Recreational water quality and health: Cyanobacteria and their toxins

Learn about the risks of cyanobacteria and their toxins, and how to keep yourself safe.

On this page

About cyanobacteria

Cyanobacteria are also known as blue-green algae. They're a type of bacteria that occur naturally in fresh and marine bodies of water. There are many types of cyanobacteria, and some can produce harmful toxins called cyanotoxins.

Cyanobacteria can grow quickly in freshwater lakes and rivers under favourable conditions that can include:

These conditions can sometimes lead to large cyanobacteria blooms. When a bloom containing toxin-producing cyanobacteria develops, toxins can reach hazardous levels.

Marine waters generally don't have problems with cyanobacteria blooms.

Health risks

You can be exposed to cyanobacteria and their toxins by:

If you're in an area containing blooms, this can happen while you're:

Cyanobacteria and their toxins can cause adverse health effects. In people, exposure to water containing blooms can cause:

The type and severity of illness depends on:

Children are more at risk because they're more likely to accidentally swallow water and often spend more time in water than adults.

It's rare for people to die from exposure to cyanobacteria or their toxins. There have only been a few deaths reported worldwide.

There have been many more reports of animals dying after:

These animals include livestock, wild animals, and pets, particularly dogs. Animal deaths usually result from a rapid onset of respiratory failure.

Signs of cyanobacteria blooms

Not all cyanobacteria blooms look the same. Cyanobacteria blooms on the surface of the water are generally green or blue-green and can look like spilled paint or fine grass clippings. Surface cyanobacteria blooms can also come in different colours, such as red or brown, which are not as common.

It can be difficult to tell the difference between a cyanobacteria bloom and other blooms on the surface of the water, such as green algae blooms.

Some blooms can spread through the water, making it look like the water has changed colour.

Benthic cyanobacteria grow on the bottom surfaces of water and form dense mats of cyanobacterial material. These benthic mats range in colour from black to brown to green. They can become detached from the bottom and accumulate along shorelines.

Other signs of a toxic bloom could include:

It's impossible to tell if a cyanobacteria bloom or benthic mat contains toxins by looking at it.Treat any cyanobacteria bloom like it's toxic as a precaution.

Learn more about identifying blooms:

Protect yourself and your pets

Check online or at the recreational water area for swimming and contact advisories for cyanobacteria blooms. Follow the advice and recommendations.

Learn what cyanobacteria blooms look like. Not all recreational water areas monitor for blooms, so they may not post advisory signs.

Avoid activities in areas with a bloom. This includes areas where there is visible cyanobacterial material or benthic mats.

Talk to your healthcare provider if you're sick and suspect it's the result of exposure to a bloom.

If you or your pet accidentally touch a bloom, shower or rinse with tap water as soon as you can.

Do not let your pets:

Talk to your veterinarian if your pet shows abnormal behaviour or signs of distress and you suspect they were exposed to a bloom.

If you suspect the water has a cyanobacteria bloom, report it and stay out of the water. Check with your jurisdiction for online reporting forms or toll-free numbers. If no specific guidance is available, report the suspected bloom to your local public health unit.

When reporting a suspected cyanobacteria bloom, include a description or a picture of the bloom if possible. Report any illnesses in people or animals that you suspect are related to contact with the water.

Managing recreational water areas

Health Canada publishes guidelines for Canadian recreational water quality. They provide information on cyanobacteria and their toxins, including:

To check recreational water areas for cyanobacteria and their toxins, authorities can:

Not all recreational water areas in Canada need to be monitored, as some are unlikely to develop blooms. Areas with higher risk for bloom development can include those:

In waters were there is a visible bloom, a swimming or contact advisory should be issued as a precaution. Advisories should remain in place until authorities say the area is acceptable for recreational activities again.

Page details

Date modified: