Cyanobacteria sites

​Environment Southland currently monitors for cyanobacteria blooms across the region. Monitoring is done to establish any public health risk associated with swimming in lakes and rivers.

From August - September 2017, monitoring showed a high abundance of cyanobacteria in Waituna Lagoon. Read our Q&A to find answers to some of the frequently asked questions at the time, and about cyanobacteria in general.

There are two types of cyanobacteria – Benthic (rivers) and Planktonic (lakes). You are more likely to come in contact or notice benthic cyanobacteria as the mats detach from the riverbed, accumulating as scums along the river's edge or becoming exposed on the river's edge as the river level drops.

In this video, Dr Wood from Cawthron Institute talks about toxic algae in New Zealand rivers and what we need to look out for to keep ourselves safe.

Cyanobacteria, sometimes called blue-green algae (even though they are actually bacteria), can be seen when they form large unattractive brown, green or black mats on river beds. Cyanobacteria can produce toxins (called cyanotoxins) that are harmful to animals and humans when eaten, even licked, or when water containing the toxins is swallowed. Some people will have an allergic reaction to just touching these mats. These forms of cyanobacteria usually grow when there are stable, low flows and are therefore more abundant in summer – when people are more likely to come into contact with the water.


benthic cyanobacteria 2.jpg 

makarewa cyano 004.jpg

Toxic cyanobacteria are now known to be widespread in New Zealand waterways. Cyanobacteria are widespread in Southland, being present in 96% of monitored sites in 2009.

Southland's most common cyanobacteria species are Phormidium, which is known to produce neurotoxins (anatoxin-a and homoanatoxin-a) linked to reported dog and stock deaths in 1999/2000 in Southland. Since then there have been no confirmed cases of cyanobacteria-related animal deaths.

National guidelines for cyanobacteria in recreational waters have been developed.

Not enough is known about when and why cyanotoxins are produced, so a precautionary approach is recommended: people, dogs and stock should avoid drinking river water in summer, and should avoid touching, licking or eating the cyanobacteria mats.

Monitoring information is not yet available online. Contact Environment Southland for the current results and more information. 

General advice on potentially toxic cyanobacteria 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 cyanobacteria mats or 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 river site. 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:

Not every river entrance is monitored.

For rivers, it is not possible to inspect all reaches of river therefore members of the public need to know how to identify cyanobacteria mats, and how to prevent themselves and their animals coming into contact with the potentially toxic cyanobacteria mats.

Caution: If the bed of a river is covered in thick dark brown or black mats that have a velvety texture and earthy/musty smell; it is better to be cautious and totally avoid that river site.

For lakes, 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 the warmer summer months.

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

Hence, even if no warnings are in place for a river, lake or bay, you should still assess the situation carefully before entering the water.

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) that come on within minutes of contact with the water, you should avoid further contact with the water and, if symptoms persist, contact your doctor.

Cyanotoxin health symptoms

Swimmers have been known to develop adverse reactions after bathing and showering in water containing potentially toxic cyanobacteria 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/river site.

What causes cyanobacteria algal blooms?

The presence of extensive mats of cyanobacteria is linked with environmental conditions conducive to their growth. Favourable conditions include the right combination of warm temperatures, sunlight, low or stable river flows, and nutrients. The occurrence of mats or algal blooms is a natural phenomenon, but human activities, such as taking water from rivers or adding nutrients and sediment to waterways, can make things worse.

Are cyanobacteria always toxic?

There are several species of cyanobacteria that may or may not be toxic, depending upon prevailing environmental conditions. However, if potentially toxic cyanobacteria are present in large numbers, you should presume that the water may be unsafe for contact recreation or consumption.
Some algae have toxins in their cells, and can be harmful if they are consumed. Such algae present a risk to dogs which may eat algal mats, or ingest algae when they drink water from a watercourse. Other cyanobacteria may release toxins into the water surrounding them, which can affect those that contact or drink the water.

Other Frequently Asked Questions

How do I know if the water contains toxin-producing cyanobacteria?

Identification of cyanobacteria requires a microscope, and its presence alone does not confirm cyanotoxin production, as not all species produce cyanotoxins and not all toxic species produce toxins continuously. Cyanotoxins are identified using a range of laboratory tests. The factors that trigger toxin production in cyanobacteria are not completely understood.

What risk do the cyanobacteria pose to stock and domestic pets?

Dogs are particularly susceptible to poisoning from both mat-forming and free-floating cyanobacteria as they enjoy being in the water and can consume these algae intentionally or by accident. Livestock are also at risk from poisoning from cyanotoxins and should be provided with alternative drinking water. Symptoms of poisoning in animals exposed to the type of cyanotoxins present in Phormidium mats include lethargy, muscle tremors, fast breathing, twitching, paralysis, convulsions. In extreme cases, death can occur within 30 minutes after signs first appear. If you are concerned, contact a veterinarian immediately.

Who should I call if I think my animal is sick?

If you are concerned about your animals, you should contact a veterinarian immediately. You or your vet can report any animal illness resulting from contact with the cyanobacteria to your local council.

What are the health risks to humans from toxin-producing cyanobacteria?

People swimming or showering in water with increased levels of algal bloom have been known to develop allergic reactions – asthma, eye irritations, rashes, blistering around the mouth and nose and gastrointestinal disorders including abdominal pain, cramps and diarrhoea.

Any reaction depends on the type of cyanobacteria, the type of cyanotoxins present, and the concentration of the toxin in the water. The higher the concentration of cyanobacteria and cyanotoxins and the longer contact with the water, the more severe the symptoms are likely to be.

Who should I call if I think I have experienced a reaction?

If you think you have experienced a reaction after exposure to water containing cyanobacteria, see your GP and tell him or her that you think you have been exposed to potentially toxic cyanobacteria.

Is it safe to drink water containing toxin-producing cyanobacteria?

No. Toxins are not removed by boiling, normal filter systems, or by adding household disinfectant.

Is it safe to swim in water with toxin-producing cyanobacteria?

You should avoid any skin contact with the water and avoid swallowing the water. The higher the concentration of cyanobacteria and cyanotoxins and the longer time in the water, the more severe the symptoms are likely to be.

Can I eat fish or shellfish from water with toxin-producing cyanobacteria?

Eating mussels and other shellfish from affected areas should be avoided as they can concentrate the cyanotoxins produced by the cyanobacteria. If you choose to eat fish from waters containing toxic cyanobacteria, you should eat them in moderation. Avoid eating the liver and kidney of the fish, as this is where accumulation of cyanotoxins may be the greatest. Fish may taste earthy. Avoid contact with the water while fishing and wash all fish in clean water.

Is it safe to boat or canoe in water with toxin-producing cyanobacteria?

How safe boating and canoeing are depends on the amount of direct contact with the water. If you swallow the water or your skin is in contact with the water while boating or canoeing, you are at risk from a reaction to any cyanotoxins that may be present. The higher the concentrations of cyanobacteria and cyanotoxins and the longer that people are in contact with the water, the more likely a reaction is to occur. Wash boats or canoes and life-jackets down with clean water after use.

Will wearing a wetsuit protect me?

No, wearing a wetsuit will not protect you. The cyanobacteria may accumulate in the collar and cuff areas of wetsuit and rub against your skin. This may cause a strong skin reaction in these areas. If you do choose to wear a wetsuit and go into the water, take care to rinse any cyanobacteria off the wetsuit with fresh water.

Can I water my garden wit​h water that contains toxin-producing cyanobacteria?

Yes. Fruit and vegetables do not appear to absorb the toxins. However, fruit and vegetables should be washed well in clean water, as the cyanobacteria may form a residue on the surface, which can remain toxic even when dry.

Can I use water containing toxin-producing cyanobacteria to put out fires?

Avoid taking water from affected areas. If you do take water, stand away from sprays to avoid contact with, or inhalation of aerosols.

Where can I get more information?

  • Contact your local council with queries relating to animals and water supply
  • Contact Public Health South with queries relating to human health
  • Check out the New Zealand Guidelines for Cyanobacteria in Recreational Fresh Waters prepared for the Ministry for the Environment and the Ministry of Health here

Questions and answers courtesy of

Page reviewed: 22 Sep 2017 12:06pm