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Research at the Werribee wetlands

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Dr Danny Rogers discusses some of the features of these small but very rich tidal flats that owe their very existence to the wise management of the Western Treatment Plant which, after more than a century still serves well both the human population of Melbourne as well as a great diversity of wildlife.     
What is it about tidal flats and wetlands that makes it attractive (or otherwise) for birdlife? Even leading shorebird researcher, Dr Danny Rogers, still wonders about the answer to this question. In part it is to do with geographical location so that the closer the tidal flats are located to the equator the greater the diversity of benthos (the tiny invertebrates living in the sediments upon which the shorebirds feed).
The further the flats are from tropical zones, the more the diversity of benthic life falls off but at the same time the actual numbers of benthic animals and even their size tends to increase.
Clearly there are many exceptions to this general rule. For example, Roebuck Bay on the Kimberley coast is a prime site for migratory and local birds, unlike its near neighbour, Eighty Mile Beach.

On the outskirts of Melbourne, for well over 80 years, the Werribee wetlands and tidal flats have been a powerful magnet for a wide variety of shorelbirds (among a host of other wildlife). Here, there is no doubt about the source of the attraction – the treated effluent from Melbourne Water's Western Treatment Plant.    
While over time the methods of treating the waste have changed from grass filtration to lagoons and ponds, the nutrient rich end product continues to be discharged into the tidal flats on Port Phillip Bay, promoting benthic proliferation.

Wader flocks, Werribee wetlands

For example, Danny Rogers' recent research into shorebird ecology at Werribee, has shown that the biomass in the flats can be as high as three to four thousand animals per square metre.
The Western Treatment Plant at Werribee (which is probably the largest plant of its kind in the world) has been a long standing example of far thinking environmentally sustainable and practical management of human waste and importantly, one where human waste is being turned to the advantage of many other species.

The overall numbers of birds using the site seems to have remained fairly constant over many years, although numbers for some individual species have changed.     
For example, Curlew Sandpiper numbers have dropped sharply in recent years (now 30 to 40 percent of 1980 numbers). Since this trend has been reported across south eastern Australia, this is more likely to reflect problems somewhere else along these migratory birds' flightpath which extends up to Northern Siberia.
Sharp-tailed Curlew Sandpiper

Part of Danny Rogers' current research is to determine how shorebirds decide where to feed in the WTP in order to be able to predict what changes might occur if the treatment processes are altered.     
Early results show that shorebirds occur in the greatest numbers in the areas where there is the greatest density of food (ie benthic life) occurs.
Perhaps not surprising, but, in addition, at these flats, the abundance of benthos in different areas is constantly changing and the shorebirds are very quick to locate where the new hot spots are located.  

Red-necked Stint eating polychaete worm

Traditionally the treated effluent has been discharged out onto the flats at four sites. With changing treatment practices, most of the material is now discharged at only two points.    
As a result, Melbourne Water is experimenting with the use of multiple outlets, trying to disperse the effluent over a larger area by making small artificial outlets. This seems to be an effective enricnment approach since samples of mud taken from around these outlets have shown dramatic increases in benthic density – not to mention increased presence of shorebirds at the sites.

Werribee Avocets

Other ongoing research by Melbourne Water involves studies of mixing zones – trying to work out where the influence of the outlets on benthic life changes; and ways to monitor the seasonal changes in benthos at the flats.
Werribee is a credit to the long-term planning from its inception. With the ongoing research interest, it is likely that it will continue to be an outstanding example for a long time to come.

Text: V.B. January 2009    Images from Dr Danny Rogers.

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