer as controlled by physical transport processes and by the phosphorus uptake capacity of the biota. We show also that carp resuspension is faster than diffusion (6 v. 28 days) in restoring phosphorus concentrations in the water column after perturbation by rapid algal drawdown.

Submerged woody habitat provides the major structure around which ecological processes operate in many lowland rivers. Colonisation by macroinvertebrates was measured in a south-eastern Australian river over a 32-day period in an experiment testing the hypothesis that wood type influences the invertebrate assemblage structure. The wood types were green wood, dry wood, and dry but previously waterlogged wood. All wood used was river red gum (Eucalyptus camaldulensis). Macroinvertebrates colonised previously waterlogged wood more rapidly than green or dry wood. The assemblage structure varied significantly over the sampling period, with copepods and cladocerans numerically dominating the assemblage during the first few days after the introduction of the wood. The assemblage became more diverse through time and was numerically dominated by dipterans, ephemeropterans and trichopterans. The results indicate that there was little difference in the time taken for macroinvertebrate colonisation after wood introduction when using either green or dry wood. This has implications for large-scale restoration projects, where green wood is likely to be a more readily available option for reintroduction than dry wood.

Since European settlement, vast amounts of wood have been removed from Australian rivers. In recognition of its ecological value, including as habitat for aquatic invertebrates, wood is being reintroduced but with little understanding of optimum placement (pools v. riffles) or structural complexity to enhance invertebrate diversity. We hypothesised that complex woody debris would support higher numbers and more macroinvertebrate taxa, especially in riffles. Wood substrates of two complexities but similar surface areas were introduced into pools and riffles at three sites along the Hunter River, Australia. After 30 days, more taxa and individuals occurred on the complex substrates in pools and riffles at all sites. Substrates in riffles usually supported more taxa and individuals but responses were site-specific. Community composition varied among sites, substrates and habitats. Complex substrates, especially in riffles, trapped drifting organic matter that increased abundance and taxa richness but did not alter overall trends among substrates or habitats within sites. However, densities of rheophilic (flow-loving) taxa were reduced by entrained organic matter. Our results indicated that complex woody debris introduced into riffles could enhance diversity and abundance of macroinvertebrates in the Hunter River. However, these conclusions from a short-term, small-scale experiment need validation from longer-term, large-scale river rehabilitation projects.

The spatial and temporal dynamics of physical habitat in rivers is driven by the interaction between channel morphology and discharge. However, little is known about how altered discharge affects the dynamics of habitat patches such as slackwaters. This study investigated the influence of discharge on the availability, stability, quality, and diversity of slackwaters in a southeastern Australian lowland river. The area, spatial configuration, permanence, and within-patch characteristics of slackwaters of two reaches in a regulated section and two reaches in a largely unregulated section of the river were compared. There was less slackwater area and it was less permanent at higher discharges and in the two regulated reaches than at lower discharges and in the largely unregulated reaches. Individual slackwaters were more homogenous in relation to within-patch characteristics in the regulated than in the largely unregulated reaches. However, variability in the spatial configuration of slackwaters and within-patch characteristics and diversity at the reach scale were not related to discharge. We suggest that channel morphology, rather than discharge, is the main driver of these characteristics.

Caridean shrimps are an important component of lowland river ecosystems and their distributions may be affected by river regulation. We studied the mesoscale distributions of Paratya australiensis, Caridina mccullochi and Macrobrachium australiense in five lowland rivers of the Murray–Darling Basin, south-eastern Australia. We distinguished habitat patches according to water-current velocity and channel location – still littoral (SL), slow-current-velocity littoral (SCVL) and moderate-current-velocity channel (MCVC) – and investigated ontogenetic shifts in habitat use. We sampled seven reaches for shrimp in March 2003 and December 2003 using a modified backpack electrofisher. Paratya australiensis occurred in all habitats but was mostly associated with SL. All life stages of C. mccullochi utilised SL and SCVL, and only a few adults were collected from areas with greater than slow current velocity. The habitat preference of M. australiense changed with development: larvae only occurred in SL, but adults and berried females strongly preferred MCVC. Low flows and slow water currents are characteristic of lowland rivers in southern Australia during summer and autumn (December–April), the period during which shrimps’ larval development and juvenile recruitment occurs. Caridina mccullochi and M. australiense may rely on still and slow-current-velocity habitats during larval development and juvenile recruitment and to facilitate upstream movements.

Bacterial production is important in aquatic carbon cycles because it represents a key component whereby dissolved and particulate carbon can be recycled back into food webs. Despite its acknowledged importance, few studies have examined bacterial production in lowland rivers. Since studies have suggested bacterial production is closely related to some carbon pools, we anticipated this to be the case in the Murray River, but that the timing and type of carbon inputs in the Murray River may lead to bacterial dynamics that differ from studies from other sites. Bacterial abundance and production were measured at three contrasting sites of the lowland Murray River, south-eastern Australia, over an 18-month period. Bacterial abundance varied across the three sites on the Murray River and was correlated with chlorophyll a concentrations but not with temperature, nutrients, particulate organic carbon and dissolved organic carbon concentrations. Bacterial production also varied across the sites. Lowest production was at the site most immediately downstream of a large reservoir, with production generally ranging from 0.88 to 8.00 μg C L−1 h−1. Bacterial production in a reach within a large forest ranged from 4.00 to 17.38 μg C L−1 h−1. Production at the reach furthest downstream ranged from 1.04 to 23.50 μg C L−1 h−1. Bacterial production in the Murray River was generally greater than in the European River Spree, reaches of the Meuse and Rhine without immediate impacts from major urban centres and the Amazon River, but was similar to the concentration measured in the Mississippi and Hudson Rivers. Bacterial production was closely correlated with chlorophyll a concentration and total phosphorus, but not with temperature, dissolved organic carbon, particulate organic carbon or inorganic nitrogen. Despite the differences in production and respiration measured at different sites across the Murray River, bacterial growth efficiency was very similar at the three sites. Bacterial populations in the Murray River appear to be influenced by reach-specific conditions rather than broad-scale drivers such as temperature, carbon and nutrient concentrations.

Despite the perceived importance of floodplain inundation to the functioning of lowland rivers, there is limited understanding of the contribution that floodplains make to the main river channel during floods. In 2010, substantial flooding occurred throughout south-eastern Australia, which provided an opportunity to quantify the export of biological material and nutrients from a floodplain back in to the main river channel. We quantified the amounts of zooplankton, phytoplankton, dissolved organic carbon and nutrients within the main river channel of the River Murray immediately upstream of the Barmah–Millewa Forest, and at two sites immediately downstream of the forest during two flood events in July and October of 2010. Results demonstrated that although a smaller flood event in July did not contribute substantially to an increase in the measured parameters, a much larger flood in October contributed 0.4 tonnes (t) of phytoplankton; 7t of zooplankton and 300t of dissolved organic carbon. This suggests that small floods will provide minimal resource subsidies back into the main channel after the cessation of flooding. In comparison, larger floods that result in large volumes of floodplain water returning to the river will provide substantial subsidies of terrestrially derived resources.

The timing and composition of organic matter (OM) inputs to rivers are important as carbon plays a major role in river functioning. Management of Australian rivers since European settlement has altered inputs of organic matter to these systems. Heterotrophic microbes play a critical role in the transformation of OM in rivers, allowing transfer of carbon to other biota. Alteration to the proportions of OM from different sources affects microbial functioning due to differences in OM composition. Macrophytes can represent important sources of carbon to rivers, however their inputs and in-stream processing are poorly understood. The aim of my study was to examine inputs and microbial processing of macrophyte OM in Australian lowland rivers under different flows. Distributions of dominant macrophytes (Typha orientalis, Phragmites australis, Vallisneria gigantea and Persicaria prostrata) were mapped in three lowland river reaches in south eastern Australia. Integration with flow data in a GIS allowed the determination of macrophyte inundation patterns under different flows. Resource allocation (biomass and nutrients), live and dead shoot densities and litter production were monitored in the field over 18 months. DOM release from different macrophyte tissues was examined in the laboratory and leachate composition was assessed using nutrient and spectral analyses. Responses of riverine microbial communities to different OM sources were assessed from substrate-induced respiration and enzyme activity experiments and field measurements of respiration and enzymatic responses to varied OM inputs. Finally, all data were integrated into a model of microbial responses to macrophyte OM inputs induced by different flows. Large populations of macrophytes occurred at all three sites, at bed level, on in-channel benches and on banks. Bank slope, channel heterogeneity and the vertical distribution of macrophyte beds all affected macrophyte inundation patterns. Substantial differences in biomass allocation, nutrient dynamics and litter composition were observed among different plant growth forms and over time. While leaves represented the major shoot component in litter for all species, stems and reproductive structures were also important in some species. Aside from the litter pool, translocation to rhizomes represented a major sink for annual production in emergent plants. Patterns of shoot density and litter production over time varied among species, providing a source of variation for particulate, and hence dissolved OM inputs upon inundation. The majority of DOM release from POM occurred within 24 hours of inundation. Growth form, tissue type (blade, stem, etc.) and status (live or dead) affected rates, quantities and composition of DOM release, with implications for microbial utilisation. Both overall activity and patterns of carbon utilisation in riverine microbes changed in response to altered OM inputs. Patterns of microbial carbon use were shown to be specific to the carbon source which induced them. Modelling showed that flow regulation had a major impact on OM inputs and microbial metabolism, through the effects of flow variability on macrophyte vertical distributions, macrophyte bed inundation and dilution. Positive relationships between discharge, DOM inputs and microbial metabolism were observed at the most highly regulated site (drought < current < historic < flood). While a similar pattern occurred at the less regulated site in terms of total loading, dilution effects resulted in a reversal of this trend on a reach volume basis. Microbial metabolism and DOM inputs were restricted to summer/autumn under regulated flows compared to a greater emphasis on winter/spring inputs and microbial activity under unregulated flows. Continual OM inputs during winter with pulsed inputs in spring under natural flows probably benefit larger, slow-growing macro-invertebrates. River regulation promotes pulsed macrophyte OM inputs during spring/summer, potentially favouring riverine microbial and zooplankton production, although at lower levels due to the overall reduction in OM inputs. The predictive model of macrophyte OM inputs and microbial responses developed throughout this thesis represents a major step forward in our understanding of macrophytemicrobe interactions and our ability to manage our river systems. This work has shown that flow manipulation can be used to influence macrophyte organic matter inputs to rivers and microbial responses, affecting whole stream metabolism and food web interactions.

Rivers around the world are under increasing pressure from a variety of human activities. Effective management of riverine landscapes requires an ecosystem approach and one that recognises the complex interactions between their physical, chemical and biological components. Perceptions of pattern and process are central to our understanding of riverine landscapes. Pattern and process operate over multiple scales to produce heterogeneous mosaics of landscape patches that change over time. Hierarchical patch dynamics provides a useful approach to unravel pattern and process at multiple scales in riverine landscapes. This thesis adopts a hierarchical patch dynamics approach to investigate floodplain sediment and nutrient dynamics within the Barwon-Darling River in South Eastern Australia. The flow regime of the Barwon-Darling River is highly variable. As a result, it has a complex channel cross section featuring inset-floodplain surfaces that occur at multiple elevations within the channel trough. These surfaces formed the focus of this study. The texture of inset- floodplain surface sediments displays a patchy spatial distribution and one that did not reflect lateral or longitudinal gradients within this floodplain landscape. Rather a sediment textural patch mosaic was identified. Nutrient concentrations associated with the surface sediments of the inset-floodplains were also shown to vary significantly resulting in a nutrient patch mosaic. This spatial nutrient mosaic was enhanced by factors including the surface elevation of the floodplain surface. Sediment and nutrient exchange between the river channel and inset-floodplain surfaces was measured during several flows in 2001, 2002 and 2005. Pin and sediment trap data showed that significant quantities of sediment were exchanged between the river channel and floodplain surfaces during inundation with both cut and fill processes occurring. Patterns in sediment exchange appear to be related to local sediment supply and seasonal sediment exhaustion, rather than the top down geomorphic constraints considered. These material exchanges resulted in a change to the spatial configuration of the sediment textural patch mosaic. Distinct new sediment textural patches were created following inundation, while other patches were lost post inundation and other patches changed sediment textural character to move into pre-existing patches. Thus a truly dynamic sediment textural mosaic exists within this floodplain landscape. Nutrient concentrations associated with floodplain sediments also changed over time. While nutrient concentrations increased after the December 2001 flow event, they generally decreased after the March 2002 event, highlighting their dynamic nature over time. The spatial distribution of nutrient concentrations also varied over time, with a 40 percent change to the nutrient mosaic as a result of the March 2002 flow event. In addition to the influence of the changing physical template (sediment texture mosaic), nutrient concentrations were shown to be influenced by rainfall processes on non flooded surfaces, and also a number of top-down constraints and bottom-up influences operating over multiple spatial scales. Overall, the inset-floodplains studied in this thesis acted primarily as sediment and nutrient sinks, and were a source for dissolved nutrients. Nutrient exchange was associated with the exchange of sediments in this riverine landscape, over both inter-flow and decadal timescales. It was demonstrated that water resource development within the catchment reduced the number, magnitude and duration of flow events down the Barwon-Darling River and as a result reductions in the exchange of sediment, associated and dissolved nutrients between inset-floodplains and the main river channel were calculated. The greatest reductions were with the release of dissolved nutrients (42-25 percent) and the exchange of sediment and associated nutrients from high level surfaces (43 percent). Effective conservation and management of riverine ecosystems must occur at the correct scale. This study identified potential nutrient hotspots at several scales in the Barwon-Darling floodplain landscape that could be targeted by management. The low predictability of the location of nutrient hotspots at the inset-floodplain scale over time means that environmental flows should be targeted at high level surfaces (<25 000 MLD-1) that provide long term sources of carbon to the river channel. Conserving flows of this magnitude will also reinstate flow variability, an important facet of the Barwon-Darling River?s hydrology that has been changed by water resource development. The research presented in this thesis highlights the importance of not only considering pattern and process at multiple scales, but also the way in which these processes influence landscape patterns over time, leading to the identification of the appropriate scales that can best be targeted for the conservation of these systems.

In lowland river systems, in-channel, slow-flowing or still-water areas (still-water patches, SWPs) are considered to be important habitats for many organisms, particularly the early-life stages of fish and shrimp. However, the distribution of the early life-stages of fish and shrimp among these habitats appears to be very patchy and studies suggest that the quality and diversity of microhabitat conditions within SWPs and the accessibility of SWPs to spawning adults and dispersing young may be important determinants of their suitability as nursery habitat. The aims of this thesis were to examine the utilisation of still-water patches by fish and shrimp in a lowland river in relation to habitat suitability and accessibility, with particular emphasis on early-life stages. To determine the factors influencing habitat selection among SWPs, the environmental variability in SWP habitat, and both the distribution and the movement patterns of fish and shrimp, were examined in the Broken River, a lowland river in south-eastern Australia. SWP habitat was found to be highly spatially and temporally variable in the Broken River. SWPs differed in relation to permanence, accessibility and microhabitat variables, and all life-stages of fish and shrimp were found to be significantly spatially aggregated among SWPs. This suggests that individual SWPs may differ in their suitability as habitat, and/or in their accessibility to dispersing organisms and indicates either differential rates of retention, movement into SWPs, spawning effort or survival among SWPs for these organisms. Significant associations were found for all species and life-stages in relation to the microhabitat characteristics of SWPs. The two introduced species, carp and gambusia, were found to have fewer associations, which suggests that these species are habitat generalists. Cover and SWP morphology variables were shown to be important for all native species. Significant, positive associations were found for most species and life-stages with large, deep, SWPs containing instream cover, however, the extent of cover preferred was variable. It was hypothesised that large, deep SWPs that contain instream cover are more environmentally stable and provide better foraging efficiency and reduced competition for space, whilst also providing refuge from predators and, that they may be easier to locate than smaller patches. Specific associations with microhabitat variables differed among all species and life-stages, and this was attributed to differences in diet and predation rates. Consequently, generalised microhabitat relationships for particular life-stages or species could not be identified and the results from this thesis suggest that a diversity of microhabitat conditions are required to meet the differing requirements of various life-stages and species. Significant associations were also found for most groups in relation to the accessibility characteristics of SWPs, indicating that the ability of individuals to access SWPs is an important factor in determining their distribution among SWPs. This further suggests that movement is an important factor in the distribution pattern of fish and shrimp among SWPs. Significant associations were found for most groups in relation to patch isolation, adjacent hydraulic habitat and entrance conditions, indicating that landscape composition and configuration as well as boundary conditions may be important determinants of organisms being able to locate suitable patches. Associations with accessibility variables differed among species and life-stages, and may be attributable to differences in movement capabilities. Field manipulations of instream cover and entrance depth were conducted to further examine the habitat associations found. The results confirmed a positive relationship between instream cover and fish and shrimp abundances. No species, however, responded consistently to the manipulation of entrance depths, and this was attributed to water level rises throughout the experiment and/or the correlation of entrance depth with SWP depth. However, the results from the field manipulations suggested that deeper habitats are able to be exploited by small-bodied adults and larvae when significant levels of instream cover are also available as refuge from predation. In order to confirm the importance of movement in the selection of SWP habitat by fish and shrimp, the movement patterns of fish and shrimp into and out of SWPs were investigated. Whilst the results from this aspect of the study were inconclusive for fish, the results for shrimp confirmed that adults and larvae moved routinely into and out of SWPs. However, for all shrimp species, movement appeared to be limited to a certain period of larval development, indicating that SWP quality and stability may be more important at particular stages of development than others. The results of this thesis have demonstrated the importance of SWP quality and stability for fish and shrimp in the Broken River and have shown that habitat preferences vary among individual species and life-stages. Consequently, in order to manage for multiple species and life-stages, consideration must be given not only to the availability of SWPs, but also to their stability over time and to the availability of a diverse range of microhabitats. In addition, consideration must also be given to the accessibility of SWPs and this will require a greater knowledge of the specific spawning and dispersal requirements of the organisms which utilise these patches, in combination with a greater understanding of the impacts of flow modification on riverine landscape composition and configuration.

Hydraulic conditions (velocity, depth, turbulence) strongly influence the distribution and abundance of organisms in rivers. A diverse hydraulic environment should foster biodiversity, because organisms have different hydraulic preferences. In fact, the relationship between spatial hydraulic diversity and biodiversity is largely presumed, and not well-supported by empirical studies, but it underpins efforts in river restoration and conservation. This is particularly so at the reach scale, indicating a stream- or river-section with large-scale homogeneous geomorphic and hydrological conditions and smaller-scale habitat patches, as perceived by organisms in the community under study. This thesis considers the factors that create spatial hydraulic diversity, and the ways that fish respond. It presents a method to characterise hydraulic diversity, and uses this to describe temporal and spatial changes between reaches. It also demonstrates the use of hydraulic modelling for comparing reaches. Finally, it assesses the Acoustic Doppler Current Profiler (ADCP) as a method to describe hydraulic conditions in a large, open river channel. Swimming ability tests were applied to three small freshwater fish, the pelagic Australian smelt (Retropinna semoni) and common galaxias (Galaxias maculatus ) and the demersal flathead gudgeon (Philypnodon grandiceps). The latter species was the weaker swimmer, but the tests indicated that behaviour also should be considered. A laboratory experiment was designed to investigate how two species with contrasting ecological habits (common galaxias, flathead gudgeon) behave in a diverse hydraulic environment. Habitat choices and activity were monitored in a constructed sinuous channel at three discharges over a 3-hour period. The galaxias favoured the pelagic habitat, and spent 20-60% of the time cruising, whereas the flathead gudgeon preferred the demersal habitat and spent <6% of the time cruising. The flathead gudgeons could access their preferred habitat at all discharges, but the common galaxias were limited by their swimming ability at the highest discharge. Several methods to characterise reaches were compared for eight 3-D model reaches representing the effects of channel form, wood and aquatic plants. The variogram (a measure of the variance between samples as a function of distance) emerged as a superior method because it indicates hydraulic diversity, incorporates the spatial arrangement of hydraulic patches, and facilitates comparisons between reaches. The ADCP proved a quick, reliable means to measure depth and 3-D velocity in rivers. It was effective only in depths >1.5 m, but modified instrumentation may overcome this limitation. Six reaches, including weir-pool and free-flowing sections, were compared at two discharges in the River Murray, Australia. Variograms derived from the ADCP data clearly demonstrated spatial differences between the sections, but temporal differences were less well-defined, suggesting that reaches may retain characteristic hydraulic patterns despite changes in discharge. Opportunities for further research include: the issue of optimal levels of hydraulic diversity for fish and other biota; use of variograms as a tool for field studies of aquatic biota; and measuring reach-scale hydraulic diversity and biodiversity before and after reach manipulation (e.g. the placement of wood), to elucidate the effects of changes in spatial hydraulic diversity on reach biodiversity.
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Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2008