Toxic algal bloom in Lake Forsyth

Friday October 20, 2017

​Community and Public Health (a division of Canterbury DHB) has issued a health warning after potentially toxic blue-green algae (planktonic cyanobacteria) was found in Lake Forsyth/ Te Roto o Wairewa.

People and animals, particularly dogs, should avoid Lake Forsyth/Te Roto o Wairewa until the health warning has been lifted.

Recent testing indicated high numbers of the potentially toxic cyanobacteria Aphanizomenon were present in the water of Lake Forsyth/ Te Roto o Wairewa. The cyanobacteria is not always visible, but this filamentous algae can form dense blooms near the surface.

Canterbury Medical Officer of Health Dr Alistair Humphrey says the algal bloom can produce toxins harmful to humans and animals. So people should avoid contact with the water until further notice.

"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips. If you experience any of these symptoms visit your doctor immediately and please let your doctor know if you have had contact with the lake water," Dr Humphrey says.

No one should drink the water from the lake at any time, Dr Humphrey says. Boiling the water does not remove the toxin.

Animals that show signs of illness after coming into contact with algal mats should be taken to a vet immediately.

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 cyanobacteria blooms are visible to the naked eye and toxins can persist after the blooms disappear.
  • Cyanobacteria concentrations can change quickly with changing environmental conditions (such as wind). Avoid contact with the water if a health warning is in place.

Visit the Environment Canterbury website for further information on water quality in Canterbury.

You can also contact Community and Public Health for more information (03 364 1777).

Page last reviewed: 20 October 2017
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