Factsheet: Toxic algae

What are toxic algae?

Toxic algae is a common name for toxic cyanobacteria.  Cyanobacteria are microscopic organisms that play a very important role in many land and aquatic ecosystems.  Cyanobacteria are naturally occurring and live in a range of waterways, from near-pristine to those more impacted by land-use.  Some cyanobacteria can produce toxins that are harmful to animals and humans.  

In aquatic environments, cyanobacteria can multiply and form blooms suspended in the water (known as planktonic) or dense mats attached to rocks on river beds (known as benthic).  Planktonic blooms are generally found in slow moving water-ways such as lakes, while benthic blooms usually occur in rivers.


Why can toxic algae be a problem?

Toxic algae can become a problem when they increase to high concentrations, forming ‘blooms’.  Most of the year toxic algae are present at low levels that are not much of a danger.  Blooms are more common during the summer months, when low rainfall, warm temperatures, the right level of nutrients and more sunlight create an environment where it can thrive.

Other than the general effects of algal blooms on waterways (e.g. reduction in dissolved oxygen and light in the water column), an increasing number of cyanobacterial species are known to produce toxins. These natural toxins, known as cyanotoxins, can be a threat to humans and animals when eaten, even licked, or when water containing the toxins is swallowed. The conditions for when they can become toxic is not well understood, and the levels of toxicity can only be determined by laboratory testing.  This is why it is safest to treat the blooms as if they are toxic at all times.


What does toxic algae look like in rivers?

In rivers (and occasionally lakes), toxic algae generally form brown or black mats that grow on rocks in the river bed (also called benthic). Mats that come loose from the riverbed can wash up on the banks or form floating ‘rafts’ in shallow areas. When exposed, the mats may dry out and turn a light brown or white colour and may also produce a strong musty odour.

 Phormidium 2 

Mat forming benthic toxic algae attached to rocks


Phormidium 1 

Mat forming benthic toxic algae washed up on the waters edge


Toxic algae differs from harmless bright green algae, which often form long filaments.


What does toxic algae look like in lakes?

In lakes and slow-flowing waters, toxic algae grow in a free-floating (also called planktonic) form which can cause the water to become murky or cloudy. Planktonic cyanobacterial blooms are generally green in colour and can give lakes a ‘pea soup’ appearance. They can also form visible green to red films or scums on the water’s surface, especially at the water’s edge. 

Planktonic bloom

A planktonic cyanobacteria bloom in a lake

What is the impact on people? 

Exposure to high levels of toxins from cyanobacteria may result in serious illness or death in humans. The effect of cyanotoxins varies from rapid onset of nausea and diarrhoea, to, in extreme cases, more serious effects such as liver damage.  These toxins have no known antidotes.

Exposure can occur by spending time in or around water where there is a bloom, as your skin is likely to come in contact with the water, or you could ingest the water when doing things like water-skiing or swimming. Some people will have an allergic reaction to just touching the toxic algae.

Children are at higher risk than adults for illness from toxic algae because they weigh less and can get a relatively larger dose of toxin.  Toddlers are particularly at risk as they like to put things into their mouths.  Even a small amount the size of a 50 cent piece can be enough to cause serious harm if eaten.

What should I do if I’ve been exposed and I’m feeling unwell?

Anyone who suspects they are experiencing a reaction due to contact with toxic algae should seek urgent medical attention and advise the doctor of the potential exposure to toxic algae and ask them to notify Regional Public Health.

What is the impact on pets and animals?

Dogs are at the most risk as they like to play and scavenge in the water.  Dog deaths associated with toxic algae in rivers have become increasingly common around New Zealand. In most instances these deaths have been associated with dogs eating a mat forming cyanobacteria (sometimes also known as Phormidium). The mats produce a deep earthly odour that dogs seem to be attracted to. The risk to dogs is greatest when the mats become detached from the river bed and collect at the river edge where they are easily accessible.

If you suspect that your dog has eaten toxic algae, you should treat it like an emergency and contact your vet immediately. In extreme cases, death can occur within 30 minutes after the first signs of illness appear. Signs a dog has been poisoned by toxic algae include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.

Stock can wander into waterways with toxic algae and drink it, so you will need to arrange an alternative water source for them and keep them out of waterways. 

How to keep you (and your pets) safe

The best thing you can do is stay informed.  Look for the latest toxic algae monitoring results, check for any warnings or alerts, and get to know what it looks like before you head to the river or lake.  In some conditions toxic algae can rapidly bloom, so learn what to look for, particularly during hot periods of the year.

If toxic algae are present, or you think you have spotted it, then keep out of the water and avoid drinking or touching it.  If you do swim in water that is contaminated with toxic algae, rinse off with fresh water as soon as possible.  Wearing a wetsuit will not protect you.

Keep your dog on a lead, out of the water and most importantly, ensure your dog does not touch, lick or eat any algae mats in the water or at the water’s edge, or drink water from the affected areas.  If your dog has been in the water, do not let them lick their fur, and give them a wash with fresh water.   


Is it safe to drink water containing toxic algae?

No. Toxins are not removed by boiling, normal filter systems, steri-pen or UV light or by adding household disinfectant. Arrange alternative drinking water.

Is it safe to boat or canoe in water with toxic algae?

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 toxic algae that may be present. The higher the concentrations of toxic algae 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.

My river has long stringy green algae present, is this dangerous?

No, this algae is typically a non-toxic green algae which is present in rivers when conditions get warm and sunlight hours longer, similar to your grass growing faster during summer.

How is the risk from toxic algae assessed at swimming spots?

The risk from toxic algae present at rivers and lakes is assessed by measuring either the percentage cover of the mat-forming cyanobacteria on rocks, or the numbers of cells of cyanobacteria suspended in the water.  These are compared to national guidelines for New Zealand recreational areas to determine the risk to human health.

How is toxic algae measured?

Blooms are much more common in summer than in winter months. Therefore, this is when routine council monitoring usually occurs.

Benthic toxic algae attached to the riverbed are visually assessed with an underwater viewer called a bathyscope.  A transect is run across the river and the percentage cover is recorded (see pictures below). This is done 4 times within a river area, and an average percent cover of toxic algae cover is recorded.

In addition, the river edge is scanned for detached or exposed mats, and scums and foams that may have collected at the river margins. Recreational users and animals have an increased risk of coming in contact with detached mats.

 percentage coverPeriphyton viewer

Examples of how to determine percentage cover of benthic cyanobacteria and what an underwater viewer looks like.

Planktonic cyanobacteria are assessed by collecting a sample of water from the surface and counting the number of cells present, or the biovolume of the individuals cells (the amount of space within a cell).  Because different planktonic cyanobacteria may contain different amounts of toxins based on their cell size, biovolume provides a better estimate of potential risk than simple cell counts.

The amounts of mat forming (benthic) and planktonic (suspended) cyanobacteria are assessed using the New Zealand guidelines for cyanobacteria in recreational fresh water: interim guidelines

Where to get further information

  • New Zealand guidelines for cyanobacteria in recreational fresh waters: Interim guidelines
  • Landcare Research resource on freshwater algae identification 
  • Cawthron Institute - Cawthron Institute scientist Dr Susie Wood is a world expert in toxic algae. In this video Dr Wood talks about toxic algae in New Zealand rivers and what to look out for to keep you, your loved ones, and your pets safe.