Fran Sheldon

Flow variability and the ecology of large rivers

Ecological processes in large rivers are controlled by their flow variability. However, it is difficult to find measures of hydrological variability that characterize groups of rivers and can also be used to generate hypotheses about their ecology. Multivariate analyses of the hydrographs of 52 rivers worldwide revealed distinctive patterns of flow variability that were often correlated with climate. For example, there were groups of rivers that corresponded broadly with ‘tropical’ and ‘dryland’ climates. However, some rivers from continental climates occupy both extremes of this range, illustrating the limitations of simple classification. Individual rivers and groups of rivers may also have different hydrographic ‘signatures’, and attempts to combine measures of hydrological variability into indices mask biologically significant information. This paper identifies 11 relatively independent measures of hydrological variability that help categorize river types and are each associated with aspects of fish biology. Ways are suggested by which the Flood Pulse Concept can be expanded to encompass hydrological variability and accommodate differences among groups of rivers from different climatic regions. Such recognition of the complex role of hydrological variability enhances the value of the concept for river conservation, management and restoration.

Conservation value of variable connectivity: aquatic invertebrate assemblages of channel and floodplain habitats of a central Australian arid-zone river, Cooper Creek

Rapidly expanding water resource development in arid and semi-arid zones of Australia threatens the flow regime and ecological integrity of the few large dryland rivers and their immense floodplains. Efforts to manage and conserve the surface waters of these rivers are hampered by limited scientific data on the ecology of their flora and fauna and on their responses to the high natural variability of flow regime that typifies dryland rivers. Irregular floods connect channel and floodplain wetlands to differing degrees and for varying periods of time but the ecological significance of this connectivity is poorly understood. On Cooper Creek, a large dryland river in central Australia, we explored the degree to which assemblage composition varied with connectivity and hydrological regime. Shortly after protracted regional flooding, we sampled aquatic macroinvertebrate assemblages from the principal microhabitats in 12 channel and floodplain wetlands. Ephemeral and temporary lakes tended to have fewer taxa than semi-permanent channel or terminal lake habitats. Although hydrological connection had only recently been lost for some wetlands, there was already evidence of divergence in aquatic macroinvertebrate assemblage composition. Disruption of the natural variability in connectivity and hydrological regime by excessive water abstraction or river-flow regulation threatens the ecological integrity and aquatic macroinvertebrate biodiversity of dryland rivers. Preservation of the irregular flow regime and sporadic connectivity underpins conservation of the mosaic of floodplain wetlands that play such a crucial role in the ecosystem functioning of rivers such as Cooper Creek.

Mechanistic effects of low-flow hydrology on riverine ecosystems: ecological principles and consequences of alteration

Abstract.  Alterations to the natural flow regime affect the structure and function of rivers and wetlands and contribute to loss of biodiversity worldwide. Although the effects of flow regulation have been relatively well studied, a lack of synthesis of the ecological consequences of low flows and droughts impedes research progress and our grasp of the mechanistic effects of human-induced water reductions on riverine ecosystems. We identified 6 ecologically relevant hydrological attributes of low flow (antecedent conditions, duration, magnitude, timing and seasonality, rate of change, and frequency) that act within the temporal hierarchy of the flow regime and a spatial context. We synthesized the literature to propose 4 principles that outline the mechanistic links between these low-flow attributes and the processes and patterns within riverine ecosystems. First, low flows control the extent of physical aquatic habitat, thereby affecting the composition of biota, trophic structure, and carrying capacity. Second, low flows mediate changes in habitat conditions and water quality, which in turn, drive patterns of distribution and recruitment of biota. Third, low flows affect sources and exchange of material and energy in riverine ecosystems, thereby affecting ecosystem production and biotic composition. Last, low flows restrict connectivity and diversity of habitat, thereby increasing the importance of refugia and driving multiscale patterns in biotic diversity. These principles do not operate in isolation, and many of the ecological pathways that are affected by low flows are likely to overlap or occur simultaneously, potentially resulting in synergistic and complex effects. Last, we outlined major human-induced threats to low-flow hydrology and how they act upon the ecologically relevant hydrological attributes of low flow to affect potential changes in riverine ecosystem integrity. The mechanistic links described in this synthesis can be used to develop and test hypotheses of low-flow hydrological–ecological response relationships in a cause–effect framework that will have value for both research and river flow management. Continued experimental research and ongoing consolidation of ecological information will improve our understanding and ability to predict consequences of low-flow alteration on river, floodplain, and estuarine ecosystems.

Water resource development and hydrological change in a large dryland river: the Barwon–Darling River, Australia

Water resource development has had a major impact on the hydrology of the Barwon–Darling River, a large dryland river in southeast Australia. Flows are highly modified through the presence of nine headwater dams, 15 main channel weirs and 267 licensed water extractors. Median annual runoff has been reduced by 42% over a 60-year period. Small flood events (e.g. Average Recurrence Interval of <2 years) have suffered the greatest impact with reductions in magnitude of between 35 and 70%. At a number of stations, the seasonality of flows has also been affected with a distinct shift in seasonal flow peaks relating to irrigation diversions. Overall, flows show a marked increase in predictability and consistency (sensu Colwell R.K. 1974. Predictability, constancy and contingency of periodic phenomena, Ecology 55, 1148–1153). There has also been a change in the shape of the hydrograph. Both long- and short-term hydrological changes in the Barwon–Darling, associated with water resource development, may prove to be critical for the ecological health of the system.

Changes in biofilms induced by flow regulation could explain extinctions of aquatic snails in the lower River Murray, Australia

Notopala sublineata, Notopala hanleyi(Viviparidae) andThiara balonnensis (Thiaridae) are prosobranchgastropodsthat were once abundant in the lower River Murray. These andothersnail taxa have declined markedly over the last 50 years,coinciding with increased flow regulation by dams and weirs.Inthis paper we speculate that the decline may be linked tochangesin the nature of food resources. Before regulation, most ofthebiofilm biomass in the lower Murray probably was microbial, asfluctuating water levels and high turbidity would havemaintainedthese communities in a state of early succession. Bystabilisingseasonal water levels, we suggest that regulation has promotedthegrowth of filamentous algae, perhaps at the expense ofbacteria.Evidence from gut and faecal pellet analysis, and fromanalysis ofcarbon stable-isotopes, suggests that the gastropod taxa aredetritivores, feeding mainly on amorphous organic detritus.Algaehave a relatively high C:N ratio (low nutritional value) andmay bean inadequate food to maintain female growth and reproduction,especially in viviparous snails.

The macroinvertebrate fauna of an Australian dryland river: spatial and temporal patterns and environmental relationships

Waterholes within the dryland Cooper Creek, Lake Eyre Basin, Australia, are connected only during floods and are typically isolated for long periods. Spatial changes in the macroinvertebrate assemblages of 15 of these waterholes belonging to four regions were explored and these changes were related to environmental aspects of the waterholes measured at four spatial scales: floodplain, waterhole, within waterhole and sample habitat. To explore temporal patterns, one region was sampled on four occasions differing in time since connection. Spatial patterns were characterised by ‘differentiation by distance’ whereby samples collected closer to each other in the landscape were more similar in assemblage composition than those collected further apart. Thus, there were significant differences between the assemblages of the four regions. Although there was a correlation between macroinvertebrate spatial patterns and a combination of local habitat, geomorphology and water chemistry attributes, it appears unlikely that these variables were responsible for the faunal differentiation by distance. Temporal variability was larger than spatial variability and temporal assemblage patterns were best explained by the ‘connectivity potential’ of waterholes, reflecting the position of individual waterholes within the broader channel network and long-term connectivity relationships, rather than the actual time since hydrological connection.

An ecosystem approach for determining environmental water allocations in Australian dryland river systems: the role of geomorphology

The allocation of water for environmental purposes is a key management issue in many dryland regions. Many different methods have been developed for determining environmental water requirements but these are not directly applicable to dryland rivers because of inherent flow and habitat variability. An ecosystem approach for determining environmental water allocations in dryland regions is presented in this paper. This four-step process involves (1) a hierarchical characterisation of the river system, to assess what mesohabitats are present and where they are located; (2) the determination of flows that would inundate these habitats and perform other key ecological processes; (3) hydrological analyses in which the key hydrological signatures of the river are identified and the impact of water resource development on these is determined; and (4) the derivation of a water management decision tree that enables managers to allocate water to consumptive users during individual flood pulses (events). It is recommended that the flood pulse should be the focus for environmental flow management in dryland regions. If rivers are indeed nested hierarchies, then a change in hydrological behaviour at the scale of a flood pulse will, with time, extend throughout the hydrological hierarchy. Current environmental flow management strategies in dryland river systems are essentially focused at the flow regime and history scale; this is inappropriate given the inherent flow variability of these systems. The ecosystem approach is outlined for the Condamine-Balonne River, a large dryland system in Australia.

Spatial distribution of littoral invertebrates in the lower Murray-Darling River system, Australia

The abundance and richness of macroinvertebrates in the lower Murray and Darling rivers were examined at a macroscale (rivers), mesoscale (billabongs, backwaters, channel) and microscale (vegetation, snags, substrata). In the Darling, insects dominated (85% of taxa, 81% of individuals); the richest taxa were Diptera (26 taxa) and Coleoptera (15 taxa) and the most abundant were Hemiptera (47%) and Diptera (35%). In the Murray, insects again dominated (84% of taxa, 52% of individuals), particularly Diptera (22 taxa), Coleoptera (12 taxa) and Hemiptera (9 taxa), but there were more crustaceans (9% of taxa, 47% of individuals, particularly the atyid shrimp Paratya australiensis). Both assemblages were uneven: in the Darling, >50% of biomass was Micronecta spp. (Corixidae), Dicrotendipes sp. (Chironomidae) and Macrobrachium australiense (Palaemonidae); in the Murray, 70% of biomass was P. australiensis and Caridina mccullochi (Atyidae) and the insects Micronecta spp. (Corixidae) and Chironomus sp. (Chironomidae). Abundances generally were greatest in the Murray. Hydrologic and geomorphic factors influenced assemblages at the macroscale, whereas microhabitat diversity dominated at the mesoscale.

Cryptic species and morphological plasticity in long-lived bivalves (Unionoida: Hyriidae) from inland Australia.

Molecular (mitochondrial DNA, isozyme) and morphological diversity of freshwater mussels (Family Hyriidae) was examined at 21 sites encompassing four large river systems, across southwest Queensland, Australia. Evidence was found for two major morphological groups. One group, which occurred in every river system, closely matched a recognized species (Velesunio ambiguus) both morphologically and in a well‐supported lineage within a mitochondrial phylogeny generated from partial cytochrome c oxidase subunit I (COI) sequences. The second group most closely matched Velesunio wilsonii in shell morphology but formed three deeply divergent mitochondrial DNA lineages. All four lineages occurred sympatrically in some areas and displayed corresponding fixed differences at nuclear allozyme loci, which suggests an absence of recent hybridization and the presence of separate species.

Identifying the spatial scale of land use that most strongly influences overall river ecosystem health score.

Catchment and riparian degradation has resulted in declining ecosystem health of streams worldwide. With restoration a priority in many regions, there is an increasing interest in the scale at which land use influences stream ecosystem health. Our goal was to use a substantial data set collected as part of a monitoring program (the Southeast Queensland, Australia, Ecological Health Monitoring Program data set, collected at 116 sites over six years) to identify the spatial scale of land use, or the combination of spatial scales, that most strongly influences overall ecosystem health. In addition, we aimed to determine whether the most influential scale differed for different aspects of ecosystem health. We used linear-mixed models and a Bayesian model-averaging approach to generate models for the overall aggregated ecosystem health score and for each of the five component indicators (fish, macroinvertebrates, water quality, nutrients, and ecosystem processes) that make up the score. Dense forest close to the survey site, mid-dense forest in the hydrologically active near-stream areas of the catchment, urbanization in the riparian buffer, and tree cover at the reach scale were all significant in explaining ecosystem health, suggesting an overriding influence of forest cover, particularly close to the stream. Season and antecedent rainfall were also important explanatory variables, with some land-use variables showing significant seasonal interactions. There were also differential influences of land use for each of the component indicators. Our approach is useful given that restoring general ecosystem health is the focus of many stream restoration projects; it allowed us to predict the scale and catchment position of restoration that would result in the greatest improvement of ecosystem health in the regions streams and rivers. The models we generated suggested that good ecosystem health can be maintained in catchments where 80% of hydrologically active areas in close proximity to the stream have mid-dense forest cover and moderate health can be obtained with 60% cover.