All About Algae: What You Should Know About Harmful Algal Blooms

What do you think of when you read ‘algae’? Probably the green slime that makes rocks slippery as you enter the water, or the scum that collects on the walls of your fish tank. But there is so much more to algae than something you have to clean from your betta fish’s home!

Algae play a critical role in aquatic environments, benefitting people and wildlife in numerous ways. However, excessive growth of algae can lead to nuisance, or even harmful, algal blooms. This month we explore “harmful algal blooms”, commonly referred to as “HABs” for short.

Appreciating Algae

What we call “algae” is actually a diverse group of organisms. Thousands of different algal species all share the fact that they are simple aquatic organisms that photosynthesize, just like plants on land. This means that they create their own energy from sunlight, taking in carbon dioxide and producing oxygen.

From the icy realm of the Arctic, to warm beaches and even deserts, algae can be found all over the world. Fossils of algae date back over one billion years, meaning these species have been adapting to their habitats for eons. 

Algae form the base of aquatic food webs and are therefore vital to the health of every creature that lives in and around Georgian Bay. Algae provide food for other creatures, absorb atmospheric carbon, cycle nutrients, and produce oxygen. In fact, about half of the oxygen we breathe is produced by algae.

In the biosphere region, there are three general types of algae that you might see during the summer, each with a unique ecological function: filamentous green algae, chrysophytes, and blue-green algae.

Filamentous green algae

Filamentous green algae looks like what most people picture when they think of algae: collections of green algae that tend to form in strands. These strands are made up of multiple single or multicellular organisms, and are either free-floating in pillowy clumps, or attach themselves to submerged rocks, logs, or other objects.


Chrysophytes are unicellular microorganisms that grow in colonies, making the water cloudy and tinged yellowish-green. Chrysophytes bloom most often in spring and early summer, and tend to thrive in low-nutrient lakes.

Blue-green algae

Blue-green algae are actually bacteria that have similar qualities to algae and other plants. They are more accurately called cyanobacteria (cyan means blue-green). Blooms most often occur in the late summer and into the early fall. These blooms look bluish-green, and can be dangerous to humans and animals.

What Causes Algal Blooms?

Algae are a natural and critical part of aquatic ecosystems. Under the right conditions (warm, still water with available nutrients), algae can grow rapidly from microscopic, single-celled organisms into large blooms, easily visible to the naked eye. 

The nutrients phosphorus and nitrogen are limiting factors of algae growth, meaning algae will grow until the supply of these nutrients is used up. Therefore, nutrient pollution from sources such as fertilizer runoff, animal waste, septic systems, and stormwater runoff adds additional nutrients to a waterbody and promotes algae growth beyond what would naturally be possible. Warming water and more intense rainfall events as a result of climate change also create more favourable growing conditions.

In most cases, algae blooms are simply considered a nuisance, potentially impeding recreational opportunities, exhibiting an unpleasant odour, and creating an eyesore. In some cases, certain species of cyanobacteria come together to form clusters and release toxins that pose a threat to humans and animals, these are called harmful algal blooms (HABs). Recent studies have found that nitrogen, in particular, can increase toxin production in Microcystis blooms, but at different rates depending on the form of nitrogen.

Identifying Harmful Algal Blooms

While not overly common, there are reports of HABs in the biosphere and surrounding area every few years. In the biosphere region, HABs are made up of cyanobacteria, also known as blue-green algae. There are roughly 80 species of cyanobacteria that produce toxins and it is important to note that HABs occur only when algae are capable of producing toxins that are harmful to humans, animals, and the environment (blooms without the presence of toxins are simply nuisance blooms and not considered HABs). Unfortunately, you cannot definitively distinguish a nuisance algal bloom (one without toxins) from a harmful algal bloom (one with toxins) based on the appearance of the water alone.   

Blooms of cyanobacteria most often appear bright green, but can also present in shades of blue, red, pink, purple, or milky white. HABs vary in appearance, with descriptions including spilled paint, pea soup, floating scums, mats, sheens, clumps, and streaks. Although the presence and concentration of toxins cannot be confirmed without proper testing, the most toxic HABs typically take the form of scums or mats. In addition to being an eyesore, these blooms can emit an odour comparable to rotting eggs.

Algae blooms are a collection of living organisms that move with currents and evolve over time. Because of this, the look, size, and even toxicity of a bloom can change quickly. While HABs most commonly form in warm, nutrient-rich lakes, due to the adaptability of cyanobacteria, they can also be found in cold water, nutrient-poor areas, and in rivers and streams. 

 If you detect a suspected harmful algal bloom:

  1. Stop drinking, using, and swimming in the water.
  2. Don’t let pets near the water.
  3. Submit a report online or contact the Spills Action Centre 24 hours a day, 7 days a week: Toll-free: 1-866-MOE-TIPS (663-8477).

Negative Impacts of Algal Blooms

Algal blooms can have negative impacts on the environment, economy, and human health. 

When algae bloom, there are millions of individual organisms reproducing and then dying off. When the algae die they sink to the bottom of the lake, and are consumed by bacteria. These bacteria use up the oxygen in the water under the bloom, resulting in an under-oxygenated area – a ‘dead zone’ where other creatures can’t survive. 

HABs pose a health concern because they contain cyanotoxins. There are different types of cyanotoxins such as hepatotoxins (impacts the liver), neurotoxins (damages nerves), and dermatoxins (affects the skin). 

Humans, pets, and livestock can all be exposed to the harmful effects of algal blooms through ingestion, contaminated drinking water, and recreational activities like swimming and fishing. Cyanotoxin infection can cause skin irritation, respiratory irritation, gastrointestinal illness, neurologic symptoms, and can be harmful to the liver or kidneys. Health effects and severity of symptoms depend on duration and frequency of exposure, amount of toxin in the bloom, and personal risk factors such as age and medical conditions. 

Pets affected by cyanotoxins can exhibit symptoms of vomiting, fatigue, shortness of breath, coughing, and potentially liver failure or respiratory paralysis. Because of these severe health concerns, it is important to take proper precautions around HABs. 


  • Avoid physical contact with scums, mats, and discoloured water
  • Look for and follow posted advisory warnings
  • Follow safe fish consumption guidelines
  • Seek healthcare immediately if you become sick
  • Report HABs related illnesses to the health department


  • Rinse off animals following any water contact 
  • Provide clean water for animals to drink
  • Do not allow pets to eat any dead animals or fish found near blooms
  • Seek veterinary care if animals become sick

Maintaining a Healthy Balance

Algae are a critically important, natural part of aquatic ecosystems. However, the presence of blooms can result in a variety of negative consequences. The following are some examples of actions you can take to limit nutrient pollution and reduce the intensity and frequency of blooms.

  • Plant a buffer of native shrubs and wildflowers along the shoreline to reduce erosion, absorb nutrient runoff, and provide beauty to your property.
  • Create a rain garden to absorb water, naturally filter runoff, and provide habitat.
  • Don’t remove aquatic plants! Aquatic plants compete with algae for light and nutrients and regulate oxygen-nitrogen exchange.
  • Use natural fertilizers like coffee grounds, eggshells, and other composted organic matter in place of nitrogen and phosphorus-based fertilizers.
  • Share this information with your neighbours!

For more information on how you can help protect and enhance water quality, check out GBB’s Life on the Bay Stewardship Guide

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