Algal Blooms

The Upper Thames River Conservation Authority (UTRCA) monitors water quality throughout its reservoirs from May to September, including monitoring for algae and algal blooms.  

It is important to remember algae are natural primary producers and play an important role in the food webs of healthy lakes and rivers. They are normally barely visible in the water but, under certain conditions, can increase rapidly to form a bloom. Hot, dry weather, sunlight, still water, and a surplus of nutrients such as phosphorus and nitrogen provide the ideal conditions for algal blooms to occur. 

Blooms that occur in the UTRCA reservoirs are generally considered a nuisance. In some cases, however, these blooms may contain blue-green algae, known as cyanobacteria, which can produce toxins. 

Cyanobacteria (blue-green algae) provide food, oxygen, and even nitrogen for aquatic ecosystems. However, under certain environmental conditions — warm temperatures, excess nutrients, sunlight, and calm water — cyanobacteria can become overly abundant, resulting in dense populations known as blooms. Harmful algal blooms (HABs) refer to these accumulations of potentially toxic cyanobacteria. Cyanobacteria blooms often look like thick pea soup or areas of spilled paint on the surface of the water. 

Always assume that toxins are present in an algae bloom and avoid contact with the water. Keep those at higher risk, children, and pets, out of the water. 

Avoid drinking, bathing, or swimming in the water when algal blooms are present. 

The Ministry of Environment, Conservation and Parks (MECP) records the occurrence of blue-green (cyanobacteria) blooms in Ontario. You can report occurrences by calling the Spills Action Centre at 1-800-268-6060. 

For more information on blue-green algae, please refer to the MECP’s blue-green algae webpage. 

A male staff pours collected water into a vessel for testing

Frequently Asked Questions (FAQs) about Algae 

Algae are a large group of microscopic photosynthetic organisms. They are natural primary producers and have an important role in the food webs of healthy lakes and rivers. They are normally barely visible in the water. However, under certain conditions, algae populations can increase rapidly to form a bloom. Hot, dry weather, sunlight, still water, and a surplus of nutrients such as phosphorus and nitrogen provide the ideal conditions for algal blooms to occur. 

The breakdown of these blooms can be detrimental to the aquatic environment. As the algae and bacteria in an algal bloom die, the decomposition process uses up oxygen in the water and can create an oxygen-depleted environment, known as hypoxia. 

Blooms that occur in the UTRCA reservoirs are generally considered a nuisance. However, in some cases, these blooms may contain blue-green algae, known as cyanobacteria, that can produce toxins. 

Cyanobacteria, also called blue-green algae, are photosynthetic prokaryotic microorganisms. They play an important role in aquatic ecosystems by providing food, oxygen and under specific circumstances, nitrogen. However, under certain environmental conditions such as warm temperatures, excess nutrients, sunlight and calm water, cyanobacteria can become overly abundant, resulting in dense populations known as blooms. Harmful algal blooms (HABs) refer to these accumulations of potentially toxic cyanobacteria. Cyanobacteria blooms often look like thick pea soup or areas of spilled paint on the surface of the water.  

HABs can produce toxins, called cyanotoxins, which cause negative health effects on people and animals. Exposure to toxins can occur when consuming contaminated water, eating contaminated seafood, or accidentally swallowing water during swimming or recreation. Cyanotoxins may persist in the environment after a bloom is degraded. 

No, not all algae are toxic, and most do not pose any risk to humans or animals. There are many species of non-toxic algae, and many are naturally found in the Upper Thames River watershed. It is important to note that the presence of algae does not mean there are toxins present, but caution should be observed. 

Harmful algal bloom species require nitrogen and phosphorus to grow. Managing nutrients on the landscape is key to reducing the frequency of blooms in aquatic environments.

If the water appears greenish or becomes cloudy, preventing you from seeing your feet while standing in knee-deep water, swimming should be avoided. Pets should also avoid contact with water when it appears this way. 

If there is a visible scum on the water surface, similar to spilled paint, all contact with the water should be avoided. This includes keeping pets away from the water. When these conditions are present, there is a higher risk of getting sick or experiencing skin or eye irritation due to water contact.

Always assume that toxins are present in an algae bloom and avoid contact with the water. Keep those at higher risk, children, and pets, out of the water. 

Avoid drinking, bathing, or swimming in the water when algal blooms are present. 

The UTRCA monitors blue-green algae in our reservoirs. If you suspect you see an algal bloom, please contact us with pictures and a description of where and when you saw it. Please provide as much detail as possible in your description about the weather and location of the suspected bloom. 

The Ministry of Environment, Conservation and Parks (MECP) records the occurrence of blue-green (cyanobacteria) blooms in Ontario. You can report occurrences by calling the Spills Action Centre at 1-800-268-6060. 

For more information on blue-green algae, please refer to the MECP blue-green algae webpage. 

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