Health Warning – Algal Bloom in Lake Forsyth (Wairewa)

Monday January 10, 2011

The Community and Public Health division of the Canterbury District Health Board has issued another health warning after more potentially toxic blue-green algae (planktonic cyanobacteria) was found in Lake Forsyth/Wairewa.

People should stay out of the water at this lake until the health warnings have been lifted. Toxic algae are particularly dangerous for dogs, so they should be kept away from the water. 

The type of cyanobacteria that is currently present in high concentrations is Anabaena, which is different to the Nodularia blooms that have been a regular occurrence over recent summers.  The Nodularia blooms appear as a thick surface scum while Anabaena forms green globules that float in the water column.  Despite the difference in appearance, the potential health risks are similar and people should avoid contact with the water until further notice.

Canterbury Medical Officer of Health Dr Alistair Humphrey says the algal bloom can produce toxins harmful to humans and animals.

“Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips. If you experience any of these symptoms after contact with contaminated water, visit your doctor immediately,” he says.

No one should drink the water from the lake at any time, Dr Humphrey says. Boiling the water does not remove the toxin. Ordinary tap water from community supplies is not affected by the algal bloom. 

Animals should be taken to a vet immediately if they come into contact with the algal bloom or if they display any unusual symptoms.

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.

Environment Canterbury monitors the lake weekly during summer and the public will be advised of any changes in water quality that are of public health significance.

Facts about cyanobacteria:

  • The algae occur naturally but can increase rapidly during warmer months.

  • If the water is cloudy, discoloured, or has small globules suspended in it, avoid all contact.

  • Not all cyanobacterial blooms are visible to the naked eye and toxins can persist after the blooms disappear.

  • Cyanobacterial concentrations can change quickly with changing environmental conditions (e.g. wind). If a health warning is in place avoid contact with the water.

For further information visit the Environment Canterbury website or contact Community and Public Health on (03) 364 1777.​

Page last reviewed: 14 February 2014