Blue-green algae

Watch for blue-green algae from May to October. Blue-green algae (also called cyanobacteria) occurs naturally in surface waters like lakes, ponds, rivers and streams. The toxins in some algae can make people sick and can be fatal for pets if ingested.

Blue-green algae are microscopic, plant-like organisms that occur naturally in ponds, rivers, lakes and streams.

Blue-green algae develops when a period of hot, dry weather is followed by heavy rainfall. It’s becoming more common with warmer temperatures and more intense storms caused by climate change. Increased amounts of nutrients (like nutrients from fertilizers, septic systems and paved surfaces near the water) can also help blue-green algae grow.

Blue-green algae isn’t normally visible in the water, but it can quickly reproduce to form a large mass called a bloom or mat. Once blue-green algae appears, it’s more likely to reappear in the same body of water.

Algae blooms come in many shapes and sizes, and harmful blooms can look very similar to harmless blooms. It can be blue-green, turquoise, green, brown, red, white or mixes of these colours.

Reporting blue-green algae

If you think you see blue-green algae, you should treat it as potentially toxic. Report it to your local Department of Environment and Climate Change office or call 1-877-936-8476.

Where to find blue-green algae reports

The Department of Environment and Climate Change publishes a list of potential blue-green algae bloom sightings and also shares reports on social media. Only sightings reported to the department are included on the list.

If the potential blue-green algae is in public drinking water supplies, an inspector from the Department of Environment and Climate Change visually confirms the presence of blue-green algae and works directly with the water supply owner.

Once a blue-green algae bloom occurs, it can reoccur or move to different locations in that body of water. Reports of blue-green algae are for the whole season (May to October). Reports can help you decide if or how you use the water (but reports don’t state that the body of water is closed or that blue-green algae is widespread in the body of water).

Halifax Regional Municipality monitors and reports on the status of supervised beaches across the municipality. Watch for blue-green algae advisories by visiting and following @hfxgov.

The Nova Scotia Lifeguard Service shares notices about water quality and beach closures at @nslifeguard. You should also look for signs posted near provincial beaches.

Blue-green algae reported in 2024

Identifying blue-green algae

Planktonic blue-green algae blooms

Blue-green algae can be suspended in the water of a lake, pond, river or stream or float in a thin layer on the surface. These are planktonic algae blooms.

Blue-green algae blooms can look like spilled paint or pea soup. The algae cells are very small but can sometimes clump together and look like fine grass clippings in the water. Sometimes they also look like a thick scum on the surface. They often smell musty or grassy when healthy, and they can smell like ammonia when decomposing.

Benthic blue-green algae mats

Another type of potentially harmful blue-green algae can occur at the bottom of clear shallow areas of lakes and rivers or on the shoreline. These are benthic algae mats. The algae mats look like sheets of slimy green, brown or reddish growth, with visible bubbles or air pockets that can appear black, brown or dark green in the water. They can also look like web-like grey, brown or yellowish growth on rocks, shorelines, lakebeds or riverbeds.

They may detach from the bottom, wash up on the shoreline, and appear brown or grey once they have dried.

Animals are attracted to their odour and may try to eat them. Ingesting algae mats can be lethal, so it’s important to keep pets and children away from mats.

Health effects

For people, the effects of blue-green algae are usually short-lived. Exposure to blue-green algae when swimming can cause itchy, irritated eyes and skin. Swallowing or inhaling the water can cause headaches, fever, diarrhea, abdominal pain, nausea and vomiting (and ingesting water for long periods of time can lead to a damaged liver).

Children are at greater risk of having health effects because of their lower body weight. They also usually spend more time in the water and are more likely to swallow it.


For pets, exposure to blue-green algae can be fatal. Pets are attracted by the smell of algae mats and ingesting the mats has caused pet deaths in Nova Scotia.

Safety measures

If you think you see blue-green algae or if blue-green algae is present:

  • don’t touch the algae or the water
  • don’t drink the water
  • don’t swim, bathe, shower or play in the water
  • don’t brush your teeth with the water
  • keep children, pets and livestock away from the water (don’t let them drink or swim in the water)
  • wash yourself with clean water if you come into contact with the algae (and any items that come in contact with it)
  • use alternative water sources for drinking, cooking, bathing, laundry and watering your vegetable garden (contaminated water isn’t safe even if it’s boiled, filtered or treated with bleach, herbicides, copper sulfate or other algicides)
  • be careful with recreational water activities that generate spray (like boating) because you can inhale the toxins
  • be careful when eating fish caught in the water and don’t eat the liver, kidneys or other organs because toxins can be stored there
  • seek medical attention if you get sick
  • contact a veterinarian immediately if your pet ingests the algae

Drinking water sources

Surface water like lakes or rivers isn’t recommended as a drinking water source for people’s homes because it can get contaminated more easily than groundwater and because the water quality and chemistry is much more variable.

If you take your drinking water from a lake, river or other source of surface water, or if you have a shallow-dug well next to a lake or a river, your drinking water can be contaminated during and after the presence of blue-green algae. The highest concentrations of toxins are usually released into the water when the algae dies.

If you want to keep using surface water for your drinking water source, you should get the water tested. The best time to take a water sample is after the algae has died off and there are no longer visible signs of it. Ask the lab to test for the toxin microcystin. If the microcystin level is over the limit of 0.0015 milligrams/litre, you should find a different source of drinking water.

Water treatment systems

Most municipal water supplies use specialized treatment to manage blue-green algae.

Most residential filters don’t remove the toxins caused by blue-green algae. If you have questions about your residential filter, contact a water quality specialist. Make sure you understand what contaminants you need to treat for and look for treatment units that will treat those contaminants.

Learn more about drinking water safety: The Drop on Water factsheets.