• Change text size
• Skip to content

• About us
• Contact us

Search

• Home
  • Atmosphere
  • Biodiversity
  • Coasts and marine
  • Heritage
  • Human settlements
  • Land
  • Living sustainably
  • Parks Australia
  • Water
  • EPBC Act

• SoE
  • About SoE reporting
  • SoE 2011
    • SoE 2011 report
    • SoE 2011 in brief
    • Supplementary material
  • SoE 2006
    • SoE 2006 report
    • SoE 2006 at a glance
    • Theme commentaries
    • Integrative commentaties
    • Current or emerging issues
    • Technical reports
    • Data Reporting System

• SoE 2001
  • Report
  • Key findings
  • Fact sheets
  • Theme reports
  • Technical papers

• • SoE 1996
    • SoE 1996 report
    • Key findings brochure
    • Technical papers
  • Themes
    • Drivers
    • Atmosphere
    • Inland water
    • Land
    • Marine environment
    • Antarctic environment
    • Biodiversity
    • Heritage
    • Built environment
    • Coasts
  • Publications

Habitats and species (continued)

Estuaries

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.

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.

Table 2: Australian estuaries within each process type and condition class.

Class	Subclass	Near Pristine	Largely Unmodified	Modified	Extensively Modified	Total
Wave	Wave-Dominated Estuary Strandplain Other	28 36 40	41 13 30	62 10 22	25 1 17	156 60 109
Tide	Tide-Dominated Estuary Tidal Flat/Creek Other	57 210 40	25 43 17	9 16 23	4 15 9	95 284 89
River	Wave-Dominated Delta Tide-Dominated Delta	28 36	24 16	30 11	12 9	94 72
Other	Other	9	1	3	0	13
	Total	484	210	186	92	972

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.

• Next
• Contents
• Previous