Government of New Brunswick

Blue-green algae (Cyanobacteria) are photosynthetic bacterial organisms, naturally found in many types of water systems including lakes, rivers and wetlands.  Under the right conditions, they can increase in numbers quickly to form a bloom or floating algal mats.  Blooms can range in colour from dark green to yellowish brown.  Some blue-green algae species can produce toxins (microcystins), which can impact the health of humans and animals.

In previous years blooms have been seen in some New Brunswick lakes.  Since some algae can produce toxins that are harmful to humans and animals, public health advisories may be issued.  Advisories help to inform local recreational water users, so that they can make informed decisions on water use in the affected area. They also serve to remind users to check the water for the formation of visible blooms, scum and algal mats which pose the most risk.  Since conditions may change in a matter of hours, it is important to be aware of local water conditions.  Individuals should avoid swimming and any other recreational water-related activities in areas with blue-green algae blooms or mats.

Some blue-green algae can produce toxins, which can cause skin, eye and throat irritation. More serious health effects such as gastrointestinal illness can occur if toxins are consumed.

Public Health recommends the safety advice below for all recreational water bodies including the ones with advisories. Its important to remember that just because you don’t see it in the area your swimming, it doesn’t mean its not present elsewhere. Our safety advice is primarily targeted at humans, for animal specific information, please talk to your veterinarian.

Safety advice includes:

  • Always check the water before entering and avoid using the water if algae is present

  • Do not swallow lake or river water when swimming, and always supervise young children and pets in recreational waters.
  • Bathe or shower immediately after swimming.
  • Do not enter the water with open cuts or sores.
  • Do not use water from areas with blue-green algae blooms for drinking or cooking. Boiling the water will not remove toxins. Always obtain drinking water from a clean and safe source.
  • Fish caught from water where algae are present should have all their organs removed and be rinsed well with clean drinking water before being cooked and eaten.
  • Even if no algae are present, it is recommended you shower with clean water after being in recreational waters.

Blue-green Algal Blooms


When there is lots of food (nutrients such as phosphorus and nitrogen) in the water, blue-green algae can grow very quickly and clump together to form a bloom. A bloom will look like surface scum, foam or mat and often be blue-green in colour. Blooms can also look red, brown, green, or yellow in colour. Some blooms look like paint streaks on the water, while others may not affect the look of the water.  Fresh blooms can smell like newly mown grass; older blooms can have a foul smell, sometimes like garbage.

Blooms usually occur when it begins to get hot outside, typically in the late spring and early summer.

Blue-green algal blooms can appear quickly or overnight. On windy days algae blooms may accumulate near the shore.

Blooms can also be suspended at different depths in the water. This can make them more difficult to see. They can float up and down in the water and move to where there is more food (nutrients) and light. So even if a bloom is not floating on the surface of the water, it doesn't mean that one isn’t present.

While not all blue-green algae blooms are harmful to human health, some can produce toxins. The most commonly found toxin in North America is microcystin, which can cause skin, eye and throat irritation and more severe illness if consumed.

Benthic Algal Mats


Benthic algae are a natural and essential part of our freshwater ecosystem.  They grow on the bottom of rivers and lakes where they can form large mats. These mats can grow quickly when the water is warm, there are stable flows and lots of food (nutrients).  The algae mats look like clumps of vegetation, and can appear black, brown or dark green in the water.  On the shoreline they may appear brown or grey once they have dried. Animals are attracted to their odor.  The mats can contain a mixture of algae and cyanobacteria (blue-green algae). What is the concern with benthic algal mats?  Since blue-green algae can be present in algal mats, it is possible that toxins may be present. The algal mats can break away from the bottom of a lake or river and wash up along the shoreline, making them accessible to pets and children. They can also be attached to rocks or aquatic plants or may be found floating in the water or along the surface.

Children should not play with, and pets should be kept away from, algal mats or plants that are found floating near the shore or that may have washed up along the shoreline.

Dogs can be attracted to the odour of algae mats and may want to eat them. Dogs should not eat vegetation or floating mats found along the shores of lakes or rivers