Blue-green algae at Warragamba

Algae occur naturally in all aquatic environments, particularly during warmer weather, and can flourish under suitable light and temperature conditions when there is a good supply of nutrients in the water.

In August 2007, an extensive bloom of blue-green algae developed in Lake Burragorang, near the Warragamba Dam wall. The bloom followed nine years of severe drought when storage levels in Warragamba Dam fell to unprecedented lows.  The receding water levels also exposed 1400 hectares of sediments that were previously under water.

Heavy rainfall in June and July 2007, carried these sediments and high levels of nutrients from the catchments into Lake Burragorang. This, combined with an unusually warm August, provided the ideal environment for the algae to grow.

The Sydney Catchment Authority (SCA) continues to investigate the contributing factors of the 2007 bloom and is involved in longer term research activities to help prevent or minimise the occurrence of possible future blooms.

Since August 2007, the SCA has been monitoring and managing the raw water, along with Sydney Water and NSW Health, to continue to supply water that meets Australian Drinking Water Guidelines. In order to provide the best quality raw water to Sydney Water’s treatment plants, water supplies are being sourced from between 37 and 44 metres below the water surface.

The bloom at the dam wall dissipated at the end of December 2007, with residual algal cells remaining in the surface water. These were at quantities generally below those outlined in the Australian Drinking Water Guidelines.

At the peak of the bloom, the dominant type of algae was Microcystis, which can potentially produce algal toxins. However, genetic investigations confirmed that the algae was unlikely to produce toxins and no toxins have been detected in the raw water supplied for treatment.

Throughout the summer of 2007-08, algae continued to be present in the shallower waters in the upper end of Lake Burragorang but these numbers have now also declined with the onset of cooler weather.

The SCA continues to monitor algal levels regularly in Lake Burragorang, its other dams and in the raw water supplied to the treatment plants.

14 August 2008