Project Background

Physical Setting

The survey area extends from the Victorian/South Australian border to Batemans Bay in southern New South Wales. There is a total of 3,900 Km² of estuarine habitat in this region. The Great Divide Mountain Range closely parallels the coast in this part of Australia. Consequently, there are no large catchment areas for estuaries in the sample set. The maximum catchment area is 20,600 Km² and the average 1824 Km².

The region is subjected to seasonal rainfall with most rain in the winter. Consequently, river discharge in summer and autumn is extremely low. As many of the estuaries discharge through surf beaches during periods of low flow their mouths are frequently closed by sandbars.

In the study area we recognise four types of estuaries.

1. Tidal, estuaries with free tidal exchange (drowned River Valleys or barrier estuaries where the mouths are artificially kept open. These may or may not have a lagoon near the mouth.
2. Barred lagoon, barrier estuaries with a wide comparatively shallow body of water banked up behind the mouth. In periods of low flow the water level is close to sea level but the barrier prevents normal tidal patterns. Seawater only enters the system after the bar has been opened by flooding, during spring tides, or when strong winds push the surf over the barrier.
3. Barred stream, barrier estuaries with no associated lagoon i.e. the stream is relative narrow and comparatively deep (> 2 m). In periods of low flow the water level is close to sea level but the barrier prevents normal tidal patterns. Seawater only enters the system after the bar has been opened by flooding, during spring tides, or when strong winds push the surf over the barrier.
4. Raised Estuaries, barrier estuaries where the water level is a couple of metres higher than sea level. Usually seawater only enters the system when strong winds push the surf over the barrier.

Sampling for the RIVPAC model was undertaken in autumn when river discharge is at its lowest. At this time the following patterns were observed in these estuaries. Tidal estuaries had a classical pattern with salinity in the lower estuary equivalent to seawater, and reduced salinity in the upper reaches.

Collecting a dredge sample at the mouth of an estuary. Many south-eastern Australian esturies have sand bars blocking the entrance to the ocean for a large part of the year.

At the sampling sites the water was well mixed vertically with no salinity gradient. Barred lagoons had a wide range of salinities from hypersaline (>34 ppt) to diluted salinities (10 ppt). Usually there was a poor horizontal gradient in salinity. Sites in the lagoons, near the mouth, usually had similar salinities to the sites in the upper estuaries of the streams that flowed into the lagoon. Barred lagoons usually had good vertical mixing with no salinity gradient (Halocline) between surface and bottom waters. In contrast, barred streams where usually characterised by poor vertical mixing with a layer of fresh water on the surface even near the mouth, and saline waters near the bottom. The differences between barred lagoons and streams are presumably because wind mixing of water can occur in the lagoons. Only one raised estuary was sampled. The water even at the mouth was fresh (<0.5 ppt), water was well mixed vertically with no halocline. This estuary was included in our survey because despite the low salinity estuarine animals were still collected.