Catherine Leigh

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.

Predicting the vulnerability of reservoirs to poor water quality and cyanobacterial blooms

Cyanobacterial blooms in drinking water reservoirs present a major ecosystem functioning and human health issue. The ability to predict reservoir vulnerability to these blooms would provide information critical for decision making, hazard prevention and management. We developed a new, comparative index of vulnerability based on simple measures of reservoir and catchment characteristics, rather than water quality data, which were instead used to test the indexs effectiveness. Testing was based on water quality data collected over a number of seasons and years from 15 drinking water reservoirs in subtropical, southeast Queensland. The index correlated significantly and strongly with algal cell densities, including potentially toxic cyanobacteria, as well as with the proportions of cyanobacteria in summer months. The index also performed better than each of the measures of reservoir and catchment characteristics alone, and as such, was able to encapsulate the physical characteristics of subtropical reservoirs, and their catchments, into an effective indicator of the vulnerability to summer blooms. This was further demonstrated by calculating the index for a new reservoir to be built within the study region. Under planned dimensions and land use, a comparatively high level of vulnerability was reached within a few years. However, the index score and the number of years taken to reach a similar level of vulnerability could be reduced simply by decreasing the percentage of grazing land cover via revegetation within the catchment. With climate change, continued river impoundment and the growing demand for potable water, our index has potential decision making benefits when planning future reservoirs to reduce their vulnerability to cyanobacterial blooms.

Drying as a primary hydrological determinant of biodiversity in river systems: a broad‐scale analysis

Determining and understanding relationships between biodiversity and hydrology is a critical goal in ecology, particularly given biodiversity in the freshwater realm is in crisis. Despite the prevalence of rivers experiencing natural drying disturbances (which we collectively refer to as intermittent rivers), and projections of increased frequency and duration of drying events, the importance of drying relative to other flow-related determinants of river biodiversity remains understudied. We assessed the influence of drying on alpha- and beta-diversity using discharge and macroinvertebrate data collated from Australia and southwest Europe over broad spatial and temporal scales, providing information on current and past drying events, and combining a wide variety of flow metrics. We found clear evidence that drying acts as a strong environmental filter and is a primary hydrological determinant of alpha-diversity; even when considering both intermittent and perennial rivers, drying-event conditions were its most important predictors. Macroinvertebrate richness declined with increasing durations of drying over the long-term (Australia) and recent (Australia and southwest Europe) history of river discharge, and with decreasing predictability of event timing (Australia). Our analysis also revealed that: responses can be taxon specific due to variation in traits of resistance and resilience to drying; some taxa may respond just as or more strongly to variation in other discharge components (e.g. high- or low-flow events) than to drying; and together these phenomena may result in differing community-level responses within and across regions. Patterns of beta-diversity across the wide biogeographical range of our study suggested that convergent and divergent niche-selection processes may act in combination on aquatic communities of rivers experiencing drying disturbances. However, strong ability to disperse by flight (not by water) weakened beta-diversity patterning among rivers. Our findings can be used to improve understanding of biodiversity organisation in disturbed systems, notably in those with dendritic features, including intermittent rivers.

Dry-season changes in macroinvertebrate assemblages of highly seasonal rivers: responses to low flow, no flow and antecedent hydrology

Highly seasonal rivers can experience extended low flow, and often dry, periods. Macroinvertebrate and flow data were used to explore hypotheses on the effects of antecedent hydrology and the low-flow, dry-season period on macroinvertebrate assemblages in northern Australia. Composition differed between early and late dry seasons. Taxa were more sensitive to water quality and more rheophilous in the early dry season when their habitats were lotic than when habitats later became lentic. As flow magnitudes in the antecedent dry season and on the sampling day increased, the habitats became more oxygenated and, in turn, macroinvertebrate richness increased. Higher wet-season flow magnitudes, flow variability and rates of fall were correlated with lower richness in the following dry season. Alteration of the flow-disturbance regime that increases the likelihood of flow cessation in macroinvertebrate habitats, or extends the duration of the dry season beyond that previously experienced in these highly seasonal systems, may alter the resistance and resilience of assemblages such that the seasonal decline and recovery of biodiversity may no longer be so reliable. Given the projected increase in low-flow incidence in many regions of the world, future research needs to examine the effects of reduced flow, flow cessation and stream drying as multiple, interacting stressors on stream biota.

Understanding multiple ecological responses to anthropogenic disturbance: rivers and potential flow regime change

Human-induced alteration of the natural flow regime is a major threat to freshwater ecosystems and biodiversity. The effects of hydrological alteration on the structural and functional attributes of riverine communities are expected to be multiple and complex, and they may not be described easily by a single model. Based on existing knowledge of key hydrological and ecological attributes, we explored potential effects of a flow-regulation scenario on macroinvertebrate assemblage composition and diversity in two river systems in Australias relatively undeveloped wet-dry tropics. We used a single Bayesian belief network (BBN) to model potential changes in multiple assemblage attributes within each river type during dry and wet seasons given two flow scenarios: the current, near-natural flow condition, and flow regulation. We then used multidimensional scaling (MDS) ordination to visually summarize and compare the most probable attributes of assemblages and their environment under the different scenarios. The flow-regulation scenario provided less certainty in the ecological responses of one river type during the dry season, which reduced the ability to make predictions from the BBN outputs directly. However, visualizing the BBN results in an ordination highlighted similarities and differences between the scenarios that may have been otherwise difficult to ascertain. In particular, the MDS showed that flow regulation would reduce the seasonal differentiation in hydrology and assemblage characteristics that is expected under the current low level of development. Our approach may have wider application in understanding ecosystem responses to different river management practices and should be transferred easily to other ecosystems or biotic assemblages to provide researchers, managers, and decision makers an enhanced understanding of ecological responses to potential anthropogenic disturbance.

High potential subsidy of dry-season aquatic fauna to consumers in riparian zones of wet–dry tropical rivers

Abstract Aquatic fauna provide an important subsidy to terrestrial consumers. In the wet–dry tropics, important subsidies from rivers to riparian-zone consumers are expected in the dry season but may vary depending on riparian zone condition. We investigated the potential subsidy of aquatic fauna to consumers in riparian zones in 2 highly seasonal rivers in Australia’s wet–dry tropics. Stable carbon and nitrogen isotopes of invertebrate predators in riparian zones were closely aligned with aquatic invertebrates and emergent adult insects. Further, a considerable proportion (40–50%) of the observed vertebrate fauna in riparian zones were consumers of aquatic fauna, which included fish, crustaceans, inverte-brates, and flying adult insects with aquatic larval stages. For hydrologically disconnected waterbodies, estimates of potential insect emergence and the proportion of vertebrate species (in riparian zones) that consume these insects both increased as indicators of riparian plant regeneration and condition improved. Our findings suggest that aquatic fauna provide important subsidies to terrestrial-zone consumers (both invertebrates and vertebrates) during the dry season, and that these transfers can be moderated by riparian zone condition. The wide home and foraging ranges of some consumers also suggest that the importance of these subsidies may extend far beyond the waterbody of origin. Human activities and climate-driven alteration of flow regimes and riparian zones that reduce the availability of dry-season waterbodies or degrade their riparian zones are likely to have negative impacts on aquatic–terrestrial linkages in these systems.

Effects of extreme inflows on the water quality and phytoplankton of seven reservoirs in subtropical Australia

Abstract Differentiating the relative importance of catchments and reservoirs in driving water quality in impounded rivers is challenging because water quality reflects the integration of peak events (e.g., rainfall-driven inflows) and longer-term processes (e.g., sediment remineralization of nutrients). We examined water quality in several subtropical reservoirs following extreme summer rainfall and associated inflows to determine short- and longer-term implications in terms of reservoir vulnerability to poor water quality and blooms of cyanobacteria, as reflected by the Vulnerability Index (VI), a previously described index validated under low-flow conditions. The reservoir with the highest VI, Wivenhoe, had the highest concentrations of total suspended solids, phosphorus (total and dissolved inorganic fractions), and nitrate/nitrite following the event, whereas the reservoir with the lowest VI, Cooloolabin, had the lowest concentrations. This suggests that vulnerability to poor water quality during major inflows is driven by suspended solids and nutrients, with the VI being a robust measure irrespective of inflow conditions. By contrast, phytoplankton-related measures, including cyanobacteria proportions, in Wivenhoe were not higher than in Cooloolabin, possibly due to reduced water residence times and increased light attenuation (shallower Secchi depth) preventing bloom establishment. This finding contrasts with low-flow summers when reservoirs with the highest phytoplankton biomass align with those with the highest VI and suggests that short-term vulnerability to poor water quality associated with peak events is not driven directly by phytoplankton. While inflows increase nutrients and suspended solids, biotic responses appear delayed, highlighting the challenges of linking catchment-derived nutrient loads to phytoplankton responses in event-driven systems.

Flow intermittence and ecosystem services in rivers of the Anthropocene

Intermittent rivers and ephemeral streams (IRES) are watercourses that cease flow at some point in time and space. Arguably Earths most widespread type of flowing water, IRES are expanding where Anthropocenic climates grow drier and human demands for water escalate.However, IRES have attracted far less research than perennial rivers and are undervalued by society, jeopardizing their restoration or protection. Provision of ecosystem services by IRES is especially poorly understood, hindering their integration into management plans in most countries.We conceptualize how flow intermittence governs ecosystem service provision and transfers at local and river-basin scales during flowing, non-flowing and dry phases. Even when dry or not flowing, IRES perform multiple ecosystem services that complement those of nearby perennial rivers.Synthesis and applications. Conceptualizing how flow intermittence in rivers and streams governs ecosystem services has applied a socio-ecological perspective for validating the ecosystem services of intermittent rivers and ephemeral streams. This can be applied at all flow phases and in assessing impacts of altered flow intermittence on rivers and their ecosystem services in the Anthropocene.

Flow regimes in Intermittent Rivers and Ephemeral Streams

The defining feature of all intermittent rivers and ephemeral streams (hereafter, IRES) is that they cease flow at some time. Many IRES dry to isolated pools but flow often continues through the hyporheic sediments below the streambed. If dry conditions persist, hyporheic flows may also cease and the streambed dries completely. Consequently, the flow regimes (e.g., frequency, magnitude, duration, and timing of flow events) of IRES and the presence of water are typically more variable than in nearby equivalent-sized perennial rivers and streams. This highly variable flow regime, especially intermittence, has major implications for the physicochemistry, biota, ecological processes, and management of IRES. Flow regimes of IRES have been primarily characterized using data from gauging stations, supplemented by diverse methods such as wet-dry mapping, various forms of imagery, and modeling. Flow data are often summarized as hydrological metrics such as variance in frequency, duration, timing, and rate of onset of intermittence that have been used to classify flow regimes of many of the world’s rivers. Such classifications reveal that IRES are globally abundant and that intermittence is increasing across much of the world, largely owing to climatic drying and water abstraction.

The biota of intermittent rivers and ephemeral streams: aquatic invertebrates

Intermittent rivers and ephemeral streams (IRES) support diverse and sometimes distinctive aquatic invertebrate communities. Although flow intermittence has been linked to reduced taxa richness, the highly variable environmental conditions that characterize IRES can enhance both taxonomic and functional diversity, with different invertebrates characterizing flowing, pool, dry, and flow-resumption phases. Aquatic invertebrate communities, which include specialist taxa, have a diverse range of adaptations to promote their survival in the pools and dry bed sediments that characterize IRES channels during nonflowing phases. These tolerant organisms and recolonists from outside of IRES channels allow communities to recover once flow resumes. IRES invertebrate communities have been affected by human activity, especially where flow regimes have been altered by climate change, water resource pressures, and changing land use. Restoration initiatives, from riparian revegetation projects to conservation strategies that protect individual species, are therefore needed to safeguard aquatic invertebrate community diversity in IRES.