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Pegasus Lake Update - 23rd March - Lake Warning Issued

Algal Bloom in Lake Pegasus

23 March 2021

Canterbury District Health Board’s Community and Public Health unit has issued a health warning after evidence of potentially toxic blue-green algae (planktonic cyanobacteria) was found in Lake Pegasus. Evidence of cyanobacteria was also found on a number of beach shorelines as accumulations of scum.

People should avoid the area and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.

Dr Ramon Pink, Canterbury Medical Officer of Health, says the algal bloom can produce toxins harmful to humans and animals.

“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 Pink says.

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

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

Lake Pegasus is being monitored on a weekly basis and the public will be advised of any changes in water quality.

Facts about cyanobacteria:

  • In lakes cyanobacteria is generally suspended in the water column and typically forms a thick, bright green colouration to water and can produce scums on the water.

  • In rivers cyanobacteria appears as dark brown/black mats attached to rocks along the riverbed.

  • A low cover of the algae can occur naturally but can increase rapidly during warmer months. Algal blooms are influenced by a combination of available nutrients in the water and sediments (such as nitrogen and phosphorus), a sustained period of low and stable flows, and favourable weather conditions (e.g. increased temperature, calm days).

  • It often has a strong musty smell and algal toxin concentrations can vary over short periods.

  • Although high water levels in rivers may remove the algal bloom, detached mats can accumulate along the shore and increase the risk of exposure to toxins.

  • If a health warning is in place avoid contact with the water.

  • Although district or city councils may place warning signs, these may not be seen at the numerous river access points, hence the need for people/ dog-walkers to treat every low-flowing river cautiously.

For further information visit:

Learn what to look for in lakes and rivers: ECan - Potentially toxic cyanobacteria (pdf)

Or contact Community and Public Health on (03) 364 1777:

Or you can read our Frequently Asked Questions factsheet

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