Detecting impacts and setting restoration targets in arid-zone rivers: aquatic micro-invertebrate responses to reduced floodplain inundation
1. Water extraction from arid-zone rivers increases the time between floods across their floodplain wetlands. Less frequent flooding in Australian arid-zone rivers has impaired waterbird and fish breeding, killed riparian vegetation and diminished invertebrate and macrophyte communities. Restoration currently focuses on reinstating floods to rejuvenate floodplain wetlands, yet indicators to measure the success of this are poorly developed. 2. We explored the application of criteria for ecologically successful river restoration to potential restoration of floodplain wetlands on the Darling River, arid-zone Australia. Using emergence of micro-invertebrates from resting eggs as an indicator, we compared responses of taxa richness, densities and community composition in floodplain lakes with different inundation histories. 3. Increased drying of floodplain lakes reduced the number of micro-invertebrate taxa. Several key taxa were absent and faunal densities (particularly cladocerans) were reduced when the duration of drying increased from 6 to 20 years. 4. A conceptual model of the ecological mechanisms by which restoration of flooding regime could achieve the target of preserving micro-invertebrate community resilience predicts that reducing the dry period between floods will minimize losses of viable resting eggs. Protection of this 'egg bank' permits a boom in micro-invertebrates after flooding, promoting successful recruitment by native fish and waterbirds. 5. Synthesis and applications. In arid-zone rivers, micro-invertebrate densities and community composition are useful indicators of the impact of reduced flooding as a result of water extraction. Critical to successful native fish recruitment as their first feed and as prey for waterbirds, micro-invertebrates are a potential early indicator of responses by higher trophic levels. Taxon richness, density and key taxa present after flooding, all indicators of resilience, can be incorporated into targets for arid-zone river restoration. For example, one restoration target may be microcrustacean densities between 100 and 1000 L⁻¹ within 2–3 weeks after spring flooding. These criteria can be applied to measure the ecological success of restoration projects seeking to recover natural flood regimes. Given the high economic cost of water in arid zones, convincing demonstrations of the ecological success of environmental water allocations are crucial.
Developing Indicators for Floodplain Wetlands: Managing Water in Agricultural Landscapes
Floodplain wetlands in arid and tropical environments are intermittently or seasonally flooded, drying for months and up to years in arid wetlands between floods. Aquatic food webs in these systems are adapted to this variable water regime; pulsing in productivity and diversity after floods, yet dependent on nutrients generated during dry cycles. In floodplain wetlands, upstream dams and irrigated agriculture have reduced floods and extended drying, leading to changes to natural levels of productivity and diversity. Environmental flows can contribute to the sustainable management and restoration of degraded wetlands in agricultural landscapes. Decisions on the timing, frequency, duration and magnitude of environmental water allocations depend on sound knowledge of ecological thresholds, and indicators to measure success or failure of management interventions. Thresholds are discussed for concentrations of carbon, ecosystem metabolism and microinvertebrates in arid wetlands as indicators of key ecosystem functions. This approach is also relevant to floodplain wetlands in tropical climates where water managers must balance environmental and agricultural needs in the midst of the uncertainty of climate change.
A common parched future?: Research and management of Australian arid-zone floodplain wetlands
Wetlands in arid and semi-arid areas face intensifying pressure for their water resources yet harbor unique biota and ecological processes that rely on the "boom and bust" regime of alternating flood and drought. Recent research in Australia has revealed that models of ecosystem processes derived from northern temperate zone wetlands are often inapplicable to arid zone wetlands, confounding efforts to manage or protect these threatened habitats. We review four case studies from inland Australia that demonstrate different degrees of successful management, aiming to draw out lessons learned that will improve our sustainable use of these delicate systems. Inappropriate extrapolation across scales that ignores the inherent spatial and temporal variability of arid-zone wetlands, "reactive" rather than "collaborative" research and management, and a reluctance to adopt functional indicators to complement state variable are several common themes. We are optimistic that managers and researchers are collaborating to tackle these issues but warn that a parched future faces some wetlands where jurisdictional boundaries hamper their effective management or entrenched beliefs and community distrust of managers threaten ecologically sustainable resource use. In arid areas where water is so precious, environmental allocations are costly and their long-term effects are difficult to identify against a backdrop of high inherent variability. Preservation of this variability is the key to successful management of these "boom and bust" systems but diametrically opposes the desire for regulated, reliable water supplies for human use. Social and institutional acceptance and change now appear to be greater barriers than limited ecological understanding to effective management of many "parched wetlands" in Australia.
Microinvertebrate community response to changing water regimes in the Macquarie Marshes, NSW, Australia
Dryland rivers in arid regions of the world support dependant ecosystems under highly variable conditions. The biota of dryland rivers and floodplains has adapted, over time, to the natural cycles of flooding and drying that characterize these environments. Aquatic microinvertebrates form a critical link in the transfer of energy from primary producers to fish and waterbirds that thrive when the rivers rise and floodplains are inundated. Importantly, aquatic microinvertebrates contribute to resiliency of dryland river biota through their ability to form drought-tolerant resting stages during long dry periods. I examined the response of aquatic microinvertebrate communities to changes in environmental conditions, including water regimes, floodplain processes, and habitat availability in the Macquarie Marshes in the arid-zone of western New South Wales, Australia. The Macquarie Marshes are temporary wetlands which have, historically, supported rich and diverse ecological communities such as huge bird colonies of international significance. While these communities have waxed and waned over time with natural flood and drought cycles, anthropogenic changes, in particular modification to the water regime through river regulation and extraction are now altering the normal ecological cycles in the Marshes. ...This study showed that when flooded, the Macquarie Marshes support huge populations of microinvertebrates and correspondingly high numbers of dormant resting stages in dry sediments once the waters have receded. Changes to water regimes that reduce the frequency of flooding and change temporary creeks into semi-permanent water bodies will have a detrimental effect on microinvertebrate communities.
The potential role of waterbirds in dispersing invertebrates and plants in arid Australia
1. The role of waterbirds as vectors of plants and invertebrates within and between arid-zone wetlands is poorly understood. We present the first detailed study of passive dispersal by nomadic birds in Australasia. We investigated the numbers and types of invertebrate and plant propagules within freshly collected faecal samples as well as their viability. We compared dispersal among Grey Teal ('Anas gracilis'), Eurasian Coot ('Fulica atra') and Black Swan ('Cygnus atratus') in the Macquarie Marshes, a complex of temporary to semi-permanent wetlands in New South Wales. 2. When faecal samples (n = 60) were inundated in the laboratory and monitored over 3 weeks, ciliates (75% of samples), nematodes (22%), ostracods (13%) and rotifers (5%) were recorded, with higher taxon richness in coot samples. Faecal samples (n = 71) were also sieved to quantify intact propagules, and ostracod eggs (70% of samples), large branchiopod eggs (31%) and bryozoan statoblasts (31%) were the most abundant invertebrates. Diaspores of 19 plant taxa were recorded, 14 of which were germinated in the laboratory or shown to be viable at the end of germination trials. The abundance and diversity of invertebrate propagules was highest in coot samples, whereas the abundance and diversity of diaspores was highest in teal samples. 3. One Australian Pelican ('Pelecanus conspicillatus') sample was obtained and found to contain more taxa and far more propagules than any sample from other waterbirds, suggesting that piscivorous birds might have an important role in the indirect dispersal of propagules ingested by fish. 4. Our results support a role for birds in explaining the distributions of cosmopolitan plant genera such as 'Lemna', 'Typha', 'Myriophyllum' and 'Nitella'. The alien plants 'Ranunculus sceleratus', 'Medicago polymorpha' and 'Polygonum arenastrum' were recorded, demonstrating the potential role of waterfowl in the spread of exotic species. As the frequency and duration of flooding of arid-zone wetlands decreases owing to human activities, the importance of waterbirds in facilitating recolonisation of temporary wetlands is likely to increase.
Connectivity in a dryland river: short-term aquatic microinvertebrate recruitment following floodplain inundation
Periodic hydrological connectivity among fragmented floodplain habitats governs the persistence of aquatic biota. In dryland rivers, unpredictable flooding interspersed with low flows and drying drive their "boom and bust" ecology. During drying, aquatic habitats contract and fragment. Flooding connects and expands habitats, triggering productivity booms in waterbirds, fish, and plants. Microinvertebrates form the base of the food web and also flourish after floods, but their colonization pathways remain unknown.To determine the relative contribution of floodwater-transported propagules vs. emergence of in situ resting stages from inundated sediments, we studied short-term recruitment of microinvertebrates after flooding in the dryland Darling River, Australia. Lakes open to transported and emerging in situ microinvertebrates were compared with lake and laboratory microcosms closed to transported microinvertebrates. Floods imported most early colonists to floodplain lakes. In situ emergence from sediments was less important until weeks later but potentially sustained production over a longer period of inundation. Some taxa primarily colonized lakes via emergence whereas others were only transported to lakes. A spatially nested ANOVA approach spanning patches within lakes to among reaches revealed contrasting variability across hierarchical scales for taxonomic richness and densities of microinvertebrate taxa. This was most marked at the reach scale, possibly reflecting seasonal patterns in flooding. Microinvertebrate colonization sequences in the Darling River floodplain rely on connectivity between habitats during flooding. Ecological connectivity was greater at small spatial scales (patches within lakes 100–1000 m, among lakes 104–105 m) than at larger spatial scales (reaches 105–106 m). The strength of connectivity is likely to vary across spatial scales with the magnitude, duration, and rate of rise and fall of floods.
Natural disturbance and aquatic invertebrates in desert rivers
Flowing waters in deserts vary from ephemeral rills that carry water only after irregular and episodic downpours to the lowland stretches of perennial rivers whose headwaters are fed by groundwater interflow, snowmelt, or monsoonal rains. Many deserts have uncoordinated (arheic) drainage patterns. Here, flow may depend as much on where in the desert rain fell as on the weak gradients of the poorly defined channels. In other desert areas, meandering endorheic channels end in internal basins that can contain water for long periods of time. For example, the Lake Eyre Basin is a large endorheic drainage system in Australia that fills irregularly in response to erratic incursions of moist tropical air from the north. Major floods can occur, associated with La Nina phases of the El Nino Southern Oscillation (Puckridge et al., 2000), triggering 'booms' in productivity of waterbirds, fish, and invertebrates (Kingsford et al., 1999; Timms, 1999; Chapters 2, 4, and 7, this volume).