Queanbeyan City Council made the decision to close the lake after concerned residents reported the possibility of an algal bloom.

The council undertook tests on samples of the lake water to determine the strain of algae and the quantities present on Monday.

The recorded results show high levels of potentially toxic blue-green algae (Microcystis aeruginosa and Anabaena circinalis). 

The council has warned people should not undertake recreational activities where they may be coming into direct contact with the water such as swimming, boating or fishing. 

All domestic animals should be kept away, as the potential harmful effects of blue-green algae can cause serious illness in animals.

Blue-green algae often appears as a green paint-like scum on the water and foreshore edges or clumps throughout the water, making the water appear dirty, green or discoloured. However, even areas with no apparent scum may contain the algae.

Warning signs are to be erected on Wednesday around the lake.

Blue-green algae may cause severe stomach upsets, nausea and skin irritation in people. It is suggested not to consume fish caught in water affected by a bloom. There is some evidence that small quantities of algal toxins may enter fish flesh when a bloom produces toxins.

Queanbeyan City Council will continue to monitor the algal bloom.

Information about blue-green algae blooms is available from the NSW Algal Information Hotline 1800 999 457 or visit www.water.nsw.gov.au/algal alerts

The current status of Lake Jerrabomberra is also available at www.qcc.nsw.gov.au