Lake warnings for toxic algae

Read our latest lake warnings for toxic algal blooms.

Lake warnings for toxic algal blooms

Warnings in place - Update 16 April 2013

Te Wairewa/Lake Forsyth – warning in place

Last updated 16 April 2013

There is a bloom of blue-green algae (cyanobacteria) present at Te Wairewa/Lake Forsyth, which is visible as a thick green scum on the water surface and along the lake edge. The Community and Public Health division of the Canterbury District Health Board has issued a health warning and people should stay out of the water at this lake until the health warning has been lifted. Toxic algae are particularly dangerous for dogs, so they should be kept away from the water.

Environment Canterbury will be sampling the lake on a weekly basis and the public will be informed when the health warning status changes.

Te Waihora/Lake Ellesmere – no warning

Last updated 16 April 2013

Ongoing water testing at Te Waihora/Lake Ellesmere has shown that concentrations of blue-green algae (cyanobacteria) remain below the trigger levels that are of concern to public health. Environment Canterbury will be sampling the lake on a monthly basis over the winter months.

Lake Roundabout – warning in place

Last updated 28 March 2013

There is a bloom of the potentially toxic cyanobacteria Anabaena present at Lake Roundabout, in the Ashburton basin. The Canterbury Medical Officer of Health, Dr Alistair Humphrey, has issued a health warning and advises people to avoid contact with water at this lake until further notice. Environment Canterbury will continue to monitor the lake and the public will be advised when the algal bloom has cleared.

Lake Rotorua – warning in place

A health warning remains in place for Lake Rotorua (Kaikoura) where blooms of the potentially toxic algae species Anabaena and Microcystis are present.

Facts about cyanobacteria:

The algae occur naturally all year round, but can increase in numbers rapidly during warmer months.
If the water is cloudy, discoloured, has small globules suspended in it, or forms coloured scums, avoid all contact.
Not all cyanobacterial blooms are visible to the naked eye and toxins can persist after visible 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.

General advice on lake algal blooms

If there is a health warning in place:

Never swim or take part in any activity that may result in accidental consumption or exposure to water affected by algal blooms if a health warning is in place for that area, even if there are no visible signs of a bloom. Do not let stock or dogs swim or drink from the water.

If a health warning is in place, it means that cyanobacteria (blue-green algae) are present in large numbers in that bay/lake. Cyanobacteria are capable of producing toxins that are harmful to humans and animals if swallowed or if exposed to skin (such as may occur when swimming, skiing or kayaking etc).

If there is no health warning in place:

Wind blowing onto shore may concentrate a bloom in a bay and make it unsafe for recreational contact. This can occur at any time of the year, but is more common in warm summer months. Hence, even if no warnings are in place for a lake or bay, you should still assess the situation carefully before entering the water.

Caution: If the water is cloudy, discoloured, or has small globules suspended in it, it is better to be cautious and totally avoid that lake or bay.

Even if the water appears completely normal to the naked eye, it can still have large numbers of algae present. If you experience health symptoms (such as those described below) that come on within minutes of contact with the water, you should avoid further contact with the water and, if symptoms persist, contact Community and Public Health 03 379 9480 or see your doctor.

Cyanotoxin health symptoms

Swimmers have been known to develop adverse reactions after bathing and showering in water containing blue-green algal blooms. These include allergic reactions, asthma, eye irritations, rashes and blistering around the mouth and nose, gastrointestinal disorders (abdominal pain, cramps, diarrhoea). Some of these can be caused by the mere presence of large numbers of algal cells in the water, but some symptoms are related to the release of toxins by the bloom. These toxins are colourless and odourless. They are unaffected by boiling the water and can persist in the water after the bloom has subsided. They can also be released at any stage of a bloom and may or may not be localised in a bay.