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Sampling carried out by the Otago Regional Council has shown the re-appearance of high concentrations of cyanobacteria (blue-green algae).
In January, the council lifted a health warning about algae in the lake near Dunedin.
The council today advised that visitors should not use the lake for recreational purposes and to take note of signs until the warning had been lifted. Dog owners should not let their pets swim in the lake until it is deemed safe.
Cyanobacteria can produce toxins that are harmful to people and animals if swallowed or through contact with skin. Exposure to it may cause symptoms such as skin rashes, nausea, tummy upset, and tingling and numbness around the mouth or tips of fingers.
The council said anyone experiencing these symptoms after contact with contaminated water should see their doctor.
Fish and shellfish can concentrate toxins and their consumption should be avoided. If fish are eaten, remove the gut and liver and wash in clean water.
What to look for
Lake Waihola has a history of cyanobacteria being present. The blue/green algae occur naturally but can increase rapidly during summer months, the council said.
If the water is cloudy, discoloured, or has small globules suspended in it, all contact should be avoided.
However, not all cyanobacterial blooms are visible to the naked eye and toxins can persist after the bloom has disappeared.
Cyanobacterial concentrations can change quickly with environmental conditions (eg wind).
The Otago Regional Council monitors cyanobacteria weekly at Lake Waihola during summer, and said the public would be advised of any changes in water quality that are of public health significance.
Signs alerting the public to the presence of cyanobacteria would remain in place until two consecutive clear samples indicated it was safe.