With the exception of Tasmania, Australia as a whole is deficient in surface waters. There are only a few sizable inland lakes in New South Wales and most of these, certainly the ones around the major population centres along the coast, are water supply reservoirs which are protected and not accessible to the public. There are however, numerous brackish “lakes” along the New South Wales coast many of which are important holiday centres for the population of Sydney, Newcastle and Wollongong. These so called “lakes” are in fact coas tal lagoons. A coastal lagoon is defined by Barnes (1980) as a body of brackish or salt water separated from the adjacent sea by a low lying sand or shingle barrier and these coastal “lakes” certainly fit well into this definition. The literature on the Australian coastal lagoons is sparse and partly inaccessible. The Lake Macquarie study by the CSIRO was by far the most comprehensive and resulted in the publication in 1959 of a series of 12 papers on various aspects of the lagoon’s ecology (Baas Becking, Thomson and Wood, 1959; Spencer, 1959; Baas Becking, 1959; Wood, 1959a, b; Davies, 1959; MacIntyre, 1959; Thomson, 1959a,b,c,d,e).
Typically lagoons are shallow with sandy bottoms and are usually fed by one or more short streams. Although the normal freshwater input is not great, floods do occur at irregular intervals due to the relatively large catchment areas surrounding the lagoons. The greatly increased hydraulic gradient generated by the extra quantity of water during floods frequently is sufficient to sweep away the sand barriers, allowing free interchange with the sea. Otherwise the sand barriers, which are constantly being reformed, are quite effective barriers and a lagoon may be isolated from the sea for many years at a time. Under normal conditions, there is very little exchange between the sea and the lagoon so that salinity and water level of the lagoon are functions of rainfall and evaporative loss of water. The extent of salinity fluctuation can be very large. In Tuggerah lakes, during the period 1979-1980 the measured salinity ranged from 20ppt to 42ppt (hypersaline). But salinity as low as 5ppt had been recorded at the end of previous floods (Higginson, 1865). The effect of rainfall on salinity changes can be more dramatic in a smaller lagoon. In Narrabeen Lagoon for example, each major fall of rain can cause steep vertical and longitudinal salinity gradients to be developed due to the rapid input of freshwater from the feeder creeks.