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Estuaries and wetlands

Estuaries

Estuaries are inlets or bays that have both saltwater and freshwater inflow (and may include river mouths and barrier lagoons), and thus through tide and river mixing show a variety of hydrodynamic conditions. This variation may be extreme – from turbulent freshwater floods to the raised salinity of drought – and so the species that live there must be able to withstand or avoid these extremes.

A national survey (Bucher & Saenger 1989) identified 783 Australian estuaries, nearly half of them in the wet tropics of the northern coast of Western Australia, the Northern Territory, and north-eastern Queensland (table 2.5).

Estuaries are a valuable coastal resource: they provide sheltered environments, well supplied with nutrients and rich in habitat niches. Estuaries are biologically productive areas, particularly their mangroves and seagrass beds, and provide spawning, nursery, and feeding grounds for many species of fish and bird. They are important points of human settlement as harbours for ocean and river shipping, and frequently their hinterland is suitable for crops and grazing. Estuaries are important fishing grounds, especially for recreational fishing, as well as for a range of other recreation including boating and diving. Unfortunately, estuaries and the rivers flowing into them have also been seen as useful for the disposal of wastes – industrial, treated sewage, and urban and farm run-off – but because of their restricted flushing, estuaries frequently trap

Table 2.5 Distribution and areas (square km) of estuarine habitat types

State or territory

Open

water*

Intertidal

flats

Mangroves

Seagrass

Saltmarsh

Total

New South Wales

1323

na

107

153

57

1487

Victoria

2682

444

41

364

125

3292

Queensland

4093

1574

3424

68

5322

14413

Western Australia

17825

2891

1561

11

2965

25241

South Australia

760

219

111

na

84

1173

Tasmania

1825

274

0

na

37

2136

Northern Territory

5187

821

2952

23

5005

61707

* includes subtidal seagrass beds

† includes intertidal seagrass beds

Source: State of the Marine Environment Report 1995

Figure 2.32 Two estuarine types from New South Wales

Two estuarine types from New South Wales

Source: modified from Chapman et al. 1982

and accumulate pollutants. The accumulated industrial pollution of the Derwent estuary, for instance, was of great concern to the citizens of Hobart in the 1970s. In New South Wales the Healthy Rivers Commission (2000, 2001) has recently classified 'coastal lakes' (a designation overlapping in part with estuaries), noting that, for those lakes it classifies as highly modified, the opportunity for restoration has passed, although improvement can be achieved.

 
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