Research Proposal

Ecosystem response to flow variability in a terminal floodplain-wetland

River-floodplain ecosystems, like the Narran Lakes, are important ecotones that regulate interactions in riverine systems.  They respond to disturbance (both natural and human induced) over a range of scales - from organism-level responses, through population and community changes and finally ecosystem-level changes.  The nature of these changes depends on the organism or group of organisms or ecosystem component in question.  Additionally, there will be a lag time before an ecosystem response can be detected in floodplain water bodies and the extent of this lag time will again depend on the component in question.  For many of the more familiar organisms (large fish, riparian trees), there would be a considerable lag time, with the effects of changing water regimes possibly taking decades to be detected.  Studies of similar systems (eg., Kingsford, 2000) have tended to ignore their multi-scaled functioning and the requirement for an interdisciplinary and integrated approach to study these ecosystems. 

This proposal outlines an interdisciplinary research project which will investigate ecosystems responses to flow variability in the Narran Lakes floodplain-wetland complex.  This project has been based upon the development of a series of conceptual models of the key ecological functioning of Narran Lakes.  These have enabled the identification of key knowledge gaps and hence development of this research project.  This research project will increase our understanding of terminal floodplain-wetlands in the semi arid region of Australia and more importantly allow the prediction of the response of these types of systems to disturbances – both natural and those induced by continued water resource and floodplain development.  Measurement of the effects of these disturbances will provide a basis for assessing different management objectives through out the region.

Conceptual models for the Narran ecosystem.

Healthy riverine ecosystems provide templates for the development of ecological conceptual models. The Narran River is an example of a bifurcating river that does not rejoin the other major channels in the region. This results in the formation of a terminal wetland system. The typical geomorphic features of the Narran River and lakes are bifurcating channels, with secondary channels that persist relatively independently from the main channel. An important feature of the system is that the bankfull cross sectional area decreases downstream, and most of the habitat occurs in the secondary channels. The dominant functional unit of this type of geomorphic zone is the secondary channels and the extensive floodplain surface through which they flow. The secondary channels are no less complex than the main river channels and contain flat bench surfaces, flood runners, anabranches and other floodplain features.

A conceptual ecosystem model is proposed for the Narran River and Narran Ecosystem (Figure 1).  The main components of this model are:

• Water, nutrients, carbon and living organisms are transported from upstream via the river and floodplain and enter the lake system;
• The amount and type of transported material is transformed during interactions with the floodplain;
• Because the wetland system is predominantly ephemeral, some of the biota colonise the wetlands from the river (e.g. fish) or elsewhere (e.g. waterbirds) soon after water arrives;
• Another major component of the biota will be permanently associated with the wetlands and include animals and plants that are adapted to the natural flooding regime and survive periods of drying in the egg and seed banks of the wetlands. Also adapted to a flooding regime are several critical vegetation communities that are important habitat for colonial nesting waterbirds, such as Lignum, Coolibah and Black Box;
• The major export of carbon from the system during the terrestrial phase is through grazing on vegetation by native and introduced vertebrates and invertebrates. The importance of this trophic linkage will be determined by the effect of water regime on the vegetation; and,

A key characteristic of the wetland system is spatial and temporal variability, where biotic and abiotic features will be in a state of flux.  The wetland is also an important refugium for aquatic organisms during dry periods.  This project will have links to the CRC for Freshwater Ecology’s dryland refugium project that is currently investigating the role of waterholes as refugia in several dryland river catchments.

The flooding regime is different for each component wetland in the Narran Ecosystem, and that at any one time each will have different biotic communities. Unfortunately, at this point in time we do not have the knowledge to determine which habitats, hydrological connections or metabolic processes (e.g. primary production, organic matter decomposition, grazing, and avian and fish predation) are important and which could withstand alteration, or how these factors are affected by water resource development and land use changes. A major component of this project will be to establish indicators that can be used in a monitoring program aimed at determining response to water resource development.

Preliminary observations and assessments by Thoms et al. (2002) suggest that the important habitats in the Narran Ecosystem are:

• Deep holes in the river that provide a refuge for animals in dry times and a source of colonists for the terminal wetland system;
• Woody debris from fallen riparian vegetation is a major habitat within the river channels;
• The lake bed which provides a refuge for seeds and eggs of plants and invertebrates;
• Riparian vegetation that is partially inundated during flooding provides nesting and roosting habitat for birds and habitat complexity for aquatic organisms;
• Open forests and woodlands on the floodplain provide important habitat for reptiles and birds; and,
• Cracking soils are important for small mammals, planigales in particular, frogs and reptiles.

Our conceptual model also suggests that the important connections within the Narran Eco system will include:

• Riverine–lake connections which facilitate persistence of aquatic organisms and the delivery of carbon and nutrients to the Narran Lakes system;
• River–floodplain connections which facilitate access to breeding and feeding habitats for aquatic species and for the transfer of organic matter, nutrients and sediment;
• River–floodplain connections for the maintenance of the terrestrial vegetation complexes which support high levels of terrestrial biodiversity; and,
• Wetland/lake–riparian connections that facilitate the export of carbon and nutrients from the terminal lake through predation and grazing.

The conceptual model for the Narran ecosystem highlights three important issues.  First, the need to consider the Narran Ecosystem ie, the three lakes, floodplain and river channel, as an inter-connected system.  Second, the important role of spatial and temporal variations in the flow regime in governing the availability of habitat and influencing ecological processes.  Finally, the requirement to view the ecology of the Narran Ecosystem at several different spatial scales.  River-floodplain interactions under different flow regimes are important for maintaining the character of both the main river channel and floodplain wetlands.  Water resource development upstream of Narran (Qld DNRM, 2000) has altered the flow regime in the Narran River and therefore the connectivity between the river channel, the floodplain and the terminal lakes.  Furthermore, increased base flows (due to compensatory releases from Beardmore Dam on the Lower Balonne) will also change the wetting/drying regimes of these ecosystem components.  Both hydrological processes have the potential to change the ecological character and functioning of this wetland ecosystem over varying time scales.

Research and management scales for the Narran Ecosystem

While this integrated project recognises the importance of the Narran wetlands themselves, it is also important that these wetlands are viewed as part of a mosaic of floodplains and wetlands within the Murray-Darling Basin.  Therefore, we suggest there are three scales at which ecological questions need to be framed.

Local scale

The ecological character of the Narran system and its role in providing significant refugia for a range of organisms is largely determined by the flow regime in the Narran River.  This river is being subjected to changing hydrological regimes because of upstream developments.  However, the ecological effects of changed flow regimes can only be investigated, at least in the short term, by understanding the links between hydrology and habitat availability for key biota.  These flow-habitat-biota connections (not sure what figures are being referred to?) are being recognised as an increasingly important component in the sustainability of the ecological character of our inland river systems (Thoms and Sheldon, 2000).  However, there are limited data on the role of flow in maintaining habitat–biota connections for many of Australia’s inland river systems, like the Narran.

At the local scale this project will determine the:

• habitat requirements of key biotic components of the ecosystem (plants, invertebrates, fish, water birds);
• distribution of habitats for the key biota under different flow states; and,
• importance of bottom-up control of ecosystem processes, especially productivity examining nutrient and carbon responses to wetting and drying.

Catchment Scale

The Narran Ecosystem is an ephemeral, terminal system in the lower reaches of the Condamine Balonne River, in the northwest section of the Barwon Darling system.  The ecology of this terminal floodplain lake system will be influenced by the supply of water, sediments, nutrients and carbon from upstream and through the exchanges between the river and adjacent floodplain as well as colonisation of organisms.  These ecological components will also depend upon the stage of wetting and drying in the system.  However, changes in flows in the Narran River and across the floodplain will have consequences for the ecology of the lakes.  Impacts that may arise as a result of catchment modifications may include the reduction in the abundance of zooplankton colonising the lakes after drying events because of decreases in floodplain interactions.  Fish communities in the river and hence fish communities colonising the lakes after drying events may also change as a result of increases in base flows.  There are no data to investigate, or begin to model, the response of the Narran Ecosystem to changing water resource development or land use modifications. 

This project will, through the development of a hydrological model predict habitat availability under different flow scenarios for the Narran Ecosystem.  It will also determine the significance of patterns of wetting and drying of the lakes and how these patterns are modified by changing flow regimes in the Narran River.  Data from this project will enable the development of a model that can be used to predict the influence of water resource development and changing catchment conditions on the ecology of the Narran Ecosystem.

The regional or basin scale

Wetland systems, of varying degrees of water permanency, are a feature of inland Australia (Williams, 1998).  Hence, the Narran Ecosystem must be put into this larger context.  It has been suggested (eg., Kingsford et al., 1999; Thoms et al., 2001) that the Narran Ecosystem has a unique collection of habitats that are important for migratory birds in that they provide a key refugium when other wetlands are not providing appropriate habitat.  However, this has not been quantitatively assessed. Data collected from this study would be used to construct a landscape model of the use of wetlands in the region and therefore assess the importance of the Narran system for both local and migratory birds.  This section of the project will be undertaken in collaboration with New South Wales National Parks and Wildlife Service (NPWS).  It would involve landscape-scale GIS applications to measure habitat availability, along with data from aerial bird surveys undertaken by NPWS.  These data would measure the connectivity eg., the movement of waterbirds, among regional wetlands and hence the relative importance of the Narran Lakes ecosystem in the region.

Research aims:

Our conceptual framework is that flood pulses and dry phases are particularly important for the ecology of lowland rivers and their floodplains and wetlands.  Therefore, the fundamental aims of this project concern linking ecological (both physical and biological) responses of selected components of the Narran Ecosystem to variability in flow regime.  Specifically, we aim to:

• To determine the physical and biological responses of the Narran Ecosystem to variations in the flow regime.
• Predict the responses of selected components of the Narran Ecosystem to alterations in flow regime under different water resource development and long-term climate change scenarios.  These predictive models would emphasise the links between flow regime and biota through habitat availability and changes in ecological processes such as primary productivity.
• Evaluate the ecological significance of the Narran Ecosystem in the regional context of wetlands within the northern Murray Darling Basin.
• Develop a conceptual model that links physical and biological responses of the Narran Ecosystem to past and future changes in water resource development, land-usage and climate.

Research plan:

The specific research questions that will be answered to achieve the broad aims fall into four sets.  Initially, a series of conceptual models that will allow us to link the ecological functioning of the Narran Ecosystem to changes in water resource development; land use and climate will be developed.  Indeed, the final stage of the project will be to focus on integrating the different components of the study to produce a model that will allow us to link the ecological functioning of the Narran Ecosystem to changes in water resource development, land use and climate.  The model will be up dated though out project when results become available.  This component of the project will have links for the new CRC for Freshwater Ecology ecosystem modelling project.  The second and third sets relate to the responses of the ecosystem to variability in flow regimes – the physical drivers and the responses of and interactions between the biota.  These responses reflect both local and catchment scale influences.  The fourth set of questions concerns the regional significance of the Narran Ecosystem, particularly for waterbirds, a regional scale issue. 

Physical drivers

• What is the relationship between surface water inflows and water surface areas and water depths in the Narran Ecosystem?

A number of steps will have to be undertaken in order to answer this question and these include:
1).  Determine the surface water inflows to the Narran Ecosystem.
Given the limited historical data set for the region, this project will rely on modeled hydrological data.  The Queensland Department of Natural Resources and Mines have developed a flow model for the Lower Balonne region and this includes several sites along the Narran River.  This model will be used to determine the hydrology of the lakes (flow states) plus determine the possible influence of natural climatic changes and water resource development.
2).  Derive the bathymetry of the Narran Lakes and associated floodplain.
A series of Lidar images will be used to determine the bathymetry of the lakes and floodplain.  This will allow lake and floodplain volumes to be determined.
3).  Development of a hydrology model for the Narran Ecosystem.
Commercially available software eg. MIKE 21, will be used to develop a hydrological model for the Narran Ecosystem.  This software produces a grid cell image of Narran Ecosystem and determines the relationship between surface water inflows and the area of inundation as well as water depths in each grid cell.  This model will be calibrated using remote sensing images and aerial photographs to verify the relationship between surface area inundated to inflow volumes.  Moreover, water level recorders will also be installed in all three lakes to assist with the calibration of water volumes at different stages in the Narran Lakes.
4).  Prediction of the effects of climatic change and water resource development on the hydrology of Narran Lakes.
The calibrated model will be used in conjunction with a number of different surface water inflow scenarios to determine the effect on the area of inundation and water depths in the Narran Lakes Ecosystem.

Ecological responses

Lake scale responses

• Is there an association between the frequency of inundation, distribution of water depths and plant communities in the Narran Ecosystem?

Remotely sensed images will be used to map the spatial distribution of vegetation communities. This distribution will be confirmed by ground-truthing through field surveys.  Sampling of vegetation communities will be along a gradient from above water level (shore) to maximum (or defined level) depth.  Vegetation community characteristics such as plant species presence and percent cover will be determined along these transects. This will establish links between the remotely sensed images and vegetation community composition. The remotely sensed images will also be used to assess vegetation community response to wetting through an analysis of different spectral bands eg. NDVI (Sims and Thoms, 2002).

A major output from this component of the study will be maps that detail the distribution of vegetation communities and their responses to flooding.  This will enable the prediction of the distributional response of vegetation communities under various flow scenarios.

Habitat specific responses.

• What is the distribution of key biota in relation to habitat, both abiotic eg. water depth, and biotic eg., vegetation?

Habitat-specific sampling will be used to track the distribution of key biota across different flow states within the Narran Ecosystem (examples of these flow states are filling, full and draining).  Targeted habitats, including open water, vegetated and unvegetated littoral zones, will be sampled.  The relative availability of these habitats will depend on the flow state within the Lakes.  For example, relative amount of unvegetated to vegetated habitat will decline as water recedes and lignum forests dry out.  This component will focus on water birds and fish, but will also examine distributions of likely food sources for these groups, such as zooplankton and algae.

Experimental studies of the ecological responses to re-wetting.

• What are the chemical and biological responses of sediments in the NarranEcosystem to flooding ?

Anecdotal data suggest that wetting and drying cycles are important drivers of floodplain wetland ecosystems because of the mobilisation of carbon (C) and nutrients during inundation events. However, the impact of wetting and drying cycles on nutrient dynamics in floodplain/wetland systems is poorly understood (Baldwin and Mitchell 2000).  Nevertheless we can hypothesise that during the terrestrial (dry) phase there is a build-up of carbon and nutrients in floodplain soils.  Once this soil is inundated during a flood event, shifts in both the chemistry and microbial community structure/activity mobilise this store of both carbon and nutrients.  Similarly, drying of wetland sediments during a drought event leads to the death of sediment microbiota.  Re-wetting of the dry sediments results in a release of C and nutrients from the dead biomass (Birch 1960, Turner and Haygarth 2000).  Therefore, inundation of either floodplain soils or exposed wetland sediments leads to a pulse of nutrients and carbon into the overlying water.  The pulse of nutrients stimulates autotrophic activity while the pulse of C stimulates heterotrophic productivity.  The increased productivity potentially drives increases in biomass at higher trophic levels.

We will use mesocosm experiments, based at the Northern Laboratory in Goondiwindi, to investigate the chemical and biological responses to drying and rewetting lake sediments.

Narran Ecosystem in the Landscape

• How does the Narran Ecosystem differ from other regional wetlands and how important is it for regional waterbird connectivity?

The hydrological, physical, chemical and biological (water bird abundance and composition) character of regional wetlands will be assessed.  Using these data the Narran system can be compared to other wetlands in eastern Australia, including those in the Murray-Darling Basin, to determine its relative importance to different species of waterbirds. Similarly, estimates can be made of its relative importance for breeding of different species of waterbirds with available data for other colonies in Australia.  These data will enable us to determine whether the Narran Ecosystem has unique habitat characteristics compare to other regional wetlands.

Data collected by the NSW NPWS will be combined with those collected in this project to construct a regional model of water bird wetland use.  This model will probably be a rule based model that relies on determining the probability of habitat and resource use by water birds at a range of scales. 

Conceptual Model of the Narran Ecosystem

The response of the Narran Ecosystem to variable flow regimes and changing wetting drying cycles will be integrated in a series of conceptual models.  In order to achieve this an intensive 5 day workshop will be held at the start of year 4 where the entire project team plus members of the international scientific review panel bring together all data on ecological structure and function at the local, catchment and regional scale.  These will be integrated into a model that links surface water inflows, lake water levels to habitat variability and chemical and biological responses to inundation.  These models will allow an investigation of the influence of various disturbances (natural and human induced) on the ecological integrity of the Narran system.