Cyanobacteria Blooms: Be Cautious of Potential Toxicity

Article by Amy Baxter, M.MS, B.Sc.

Earlier this month, BCLSS volunteers reported a dense algal bloom on Nicola Lake (see photo below). Tragically, the bloom was suspected to be toxic as multiple dogs have become ill or died after being in contact with the lake (Holliday, 2025).

Unfortunately, this isn’t an isolated incident. Worldwide, there is mounting evidence that cyanobacteria blooms are increasing and are likely to continue increasing in response to continued eutrophication and climate change (Huisman et al., 2018). These blooms raise concerns for both ecosystem and human health. Not all cyanobacteria are toxic, and even species that are known to produce toxins do not always produce toxins (Algae Watch BC, 2021). However, all cyanobacteria blooms should be regarded as potentially toxic and should be avoided.

The BCLSS wanted to put together some information and resources about these blooms, how to identify and report them, and what can be done.

What Are Cyanobacteria/Blue-green Algae?

Despite the name, blue-green algae are not actually algae, they are a photosynthetic bacteria. They are distinguished from other bacteria by the presence of chlorophyll-a and their ability to photosynthesize like plants in the aquatic system (BCLSS, 2022). Cyanobacteria have been around for billions of years and play an important role in aquatic ecosystems. However, under favorable conditions (e.g., warmer water temperatures, high nutrient concentrations, stable conditions), they can multiply rapidly and form blooms (Algae Watch BC, 2021). And as mentioned above, some types of cyanobacteria produce toxins called cyanotoxins that can harm people, pets, livestock, and wildlife.

In addition to human and animal health risks, cyanobacterial blooms can also harm aquatic life by disrupting food webs, reducing oxygen levels in the water, and blocking sunlight needed by other organisms (Huisman et al., 2018). This can have a number of effects throughout lake ecosystems and can even lead to conditions that enhance the likelihood of blooms in the future.

What Are the Health Risks?

Contact with cyanotoxins can lead to skin, eye, nose and throat irritation, stomach pain, diarrhea, vomiting, headaches, muscle weakness and fever, as well as severe illnesses that may result in death, such as liver or kidney damage and paralysis of skeletal and respiratory muscles (Health Canada, 2024). Human deaths from cyanobacteria exposure are extremely rare, however children are more susceptible to severe illness due to their tendency to accidentally swallow water (Health Canada, n.d). Dogs and livestock are also particularly vulnerable because they may drink affected water or lick algae off their fur. The type and severity of illness depends on multiple factors such as the length of exposure, the type and amount of cyanobacteria or toxins present and how the person was exposed (through ingestion or skin contact) (Health Canada, 2024).

How do you Identify a Cyanobacteria Bloom?

Cyanobacteria can often look similar to harmless algal blooms and microscopic analysis of cells at a certified laboratory is the only definitive way to identify a cyanobacteria or algae bloom (Algae Watch BC, 2021). However, cyanobacteria blooms often look like pea soup, paint, or surface scum and may have an earthy or musty smell.

To learn more about recognizing algae blooms and for tools and resources please visit the Algae Watch BC site.

Are All Cyanobacteria Toxic?

Not all cyanobacteria are toxic, and even species that are known to produce toxins do not always produce toxins (Algae Watch BC, 2021). However, as noted above, all cyanobacteria blooms should be regarded as potentially toxic and should be avoided.

Laboratory testing can confirm if a bloom is toxic, however, there are limitations to testing. Cyanobacterial blooms are often patchy, so higher concentrations of cyanobacteria may be present in areas that weren’t surveyed (WSDH, n.d). Toxin levels can also vary depending on the stage of the bloom; sometimes toxins can appear during the active growth or other times during die off (Clement, 2017). Cyanobacteria can also move within the water column using gas vacuoles to optimize access to nutrients or light, so a bloom might not be visible at the surface at certain times of day, even though toxins are still being produced within the water (Clement, 2017). Additionally, some cyanotoxins can remain in the water column for days to weeks after the bloom has disappeared (WSDH, 2025).  All of these factors make cyanotoxin testing difficult, so a bloom should still be regarded as potentially toxic even if toxin testing performed comes back negative.

What Factors Lead to Harmful Algal Bloom Formation?

Though cyanobacteria is a natural part of freshwater ecosystems, harmful blooms can flourish under certain conditions including but not limited to:

• High nutrient conditions, nutrients like nitrogen and phosphorus are essential for primary productivity in lakes. However, when lakes receive excess nutrients, they can encourage algal blooms.  Excess nutrients can enter waterbodies through both natural and anthropogenic processes. Some of the ways humans can cause nutrient enrichment in lakes include: shoreline development, agricultural practices, urban and stormwater runoff, and wastewater effluent (Algae Watch BC, 2021).
• Warmer water temperatures, which are becoming more common due to climate change, tend to favor cyanobacteria over other types of algae due to competitive advantages such as growing faster than other algae at warmer temperatures and their ability to migrate up and down the water column to access nutrients in the deeper waters then migrate back up to grow in the warmer, upper layers where there are higher light levels. (U.S. EPA, 2025).
• Stable conditions, blooms are more likely to form under stable water conditions with low wind (Algae watch BC, 2021). When a lake is stratified (distinct layers form, find more about this in our LakeKeepers manual) it forms conditions favorable for cyanobacteria which often dominate in these conditions (Algae Watch BC, 2021). Additionally, changes in climate variability such as a drought periods following an intense precipitation event can result in longer water residence times (duration that the lake-volume equivalent of water remains in the lake), allowing for more time for bloom formation (Chapra et al., 2017).
• Other factors, changes in rainfall patterns and more frequent extreme weather events can lead to increased nutrient runoff into freshwater systems, further supporting bloom formation (U.S. EPA, 2025). Additionally, changes in salinity and increasing carbon dioxide levels can favor cyanobacteria blooms as well (U.S. EPA, 2025).

Climate change and land use impacts are making these conditions more common which can favor the growth of cyanobacteria. Blooms can also create a positive feedback loop which encourages future blooms to occur. For example; cyanobacteria blooms die off and sink to the lake bottom, where bacteria break down the decaying cells in a process that uses up oxygen. The depleted DO levels in the hypolimnion can have many implications for the chemical, physical, and biological characteristics of lakes. For example, under aerobic (oxygenated) conditions, much of the phosphorus in a lake is bound to iron, manganese and/or calcium contained in lake bottom sediments. When bottom waters become anoxic, chemical changes occur in the sediments that result in phosphorus being released into the water column. When the lake begins to turn over, the phosphorus is mixed into the photic zone where algae and plants can utilize it, sometimes resulting in blooms. This phenomenon is known as internal loading. Iron, manganese, and hydrogen sulphide (H2S) can also be released into the water during these anoxic periods (BCLSS, 2022). This internal nutrient recycling can sustain eutrophic conditions even after external nutrient inputs are reduced (Husiman et al., 2018).

What Can We Do?

Managing cyanobacteria blooms at a long-term scale requires addressing the root cause at a watershed level, which is often excess nutrients entering lakes from agriculture, wastewater and urban runoff (Husiman et al., 2018). In some cases, in-lake treatments have helped such as mixing the water, phosphorus binding clays, sediment capping or dredging, chemical controls (such as hydrogen peroxide treatment), or biological methods such as boosting natural grazers or adjusting fish populations can help on a case by case basis (Husiman et al. 2018). However, these strategies are highly situation dependent and will often only be sustained if coupled with nutrient reduction at a watershed level.

There are also steps we as individuals can take to reduce risk as well as likelihood or severity of the blooms.

• Learn more about algae blooms and stay vigilant. If you observe a bloom that you believe could by cyanobacteria and it is impacting drinking or recreational waters, it is important to contact the local health authority and stay away from the water. In addition, please submit your observations to Algae Watch BC. Algae Watch is an educational program for Citizen science data gathering with the goals to help people recognize and identify algae blooms and relies on citizen science to help water quality experts identify and better understand cyanobacteria and algae blooms in B.C.

In addition to learning more and reporting possible blooms there are ways individuals can reduce their impacts on lakes to help reduce the likelihood of blooms including:

• Maintaining healthy riparian areas
• Reducing fertilizer applications
• Maintaining septic systems and ensuring they abide by regulations
• Using phosphate-free products
• Agricultural landowners should consider environmental farm planning. More information on Environmental Farm Plans is available here.

Cyanobacteria are a natural part of BC’s lakes, but under the right conditions they can flourish and become problematic for human, animal and ecosystem health. By working together through science, stewardship, and smart land use practices, we can help protect the health of our lakes for generations to come.

References

Algae Watch BC. 2021. Retrieved on July 10, 2025 from: https://www2.gov.bc.ca/gov/content/environment/air-land-water/water/water-quality/algae-watch

BC Lake Stewardship Society (BCLSS) 2022. LakeKeepers Manual. A Guide to Lake Stewardship and Ecology in British Columbia. Retrieved July 13, 2025, from  https://www.bclss.org/programs#lake-keepers-manual

Chapra, S. C., Boehlert, B., Fant, C., Bierman Jr, V. J., Henderson, J., Mills, D., Mas, D. M. L, Rennels, L., Jantarasami, L., Martinich, J., Strzepek, K. M., & Paerl, H. W. 2017. As cited in BCLSS, 2022. Climate change impacts on harmful algal blooms in US freshwaters: a screening-level assessment. Environmental Science & Technology, 51(16), 8933-8943. https://pubs.acs.org/doi/full/10.1021/acs.est.7b01498

Clement, B. 2017. Harmful Algal Blooms – Special Sampling and Response Actions. United States Environmental Protection Agency. Retrieved on July 13, 2025 from  https://19january2017snapshot.epa.gov/region8-waterops/harmful-algal-blooms-special-sampling-and-response-actions

Health Canada. 2024. Recreational water and health: Cyanobacteria and their toxins. Retrieved on July 10, 2025 From https://www.canada.ca/en/health-canada/services/environment/recreational-water/cyanobacteria-toxins.html#a2

Holliday, K. 2025. Nicola Lake residents raise alarm about algae bloom after dogs get sick, die . Kamloops.net. Retrieved on July 10, 2025 from:  https://www.castanetkamloops.net/news/Kamloops/559803/Nicola-Lake-residents-raise-alarm-about-algae-bloom-after-dogs-get-sick-die.

Huisman, J., Codd, G., Paerl, H., Ibelings, B., Verspagen, J., & Visser, P. 2018. Cyanobacterial Blooms. Nature Reviews Microbiology, 16(8), 471–483. https://doi.org/10.1038/s41579-018-0040-1

United States Environmental Protection Agency (U.S. EPA). 2025. Climate Change and Freshwater Harmful Algal Blooms. Retrieved on July 13, 2025 from: https://www.epa.gov/habs/climate-change-and-freshwater-harmful-algal-blooms

Washington State Department of Health (WSDH). N.d. Blue-Green Algae (Cyanobacteria) Frequently Asked Questions. Retrieved on July 13, 2025 from https://doh.wa.gov/community-and-environment/contaminants/blue-green-algae/faqs