Australia State of the Environment Report 2001 (Theme Report)
Australian State of the Environment Committee, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06751 5
Estuaries are waterways that are typically marine or brackish, but occasionally are dominated by fresh water (includes river mouths, deltas and barrier lagoons that may be occasionally or permanently open to the sea).
Estuaries are an important part of Australia's coastal environment, and are the preferred sites for many settlements and for industry and ports. Australia has over 1000 estuaries of varying size, diversity of habitat, accessibility and uses, of which 783 are regarded as major estuaries.
Australia's estuaries have important differences from those in other parts of the world (Digby et al. 1999). Drowned river valleys are poorly represented, most being in central New South Wales and Tasmania. In the south-east and the south-west, shallow barrier estuaries predominate. Along the Timor Sea and Gulf of Carpentaria most estuaries are deltas.
Well-mixed estuaries, which predominate on the east coast, are due to a large tidal range with low river runoff. A regime of high runoff and low tidal range favours stratified estuaries like those in Tasmania. Australia's rainfall patterns - highly seasonal in the wet tropics, and erratic elsewhere - means that many rivers vary dramatically in the freshwater discharge to their estuary. In fact, discharges during flood events, compared to mean runoff, are far greater in Australia than elsewhere in the world. These characteristics contribute to the widely varying physical conditions found in Australian estuaries, as summarised in Figure 7.
Figure 7: Relative importance of estuary type to its susceptibility to impacts.
Source: Commonwealth of Australia (2001a)
Water quality in estuaries is a function of the nutrient loads and processes occurring in the estuary itself (Harris 2001). In the case of Australian estuaries, the nitrogen inputs tend to be low because the freshwater runoff from the land is relatively low. Water residence times in the estuary may be long, and water quality is often a function of tidal flushing. Australian estuaries are predominantly nitrogen-limited because of efficient denitrification of the nitrogen loads. However, there are situations where estuaries can be phosphorus-limiting and where the limitation fluctuates between nitrogen and phosphorus on a seasonal basis.
There have been several studies of estuaries over the past five years, including those of the Derwent Estuary in Tasmania (Coughanowr 1997) and the Tamar Estuary in Tasmania (Pirzl and Coughanowr 1997). The Huon Estuary Study has been a three-year research program to improve knowledge and provide a scientific framework for the management of the estuarine zone of the Huon River in Tasmania (FRDC 2001). It is Tasmania's most valuable estuary; with salmon production and processing valued at $80 million. Key environmental issues in the Huon Estuary are associated with the effects and fate of nutrient and organic matter loads from the catchment, from coastal waters, and from activities in the estuary, especially salmon farming.
The key findings include:
- the water quality of the Huon Estuary is good, and that of its two principal sources - the Huon River and the D'Entrecasteaux Channel - is very high.
- nitrogen distributions and algal production in the estuary are supported primarily by inputs from coastal seawaters, thus algal blooms can be regarded as occurring naturally. However, available nitrogen from either fish farms or agricultural activities in the lower catchments may play a role in stimulating algal blooms.
- fallowing, or resting fish farm sites, allows sediment conditions to recover, but some of the added organic matter still remains one year after the cages have been removed.
A current National Land and Water Resources Audit project (Commonwealth of Australia 2001a) is assessing the condition of all Australia's estuaries and will provide management recommendations for estuaries. The project is being undertaken because there has been very little focus on the environmental aspects of estuaries in the past. For example, there is no nationally acceptable definition of 'estuary'.
The initial (the project concludes in 2002) condition assessment of 972 estuaries is that about half of Australia's estuaries are in a modified condition. The remainder that are in good condition, are relatively small and inaccessible estuaries.
|Class||Subclass||Near Pristine||Largely Unmodified||Modified||Extensively Modified||Total|
Source: Commonwealth of Australia (2001a).
Most nutrients, contaminants and sediments flow into estuaries during flood events. In some cases these events are sufficiently large to carry these materials through the estuary into the open sea.
Changes to flow regimes of rivers can affect estuaries. A reduced river flow, for example, may reduce flushing and hence affect water quality. Such changes to flow regimes could occur through the construction of tidal barriers or dams and weirs in the catchments. These barriers not only restrict water flow but also restrict fish movements. Habitat changes could also occur as a result of changes in hydrological cycles.
Current trends in estuary management include a greater emphasis on community involvement and on the incorporation of Indigenous traditional knowledge. A recent study by Smith (2001) found that approaches to Australian estuary management are highly diverse and variable. The reasons include different ministerial and departmental distributions of powers and responsibilities, together with different natural resource legislation of States and Territories.
Macquarie Harbour, an area of 290 km2 and up to 50 metres deep, is Australia's biggest estuary. It is located on the west coast of Tasmania; the Gordon and the King Rivers flow into the estuary.
The water column is typically three-layered: fresh, marine, and intermediate, trending to a salt wedge structure near the two rivers. Estuarine conditions can extend many kilometres upstream.
The main land uses in the catchment are national parks and tourism, mining, forestry, port operations, aquaculture, and hydroelectic power generation.
Since the 1880s, mine water and tailings have been discharged from the Mount Lyell copper mine via the King River, resulting in an estimated 100 million cubic metres of solids containing toxic metals settling on the banks and bed of the King River and in a delta in Macquarie Harbour. Tailings are no longer discharged, but acidic water still drains into the King River. Measurements in 1998 showed an average of two tonnes of copper per day was contained in drainage from the Mt Lyell site.
Despite this legacy, tourism attracts around 200 000 visitors a year, and fish farming is an important industry.
After the closure of the mine and cessation of alkaline tailings addition to the river, the levels of copper and acidity have increased due to remobilisation of the metals from the sediments in the river and the estuary. Typically, in areas where 100 to 200 parts per billion (ppb) were normal during mine operation, 400-500 ppb became the norm after its closure.
The bottom sediments of Macquarie Harbour contain very high concentrations of copper, lead, iron and manganese. In some surface sediments the metals exceed the binding capacity, so these are a source of metals to the water column. This secondary contamination is gradually spreading further south.
The response to this complex problem has included the Mount Lyell Remediation Research and Demonstration project, embracing the mine site, the King River and Macquarie Harbour. A three-year program was coordinated by Environment Tasmania to assess the situation and provide information for future management. Based on these results, Riverworks Tasmania, a joint Commonwealth-Tasmania program, commissioned pilot trials of treatment procedures for acidic drainage from the Mount Lyell site. Subsequently, under the Strategic Natural Heritage Plan for Tasmania, $7.5 million was allocated for the remediation of the King River and Macquarie Harbour through the construction of a water treatment plant.