A new approach to determining environmental flow requirements : sustaining the natural values of floodplains of the southern Murray-Darling Basin

Large overbank flood events play an important role in maintaining largescale ecological processes and connectivity along and across the floodplains and between the rivers and their floodplains in the southern Murray-Darling Basin. However, the regulation of rivers means that extensive overbank flooding can only occur in the rare circumstance of extreme flood events. Recent environmental water allocations have focussed on the largest floodplain blocks (‘icon’ sites) and a small set of specific values (e.g. colonial nesting waterbirds), as well as on trialling fine-scale manipulation of infrastructure (e.g. pumping) to water relatively small areas. There has been no comprehensive systematic assessment of the entire floodplain and its wider set of flood-dependent natural assets (such as ecosystems and species; herein referred to as ‘natural values’) to maximise the effectiveness of environmental water use and to catalogue values likely to be lost. This paper describes an assessment of some 220 000 ha found to support flood-dependent natural values in Victoria. We mapped the geographic distribution and estimated components of the flooding requirements (natural flooding frequency, and maximum period without flooding and minimum duration of each flooding event before significant deterioration) for each natural value. Using an example of one stretch of the River Murray, we show how the resultant spatial data can be used with floodplain inundation modelling to compare the outcomes of real or planned environmental watering events; potentially providing tools for management agencies to conserve a wider range of floodplain values than is currently the case. That is, water managers and the public can see what ecosystems and threatened species are intended to be maintained by environmental watering and what values are intended to be abandoned across the whole floodplain, rather than just seeing the small subset of values and ‘icon’ sites that are intended to be maintained. Examples are provided to illustrate how information about the location, water requirements and extent covered by potential floods for specific values can be used to build adaptive watering strategies for areas as large as the whole floodplain.

Floodplain wetlands of the Gellibrand estuary: What type of invertebrate community

Introduction. Along the south coast of Australia, wetlands on the floodplains of lowland rivers and estuaries have been severely altered by agriculture and urbanization. Efforts to restore or rehabilitate these wetlands are hampered by insufficient knowledge of the original condition of these wetlands, or their variability in time and space. This research describes the macroinvertebrate community of wetlands on the floodplain of the Gellibrand River and estuary, which has suffered comparatively few human impacts. The aim of the research was to describe the variability of macroinvertebrate communities as a baseline for the future management of these wetlands, and to contribute to the general understanding of estuary-floodplain wetlands, thereby improving the basis for their management.

The Gellibrand River has a catchment area of approximately 1200 km2 draining the western slopes of the Otway Ranges, and entering the Southern Ocean at Princetown. From a mean annual flow of 315 000 mL, 25 000 mL are removed per annum for agricultural and domestic use (O'May & Wallace 2001), and flows are closer to natural regimes than most other Western Victorian rivers. The estuary is a bar-built, salt-wedge estuary that becomes completely blocked by the sand bar in most years, during summer and autumn. Over past decades, the estuary mouth has been opened artificially in most years. to prevent flooding of agricultural land and roads adjacent to the wetlands. At its maximum, the salt-wedge penetrates approximately 10 km upstream from the river mouth, but the estuary may also be completely fresh during high winter discharge
(Mckay 2000).

The wetlands surrounding Princetown cover 119 ha and are listed as nationally important (Environment Australia 2001). This listing regards the wetlands as an important habitat for animals at vulnerable stages of their life cycle and a refuge from adverse conditions, such as drought. They are a good example of coastal brackish and freshwater marshes, with an important ecological and hydrological role as part of a large wetland
complex.

Differential macrohabitat use by birds on an unregulated lowland floodplain of south-eastern Australia

In many lowland floodplains around the world, upriver interferences to flows (weirs, dams, off-takes) have led to much reduced frequency and duration of flooding. As a result, many floodplain wetlands are now inundated relatively rarely if at all. Given regulation of most lowland rivers in southeastern Australia, we assessed use of wetlands by birds in the essentially unregulated Ovens River in northeastern Victoria. Twelve sites (0.4-1.2 ha) were studied after flooding. Four sites were 'permanent billabongs', four were temporary wetlands and the other four were randomly selected woodland sites >60 m from the nearest water body (including the river) acting as 'control' or 'reference' sites. Aquatic birds were not recorded using woodland sites, but many species were differentially associated with either billabongs or temporary wetlands. A surprising number of non-aquatic birds either exclusively or differentially were associated with wetland sites compared with woodland sites. We concluded that heterogeneous macrohabitat will increase local avian biodiversity on lowland floodplains. Moreover, densities and diversity of non-aquatic, woodland species also increased with the presence of wetlands. Temporary wetlands were used differently from permanent billabongs by birds, especially in foraging methods. This suggests that the reinstatement of major flooding on heavily regulated floodplains would be ecologically advantageous for birds by providing foraging and breeding opportunities.

Strategic review of enhancements and culture-based fisheries

Enhancements are interventions in the life cycle of common-pool aquatic resources. Enhancement technologies include culture-based fisheries, habitat modifications, fertilization, feeding and elimination of predators/competitors. Enhancements are estimated to yield about two million mt per year, mostly from culture-based fisheries in fresh waters where they account for some 20 percent of capture, or 10 percent of combined capture and culture production. Marine enhancements are still at an experimental stage, but some have reached commercial production. Enhancements use limited external feed and energy inputs, and can provide very high returns for labour and capital input. Moreover, enhancement initiatives can facilitate institutional change and a more active management of aquatic resources, leading to increased productivity, conservation and wider social benefits. Enhancements may help to maintain population abundance, community structure and ecosystem functioning in the face of heavy exploitation and/or environmental degradation. Negative environmental impacts may arise from ecological and genetic interactions between enhanced and wild stocks. Many enhancements have not realised their full potential because of a failure to address specific institutional, technological, management and research requirements emanating from two key characteristics. Firstly, enhancement involves investment in common-pool resources and can only be sustained under institutional arrangements that allow regulation of use and a flow of benefits to those who bear the costs of enhancement. Secondly, interventions are limited to certain aspects of the life cycle of stocks, and outcomes are strongly dependent on natural conditions beyond management control. Hence, management must be adapted to local conditions to be effective, and certain conditions may preclude successful enhancement altogether. Governments have a major role to play in facilitating enhancement initiatives through the establishment of conducive institutional arrangements, appropriate research support, and the management of environmental and other impacts on and from enhancements.

Photodegradation, interaction with iron oxides and bioavailability of dissolved organic matter from forested floodplain sources

Photochemical degradation of dissolved organic matter (DOM) can influence food webs by altering the availability of carbon to microbial communities, and may be particularly important following periods of high DOM input (e.g. flooding of forested floodplains). Iron oxides can facilitate these reactions, but their influence on subsequent organic products is poorly understood. Degradation experiments with billabong (= oxbow lake) water and river red gum (Eucalyptus camaldulensis) leaf leachate were conducted to assess the importance of these reactions in floodplain systems. Photochemical degradation of DOM in sunlight-irradiated quartz tubes (with and without amorphous iron oxide) was studied using gas chromatography and UV-visible spectroscopy. Photochemical reactions generated gaseous products and small organic acids. Bioavailability of billabong DOM increased following irradiation, whereas that of leaf leachate was not significantly altered. Fluorescence excitation-emission spectra suggested that the humic component of billabong organic matter was particularly susceptible to degradation, and the source of DOM influenced the changes observed. The addition of amorphous iron oxide increased rates of photochemical degradation of leachate and billabong DOM. The importance of photochemical reactions to aquatic systems will depend on the source of the DOM and its starting bioavailability, whereas inputs of freshly formed iron oxides will accelerate the processes.

Association of reduced riparian vegetation cover in agricultural landscapes with coarse detritus dynamics in lowland streams

Studies were conducted on streams flowing through agricultural floodplains in south-eastern Australia to quantify whether reductions in riparian canopy cover were associated with alterations to the input and benthic standing stocks of coarse allochthonous detritus. Comparisons were made among three farmland reaches and three reaches within reserves with intact cover of remnant overstorey trees. Detritus inputs to these reaches were measured monthly over 2 years using litter traps. Direct inputs to streams within the reserves were relatively high (550–617 g ash free dry weight (AFDW) m^–2 year^–1), but were lower at farmland reaches with the lowest canopy covers (83–117 gAFDW m^–2 year^–1). Only a minor fraction of the total allochthonous input (<10%) entered any of the study reaches laterally. The mean amounts of benthic detritus were lowest in the most open farmland reaches. Standing stocks of benthic detritus were found to be highly patchy across a large number of agricultural streams, but were consistently very low where the streamside canopy cover was below ~35%. Canopy cover should be restored along cleared agricultural streams because allochthonous detritus is a major source of food and habitat for aquatic ecosystems. Given the absence of pristine lowland streams in south-eastern Australia, those reaches with the most intact remnant overstorey canopies should be used to guide restoration.

Regional faunal decline – reptile occurrence in fragmented rural landscapes of south-eastern Australia

Many species of reptiles are sedentary and depend on ground-layer habitats, suggesting that they may be particularly vulnerable to landscape changes that result in isolation or degradation of native vegetation. We investigated patterns of reptile distribution and abundance in remnant woodland across the Victorian Riverina, south-eastern Australia, a bioregion highly modified (>90%) by clearing for agriculture. Reptiles were intensively surveyed by pitfall trapping and censuses at 60 sites, stratified to sample small (<30 ha) and large (>30 ha) remnants, and linear strips of roadside and streamside vegetation, across the regional environmental gradient. The recorded assemblage of 21 species was characterised by low abundance and patchy distribution of species. Reptiles were not recorded by either survey technique at 22% of sites and at a further 10% only a single individual was detected. More than half (53%) of all records were of two widespread, generalist skink species. Multivariate models showed that the distribution of reptiles is influenced by factors operating at several levels. The environmental gradient exerts a strong influence, with increasing species richness and numbers of individuals from east (moister, higher elevation) to west (drier, lower elevation). Differences existed between types of remnants, with roadside vegetation standing out as important; this probably reflects greater structural heterogeneity of ground and shrub strata than in remnants subject to grazing by stock. Although comparative historical data are lacking, we argue that there has been a region-wide decline in the status of reptiles in the Victorian Riverina involving: (1) overall population decline commensurate with loss of >90% of native vegetation; (2) disproportionate decline of grassy dry woodlands and their fauna (cf. floodplains); and (3) changes to populations and assemblages in surviving remnants due to effects of land-use on reptile habitats. Many species now occur as disjunct populations, vulnerable to changing land-use. The status of reptiles in rural Australia warrants greater attention than has been given to date. Effective conservation of this component of the biota requires better understanding of the population dynamics, habitat use and dispersal capacity of species; and a commitment to landscape restoration coupled with effective ecological monitoring.

Taking environmental water requirements and climate change into account in the designation and management of protected areas for floodplain ecosystems

In 2005, the Victorian government asked the Victorian Environmental Assessment Council (VEAC) to 1) identify and evaluate the extent, condition, values, management, resources and uses of riverine red gum forests and associated fauna, wetlands, floodplain ecosystems and vegetation communities in northern Victoria; and 2) make recommendations relating to the conservation, protection and ecological sustainable use of public land. The design of a comprehensive, adequate and representative (CAR) reserve system was a key part of the recommendations made by VEAC. In order to assist in the decision-making for environmental water allocation for protected areas and other public land, a process for identifying flood-dependent natural values on the Victorian floodplains of the River Murray and its tributaries was developed.

Although some areas such as the Barmah forest are very well known, there have been few comprehensive inventories of important natural values along the Murray floodplains. For this project, VEAC sought out and compiled data on flood requirements (natural flood frequency, critical interval between floods, minimum duration of floods) for all flood-dependent ecological vegetation classes (EVCs) and threatened species along the Goulburn, Ovens, King and Murray Rivers in Victoria. The project did not include the Kerang Lakes and floodplains of the Avoca, Loddon and Campaspe Rivers. 186 threatened species and 110 EVCs (covering 224,247 ha) were identified as flood-dependent and therefore at risk from insufficient flooding.

Past environmental water allocations have targeted a variety of different natural assets (e.g. stressed red gum trees, colonial nesting waterbirds, various fish species), but consideration of the water requirements of the full suite of floodplain ecosystems and significant species has been limited. By considering the water requirements of the full range of natural assets, the effectiveness of water delivery for biodiversity can be maximised. This approach highlights the species and ecosystems most in need of water and builds on the icon sites approach to view the Murray floodplains as an interconnected system. This project also identified for the first time the flood-frequency and duration requirements for the full suite of floodplain ecosystems and significant species.

This project is the most comprehensive identification of water requirements for natural values on the floodplain to date, and is able to be used immediately to guide prioritisation of environmental watering. As more information on floodplain EVCs and species becomes available, the water requirements and distribution of values can be refined by ecologists and land and water managers. That is, the project is intended as the start of an adaptive process allowing for the incorporation of monitoring and feedback over time. The project makes it possible to transparently and easily communicate the extent to which manipulated or natural flows benefit various natural values. Quantitative and visual outputs such as maps will enable environmental managers and the public to easily see which values do and do not receive water (see http://www.veac.vic.gov.au/riverredgumfinal.htm for further details).

Responses to flooding and drying in seedlings of a common Australian desert floodplain shrub : Muehlenbeckia florulenta Meisn. (tangled lignum)

We investigated the effects of flooding and drying over 6 months on growth and biomass allocation in seedlings of Muehlenbeckia florulenta Meisn. (tangled lignum), a common and widely distributed shrub of Australia's desert floodplains. We sought to determine if lignum seedlings respond to flooding or drying by altering traits or allocation patterns or instead display fixed patterns of development. Since desert floodplains are highly unpredictable and heterogeneous environments, we hypothesised that adaptive phenotypic plasticity is unlikely to have developed or be advantageous in seedlings of this species as environmental state changes are highly variable in their timing and duration and plants risk being caught out of kilter with environmental conditions. To test this, we conducted a glasshouse experiment in which lignum seedlings, grown in both clay and sandy sediments, were subjected to a range of hydrological conditions over a period of 6 months. Lignum seedlings exhibited considerable tolerance of both flooding and drying in our experiment and no mortality was observed. Growth was significantly reduced by flooding, however, and seedlings displayed extremely delayed development rather than plasticity in overall biomass allocation or any of the specific morphological variables we measured. Lignum seedlings were considerably more tolerant of drying than flooding and responded plastically by reducing leaf area ratios through reductions in specific leaf areas and leaf production and expansion. Sediment type had little effect on seedling development. Our results indicate that surface water hydrology is likely to be a major determinant of recruitment patterns in this ecologically significant species.

Flooding requirements for biodiversity values along the Victorian floodplain of the Murray Valley

Overbank flooding of rivers is a key process in the maintenance of vegetation types and the species that rely on the floodplain forests and woodlands of northern Victoria. Yet the flooding requirements of species and vegetation types are poorly known. Here we present initial estimates of the water requirements for flood dependent Ecological Vegetation Classes (EVCs) and rare and threatened flora and fauna species associated with the floodplain of the Murray River and its tributaries. Some 110 EVCs were found to be at least partly flood-dependent on the Murray River floodplains. The total current extent of these EVCs in the study area is 224 247 ha, of which 162 266 ha are on public land. One hundred and twenty-four rare or threatened plant taxa and 62 threatened vertebrate fauna taxa (excluding fish) were classified as at least partly flood-dependent. These initial estimates provide important information for land and water managers and researchers alike.

Recruitment of a keystone tree species must concurrently manage flooding and browsing

Multiple pressures (land-use change, water extraction and climate change) interact to influence biodiversity and ecosystem processes, but direct evidence for interactions among multiple pressures is limited. Floodplain forests are an acute example of how interacting pressures (river regulation, water extraction, decreasing rainfall and mammal browsing) interact to degrade native ecosystems. We conducted a 2-year field experiment to determine how flooding, browsing and sediment salinity interacted to determine in situ seedling survival and growth of the keystone floodplain tree species (Eucalyptus camaldulensis Dehnh.). On semi-arid floodplains of southern Australia, 1-year-old seedlings were planted on the banks of six ephemeral creeks, three of which were flooded with management flows before planting while the others remained dry. Four plots were established at each creek, two open to browsing and two fenced to exclude mammal herbivores. Flooding had a strong positive effect on seedling survival and height, but browsing had strong negative effects. Sediment salinity (a covariate rather than a designed effect) had a weak negative effect on seedling survival and height. The positive effects of flooding were largely offset by the negative interaction with browsing and, to a lesser extent, sediment salinity. Although flooding has been restored to some degraded floodplain forests subjected to river regulation and a drying climate, the long-term success of such actions is likely to be undermined by persistent browsing. Synthesis and applications. Management actions that focus on single pressures (e.g. infrequent flooding) and processes (e.g. mature tree survival) while ignoring other pressures are unlikely to sustain populations of keystone species, suggesting that complementary strategies (managed flooding with herbivore control) are necessary to sustain recruitment and, therefore, ensure the future health of these essential ecosystems.