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Improving understanding of estuaries and access to information

A key issue for improved estuarine management is to build knowledge and understanding at all scales from community groups to senior management in government. For effective estuarine management we need to understand:

• estuarine processes;
• linkages between catchment land use and estuary condition;
• implications of management decisions within catchments or estuaries;
• priorities for investment; and
• complexities and variation within the Australian landscape.

A series of products were commissioned by the Audit to enhance our collective understanding of the concepts and imperatives for improved estuarine management. These include:

• conceptual models that link catchment and estuarine use and estuary condition;
• conceptual models of the processes that drive estuarine ecology;
• a simple estuarine response model; and
• the OzEstuaries Database.

Combined with local understanding, these tools will assist estuarine managers to identify issues, assess management options, develop monitoring programs and establish priorities for works and investment.

Developing a understanding of the links between estuarine use and condition

The following models illustrate the anthropogenic uses of estuaries (pressures) as they link to and change the condition (state) of an estuary. They summarise the management considerations for a variety of catchment, recreational, fisheries, urban and industrial uses. The University of Queensland and the Coastal Cooperative Research Centre are refining these models in the development of an Australian Estuary Management Handbook, supported by the Fisheries Research and Development Corporation.

Figure 64: Best management practices - catchment pressures.

Figure 65: Best management practices - recreational pressures.

Figure 66: Best management practices - commercial fishing and aquaculture pressures.

Figure 67: Best management practices - urban and industrial pressures.

Simple Estuarine Response Model - a tool for decision makers (www.marine.csiro.au/serm/)

Building on the developed conceptual understanding of estuary processes and the multi-dimensional ecological interactions that then drive estuary condition, the Simple Estuarine Response Model was developed as part of the CSIRO contribution to the partnership initiative.

Model description

The Simple Estuarine Response Model consists of two parts:

• a simple circulation model which predicts physical exchanges in different estuary types, based on specification of a coarse box geometry to broadly represent estuary slope, and forcing variables (e.g. river flow and tidal amplitude); and
• a complex biogeochemical/ecological model, developed through previous coastal studies, and extended and improved for the Audit's Estuary Assessment. It is implemented on the same coarse box model geometry, and uses the exchanges predicted by the hydrodynamic model.

Modelling approach

Estuaries were divided into three hydrodynamic estuarine types, and simple coarse resolution circulation models were developed for each type.

The models are designed to investigate the response of Australian estuaries to changes in external forcings such as nutrient loads and flushing times. Models have been developed to represent each of the major estuary process types. They have been designed to allow users (scientists and managers) to examine a number of common management options (e.g. alterations to freshwater flows, nutrient loads, or oceanic exchange).

The Simple Estuarine Response Model:

• provides an indication of likely status, for estuaries where we have information on catchment modifications and loads, or on point source loads, and little or no direct measurements of water quality;
• helps to interpolate or interpret sparse data (e.g. putting scattered observations of water quality into context); and
• provides an interpretation of environmental status in terms of cause and effect.

In supporting management recommendations, the model can be used to describe the effects of a number of typical management actions for each estuary type, by performing simulations over a range of estuarine parameters and forcing scenarios. From these descriptions the user will be able to recommend the general types of management actions that are likely to be most effective for each estuarine type.

A relatively small number of estuarine parameters, that characterise the shape and physical and chemical forcing on an estuary, were identified for each. Simulations for a small but representative set of values of each parameter were run. This required some tens of thousands of model simulations.

The model was run for the equivalent of ten years for each parameter combination, by which time the model state variables had reached a repeating seasonal cycle. The initial conditions, which were the same for every simulation, represented ecology with a viable benthic community. The tenth year of the simulations represents the ecological system that the model predicts will have emerged after ten years of forcing as determined by the estuarine parameters. Model results were recorded every five days over the last year of simulation, and condensed into a standard set of indicator statistics, which can be viewed at the Simple Estuarine Response Model interface.

The Simple Estuarine Response Model interface has four main components:

1. Specification: requires the user to specify the range of estuarine parameters (which determine the estuary and the natural and human pressures on it) for which model results will be displayed. The user can choose any number of combinations of estuarine parameters, from one estuary (a single range for each parameter), to over 5000 combinations.
2. Explorer: graphs the results for the estuaries on the specification page using a coloured scatter plot. The user may choose to plot an indicator (which quantifies the response of the estuary) against another indicator or estuarine parameters. Explorer is ideal for looking at large combinations of estuarine parameters.
3. Assessment: depicts the indicators that are commonly used in assessments of estuarine conditions for the estuaries on the specification page. Assessment is ideal for looking at a single estuary while varying only one or two estuarine parameters.
4. Case studies: depicts the results of simulations for five different estuaries against data collected from those estuaries.

Case studies

Case study estuaries have been used to provide an assessment of Simple Estuarine Response Model performance in five different estuaries. The estuaries chosen include lagoons, tidal and salt-wedge estuaries from a variety of climate zones. The case study estuaries provide an example of the working of the model, and how the results can be interpreted.

The case study estuaries are:

• Brunswick River, New South Wales
• Huon River, Tasmania
• Maroochy River, Queensland
• Port Phillip Bay, Victoria
• Wilson Inlet, Western Australia

Limitations

The Simple Estuarine Response Model represents a first attempt to develop broad-brush, generic models of Australian estuaries, and the user should treat the results cautiously. In particular, it is not intended, and should not be used, to replace detailed local modelling studies or hydrodynamic modelling in developing and assessing management strategies for individual estuaries.

The models have largely been developed and calibrated in temperate and subtropical estuaries. There have been very few studies of Australia's tropical macrotidal estuaries, and model predictions for those estuaries may be less reliable. While this first attempt involves considerable extrapolation from our current knowledge base, the Audit's estuary assessment itself will provide data to assess model performance. The project will provide the first step in an ongoing cycle of model prediction, observation and refinement.

General management recommendations can be linked to the process-based classification of estuaries. It is not possible to recommend specific management actions for each individual estuary because:

• there is no detailed data for most estuaries to allow specific investigations;
• any examination of specific estuaries would have to be carried out in close collaboration with responsible State managers; and
• the time and cost to run models for a large number of specific cases would be prohibitive.

The model focuses particularly on the impacts of diffuse and point source loads of sediments and nutrients on estuaries as these represent the most significant and widespread anthropogenic stresses on Australian estuaries. The model also represents the effects of changes in freshwater run-off on circulation and flushing. The model is able to represent the effects of engineering works or dredging on tidal exchange and flushing, but only in a broad empirical sense (i.e. as changes in exchange coefficients).

The model used is an integrated physical - biogeochemical - ecological model. It represents the cycling of nutrients (nitrogen and phosphorus) through the water column and sediments, and ecological impacts on primary producers (phytoplankton, benthic micro-algae, macroalgae and seagrasses). Higher trophic levels, including fish, are not represented explicitly in models, although links between fisheries and model outputs such as primary productivity and seagrass habitat are well known.

Example of assessment of model performance - Maroochy River

General description

The Maroochy River is a tide-dominated estuary north of Brisbane.

Estuarine parameters

Estuary type: wide tidal

Depth: 2.5 m (Simple Estuarine Response Model range: shallow [1-3 m])

Fresh water replacement time: The annual mean river flow is about 1.8 m3/s. This flow would deliver a volume equal to the estuary volume in about 74 days (Simple Estuarine Response Model range: medium [days])

Tidal range: 1.5 m at mouth (Simple Estuarine Response Model range: low [1-3 m])

Climate zone: dominant summer rainfall

Point source loads: sewage treatment plants contribute about 110 tonnes nitrogen per year. This is equivalent to an areal load of about 64 mg N/m2/d. It should be noted that in reality most of the sewage load enters the estuary nearer to the mouth, whereas the model puts all loads into the head of the estuary (Simple Estuarine Response Model range: high [20-100 mg N/m2/d])

Inflow colour (CDOM): 1.0/m (Simple Estuarine Response Model range: medium (0.5-2.0/m)]

Catchment clearance: 50% cleared (Simple Estuarine Response Model range: high [20-100%])

Model performance

The model predicts almost continuous algal blooms through the year, and a very high annual mean chlorophyll-a. While algal blooms do appear to periodically be a problem, they are certainly not continuous. The Simple Estuarine Response Model may fail to capture important physical and chemical processes which limit phytoplankton growth. In particular, the model does not adequately capture the resuspension of sediment, which reduces light availability to phytoplankton, and therefore growth rates. The loss terms for phytoplankton, such as sinking and grazing may also be underestimated.

OzEstuaries database - information collation on Australia's estuaries (http://www.agso.gov.au/ozestuaries)

The estuary assessment collated information on 979 estuaries. This information makes up the OzEstuaries Database. The starting point of the OzEstuaries Database was the Australian Estuarine Database (Digby et al. 1999), containing data on 780 estuaries with a catchment size greater than 15 km² and visible on a 1:100 000 topographic map.

The States and the Northern Territory nominated smaller estuaries to add to the database. Many of the smaller estuaries are locally significant such as Dee Why, Curl Curl and Manly Lagoons on Sydney's northern beaches.

Estuarine attributes include:

• name, identification number and location;
• estuary condition;
• geomorphic classification;
• geometry;
• sedimentary environment; and
• oceanographic information such as tidal range and wave data.

Linking natural resources - the Australian Natural Resource Atlas (www.environment.gov.au/atlas)

The interactive web-based Australian Natural Resources Atlas (Atlas) presents Audit products at scales from local to regional to Australia-wide. The OzEstuaries Database and Simple Estuarine Response Model are part of the Atlas.

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