Blue-green Algae

What is blue-green algae?  

Blue-green algae, also known as cyanobacteria, are simple, single-celled organisms that can be naturally present in Canadian waterways. Blue-green algae can reproduce rapidly and “bloom” when water is warm, slow-moving and full of nutrients. Since some types of blue-green algae produce toxins, some blooms may produce toxins in quantities that could be harmful to exposed people, animals and the environment.  

Blue-green algae can vary in appearance and might resemble foam, scum, mats, or paint on the surface of the water. They can also change the colour of the water to green, blue, brown, red, pink or another colour. You may also notice it smells of rotting eggs, garbage or freshly cut grass. 

When are blue-green algae blooms considered harmful?  

It is important to note that not all blue-green algae blooms produce toxins and not all blooms are harmful. Even so, you should take precautions and assume every bloom is potentially harmful to your health because one cannot tell from the look or smell of a bloom if there are toxins and also because toxin production and toxin levels can vary over time. 

A blue-green algae bloom will dissipate with time particularly in moving water, and then normal activities and use of the affected water can resume. If Ottawa Public Health advised the posting of warning signs in the affected area, they will be removed once the risk level is back to normal. In stagnant water, warning signs may have to be left up for a long time. 

How can I be exposed?  

Can I eat fish caught in the vicinity of a bloom? 

It is probably the simplest and best approach to avoid eating fish from a blue-green algal bloom area. Please note, however, that the Ministry of the Environment, Conservation and Parks notes that recent studies show that toxins produced by blue-green algae blooms do not accumulate at high levels in fish fillets. This means there is a very low risk to human health, even from eating fish caught during peak blue-green algal bloom condition. One exception is that elevated levels of microcystins, a type of cyanotoxin, have been found in the livers of fish from waterbodies that regularly experience blue-green algal blooms, even if a bloom is not present. To eliminate this risk, follow the general advice to avoid eating any fish organs. Keep in mind that fishing is not recommended in an area where signage is posted to not use the water for recreational purposes. 

What are the symptoms from exposure to blue-green algae blooms? 

Not all blue-green algae produce harmful toxins; however, those that do create cyanotoxins which can make people and animals sick. Drinking water containing the cyanotoxins may cause headaches, vomiting, stomach pain, diarrhea, neurological symptoms such as muscle weakness or dizziness, or even liver damage. People or pets exposed to cyanotoxins by touching or swimming in contaminated water or by breathing in aerosolized water droplets may experience irritation of the skin, eyes, nose, throat or lungs. Dogs can be especially at risk because they may drink water directly or lick their fur after swimming and ingest even more toxins. 

Some individuals are sensitive to contact with blue-green algae and may develop a skin rash or eye irritation even if there is no toxin produced by the bloom. 

If you think you may have symptoms caused by exposure to blue-green algae and its toxins, talk to your healthcare provider. Similarly, if your pet exhibits signs of illness take them to a veterinarian.  

How do I report a possible blue-green algae bloom? 

Keep an eye out for posted signage in recreational water areas before entering the water. Before heading out to one of the City of Ottawa’s supervised beaches, check ottawapublichealth.ca/beaches for the daily swimming status during the supervised season. 

What is the testing process for blue-green algae blooms? 

Testing of suspect blue-green algae blooms is conducted by the Ministry of Environment, Conservation and Parks in consultation with Ottawa Public Health. Water samples are collected, sent to the lab, and results for the type of algae or bacteria and the presence of toxins and their concentration will be provided to Ottawa Public Health. Not all blooms require testing so information on the type of bloom or the level of toxins may not always be available.   

How long does a bloom last? 

It is not known how long a bloom will last. Ottawa Public Health works closely with the Ontario Ministry of Environment, Conservation and Parks to determine when warning signage for an affected area should be posted and how long it should remain in place. Toxins can still be at a dangerous level when the bloom is no longer visible. Continue to stay out of the water until the warning signs are removed. 

More information 

• Ontario Ministry of Environment, Conservation and Parks (2022). Blue-green Algae  
• Ontario Ministry of Environment, Conservation and Parks (2023). Guide to Eating Ontario Fish.