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Home > Coastal and floodplains > Estuaries > Factsheets > Water Quality

Estuaries in NSW

Estuarine Water Quality

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Water Temperature

Any discharge of water or wastewater can affect the temperature of the receiving waters, eg the discharge of cold bottom waters from dams, but the greatest impact comes from the discharge of spent cooling water from thermal power stations (see Human Impacts - Human Activities/Power Generation). For instance, the waters of Lake Macquarie are raised by up to 10^oC near the cooling water outlet from Eraring Power Station (Marshman and Hodgson, 1991).

Whilst the return flow of cooling water from thermal power stations can have a number of detrimental effects on the aquatic environment, these are not as extreme as might be thought. In general, the aquatic ecosystem is relatively resistant to thermal stress, as the following general findings indicate (FAO, 1984).

• Bacteria and phytoplankton appear to be resistant to thermal stress, even at temperatures in excess of 30^oC.
• Macrophytes are more sensitive to thermal stress. Their threshold may be less than 25^oC in temperate waters, as along the New South Wales coastline, and up to 34^oC in tropical waters.
• Zooplankton (microscopic aquatic animals) are apparently quite resistant to thermal stress.
• Benthos in temperate and sub-tropical waters are also quite resistant to thermal stress, tolerating temperatures 10^oC above ambient levels and up to a maximum temperature of 35^oC. If sensitive species are lost during peak summer temperatures, recolonisation occurs from adjacent areas.
• Intertidal communities of sandy and rocky shores are resistant to thermal stress.
• Most fish in temperate waters can tolerate a quite wide temperature range, but few are resident in water with temperatures of over 30^oC.

Despite the apparent robustness of aquatic ecosystems to the direct effects of thermal stress, higher temperatures modify the toxic effects of a number of substances such as heavy metals, ammonia, etc. Whilst the toxic effects of ammonia are reduced as temperatures increase, the effects of a number of heavy metals are increased, thereby further adding to physiological stress.

Since the 1960"s, the Electricity Commission of New South Wales has undertaken monitoring programs and has sponsored estuarine research aimed at understanding the effects of cooling water abstraction and return on estuarine lake processes. The study has covered water quality, seagrasses, algae, plankton, benthos, fish and prawns. The information has been reviewed during preparation of the Lake Macquarie Audit (SPCC, 1983) and the Lake Illawarra Audit of 1986, as well as during other reviews.

The available data indicate that thermal discharges resulting from over a quarter of a century of power station operation at Lakes Illawarra and Macquarie have not affected overall estuarine processes, although some changes have been detected in the immediate cooling water discharge zones where the temperature increase is greatest.

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