Nick R. Bond

Climate change and the world's river basins: anticipating management options

Major rivers worldwide have experienced dramatic changes in flow, reducing their natural ability to adjust to and absorb disturbances. Given expected changes in global climate and water needs, this may create serious problems, including loss of native biodiversity and risks to ecosystems and humans from increased flooding or water shortages. Here, we project river discharge under different climate and water withdrawal scenarios and combine this with data on the impact of dams on large river basins to create global maps illustrating potential changes in discharge and water stress for dam-impacted and free-flowing basins. The projections indicate that every populated basin in the world will experience changes in river discharge and many will experience water stress. The magnitude of these impacts is used to identify basins likely and almost certain to require proactive or reactive management intervention. Our analysis indicates that the area in need of management action to mitigate the impacts of climate change is much greater for basins impacted by dams than for basins with free-flowing rivers. Nearly one billion people live in areas likely to require action and approximately 365 million people live in basins almost certain to require action. Proactive management efforts will minimize risks to ecosystems and people and may be less costly than reactive efforts taken only once problems have arisen.

Using species distribution models to infer potential climate change-induced range shifts of freshwater fish in south-eastern Australia

There are few quantitative predictions for the impacts of climate change on freshwater fish in Australia. We developed species distribution models (SDMs) linking historical fish distributions for 43 species from Victorian streams to a suite of hydro-climatic and catchment predictors, and applied these models to explore predicted range shifts under future climate-change scenarios. Here, we present summary results for the 43 species, together with a more detailed analysis for a subset of species with distinct distributions in relation to temperature and hydrology. Range shifts increased from the lower to upper climate-change scenarios, with most species predicted to undergo some degree of range shift. Changes in total occupancy ranged from –38% to +63% under the lower climate-change scenario to –47% to +182% under the upper climate-change scenario. We do, however, caution that range expansions are more putative than range contractions, because the effects of barriers, limited dispersal and potential life-history factors are likely to exclude some areas from being colonised. As well as potentially informing more mechanistic modelling approaches, quantitative predictions such as these should be seen as representing hypotheses to be tested and discussed, and should be valuable for informing long-term strategies to protect aquatic biota.

Regime shifts, thresholds and multiple stable states in freshwater ecosystems; a critical appraisal of the evidence

The concepts of ecosystem regime shifts, thresholds and alternative or multiple stable states are used extensively in the ecological and environmental management literature. When applied to aquatic ecosystems, these terms are used inconsistently reflecting differing levels of supporting evidence among ecosystem types. Although many aquatic ecosystems around the world have become degraded, the magnitude and causes of changes, relative to the range of historical variability, are poorly known. A working group supported by the Australian Centre for Ecological Analysis and Synthesis (ACEAS) reviewed 135 papers on freshwater ecosystems to assess the evidence for pressure-induced non-linear changes in freshwater ecosystems; these papers used terms indicating sudden and non-linear change in their titles and key words, and so was a positively biased sample. We scrutinized papers for study context and methods, ecosystem characteristics and focus, types of pressures and ecological responses considered, and the type of change reported (i.e., gradual, non-linear, hysteretic or irreversible change). There was little empirical evidence for regime shifts and changes between multiple or alternative stable states in these studies although some shifts between turbid phytoplankton-dominated states and clear-water, macrophyte-dominated states were reported in shallow lakes in temperate climates. We found limited understanding of the subtleties of the relevant theoretical concepts and encountered few mechanistic studies that investigated or identified cause-and-effect relationships between ecological responses and nominal pressures. Our results mirror those of reviews for estuarine, nearshore and marine aquatic ecosystems, demonstrating that although the concepts of regime shifts and alternative stable states have become prominent in the scientific and management literature, their empirical underpinning is weak outside of a specific environmental setting. The application of these concepts in future research and management applications should include evidence on the mechanistic links between pressures and consequent ecological change. Explicit consideration should also be given to whether observed temporal dynamics represent variation along a continuum rather than categorically different states.

Climate-change threats to native fish in degraded rivers and floodplains of the Murray–Darling Basin, Australia

Many aquatic ecosystems have been severely degraded by water-resource development affecting flow regimes and biological connectivity. Freshwater fish have been particularly affected by these changes and climate change will place further stress on them. The Murray–Darling Basin (MDB), Australia, represents a highly affected aquatic system with dramatically modified flow regimes. This has impaired the health of its rivers, and potentially limited the adaptive capacity of its biota to respond to a changing climate. Here, we present our predictions of the potential impacts of climate change on 18 native fish species across their distributional ranges against the back-drop of past and continuing water-resource development (WRD). Because most of these species are found across a wide range of geographical and hydrological settings, we classified the MDB into 10 regions to account for likely variation in climate-change effects, on the basis of latitude, elevation and WRD. Cold water-tolerant species will be under greater stress than are warm water-tolerant species. In some regions, the negative impacts on exotic fish such as trout are likely to improve current conditions for native species. Because the impacts of climate change on any given species are likely to vary from region to region, regional fish assemblages will also be differentially affected. The most affected region is likely to occur in the highly disturbed Lower Murray River region, whereas the dryland rivers that are less affected in the northern MDB are likely to remain largely unchanged. Although climate change is a current and future threat to the MDB fish fauna, the continued over-regulation of water resources will place as much, if not more, stress on the remnant fish species.

A field and experimental study on the tolerances of fish to Eucalyptus camaldulensis leachate and low dissolved oxygen concentrations

In intermittent streams, deteriorating water quality during drying influences fish survival but the specific effects of individual variables and their interactions are poorly known. During summer 2002 and 2004, fish were surveyed in isolated pools of two lowland intermittent streams in south-east Australia. Despite a low dissolved oxygen (DO, range = 0.4–6.8 mg L–1) and high dissolved organic carbon (DOC, 16–50 mg L–1) concentrations, assemblage composition and abundance of native fish appeared unaffected. In subsequent laboratory experiments, concentrations of DO and DOC were independently manipulated to identify better the tolerance of these species to extremes in environmental conditions. At low DOC concentrations (20 mg L–1), no significant effects were observed. At high DOC concentrations (50, 70 and 80 mg L–1), an interaction was observed between DOC and DO, with significant reductions in resistance (decreased time to loss of buoyancy). At extreme DOC concentrations (99 mg L–1), the effect of DO appeared to have been overridden by a strong effect from DOC (rapid loss of equilibrium). The thresholds observed suggest these species have a high resistance to ‘blackwater events’ caused by leaching of DOC from terrestrial leaf litter. Our findings are consistent with the observed tolerances of fish occupying habitats, both in Australia and elsewhere in the world, where extreme physicochemical conditions are a regular and predictable occurrence.

When trends intersect: The challenge of protecting freshwater ecosystems under multiple land use and hydrological intensification scenarios

Intensification of the use of natural resources is a world-wide trend driven by the increasing demand for water, food, fibre, minerals and energy. These demands are the result of a rising world population, increasing wealth and greater global focus on economic growth. Land use intensification, together with climate change, is also driving intensification of the global hydrological cycle. Both processes will have major socio-economic and ecological implications for global water availability. In this paper we focus on the implications of land use intensification for the conservation and management of freshwater ecosystems using Australia as an example. We consider this in the light of intensification of the hydrologic cycle due to climate change, and associated hydrological scenarios that include the occurrence of more intense hydrological events (extreme storms, larger floods and longer droughts). We highlight the importance of managing water quality, the value of providing environmental flows within a watershed framework and the critical role that innovative science and adaptive management must play in developing proactive and robust responses to intensification. We also suggest research priorities to support improved systemic governance, including adaptation planning and management to maximise freshwater biodiversity outcomes while supporting the socio-economic objectives driving land use intensification. Further research priorities include: i) determining the relative contributions of surface water and groundwater in supporting freshwater ecosystems; ii) identifying and protecting freshwater biodiversity hotspots and refugia; iii) improving our capacity to model hydro-ecological relationships and predict ecological outcomes from land use intensification and climate change; iv) developing an understanding of long term ecosystem behaviour; and v) exploring systemic approaches to enhancing governance systems, including planning and management systems affecting freshwater outcomes. A major policy challenge will be the integration of land and water management, which increasingly are being considered within different policy frameworks.

Flow-related disturbance in streams: an experimental test of the role of rock movement in reducing macroinvertebrate population densities

Densities of hydropsychids (Trichoptera) on different-sized stones were compared before and after a winter flood, and the effects of rock movement (a likely form of disturbance during floods) on these organisms were tested. Before the flood, the density of hydropsychids was an order of magnitude higher on large than on small stones; after the flood, densities on larger stones had fallen to levels similar to those on small stones. In a four-week colonization experiment over the period during which the flood occurred, densities of hydropsychids were compared on bricks of two sizes, which were either fixed to the bed of the stream so that they could not move, or unfixed and able to move with changes in flow. Disturbance rates generally differed between small and large unfixed bricks, but all unfixed bricks moved during a large flood, regardless of size. The densities of hydropsychids on fixed and unfixed bricks were similar, and they reflected densities on natural stones after the flood, suggesting that even substrata that do not move during floods may fail to provide a refuge from the effects of high flows.

Nuptial coloration varies with ambient light environment in a freshwater fish

Visual signals play a vital role in many animal communication systems. Signal design, however, often varies within species, raising evolutionarily important questions concerning the maintenance of phenotypic diversity. We analysed nuptial colour variation within and among nine populations of southern pygmy perch (Nannoperca australis Günther) along an environmental light gradient. Within populations, larger males were redder and blacker, and better‐condition males were blacker. Among populations, red colour was positively correlated with the amount of orange‐red light present, suggesting that males are likely optimizing signal conspicuousness by producing proportionally larger and redder patches in broad spectrum environments with more orange‐red light. Signal contrast, in this regard, is maximized when red colour, appearing bright because of the prevalence of red wavelengths, is viewed against the water‐column background. Together, our results are concordant with the sensory drive hypothesis; selection favours signal adaptations or signal plasticity to ensure communication efficacy is maximized in different light environments.

Ecological risks and opportunities from engineered artificial flooding as a means of achieving environmental flow objectives

Restoration of floodplain ecosystems through the reinstatement of floods is often hampered by insufficient water as a result of competing human demands. An emerging alternative approach relies on floodplain infrastructure – such as levees, weirs, regulators, and pumps – to control water levels within floodplains without requiring landscape-scale overbank floods. This technique, albeit water efficient and capable of achieving some ecological targets, does not mimic the hydraulics, hydrodynamics, and lateral connectivity of natural floods. Engineering approaches like this may risk detrimental ecological outcomes, including reductions in biotic connectivity, river–floodplain productivity, and water quality, and thus may fail to support the range of ecological processes required to sustain healthy river–floodplain systems. Here, we review the potential benefits, risks, and mitigation options associated with engineered artificial flooding. Given the growing challenge of equitable water allocation, further research on and monitoring of engineered floods as a tool to sustain floodplain ecosystems are urgently required.

Floods Down Rivers: From Damaging to Replenishing Forces

Publisher Summary This chapter draws attention to the similarities and differences in the physical characteristics of floods and their ecological effects in upland and lowland ecosystems. It describes the physical nature of floods in upland and lowland systems and examines the ecological response and importance of floods in each of these settings. High water events, floods, as disturbances are a major force shaping the ecology of streams and rivers. In constrained streams, usually in upland areas near the source, floods are marked by their high power—high velocities and shear stress. Such floods can change the shape of the channel, moving and scouring sediments, degrading and creating habitat, and removing and damaging biota. Covering recent developments in the ecological understanding of floods, the chapter highlights the damaging and replenishing nature of floods in different ecosystems. The chapter also emphasizes on examples drawn from the Murray–Darling Basin, a system that harbors a wide variety of watercourses and has been a major focus for aquatic ecosystem research in Australia.