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Lake Superior is known for its pristine waters, but a combination of nutrient additions from increasing human activity (including farming and development), warming temperatures and stormy conditions have resulted in more frequent blooms of potentially harmful algae.

thrive in freshwater systems with warmer water temperatures and elevated nutrient inputs, typical of highly urbanized and agricultural landscapes.

Cyanobacterial blooms have the potential to produce toxins, such as and other , that can .

These toxins can create an unpleasant taste and odor in water, interfere with water treatment, cause gastrointestinal issues and liver damage in humans and have even .

Unfortunately, as the planet warms, more and more parts of Canada are experiencing harmful algal blooms. Our team has set out to try and understand the extent of the problem in Lake Superior, and what can be done about it.

Blooms in the Great Lakes

Cyanobacterial blooms are no stranger to the Laurentian Great Lakes. Each summer, Lake Erie, the smallest and warmest Great Lake, is .

These blooms happen when hot temperatures occur in areas of fresh water where nutrient runoff drains. For example, Lake Erie is ringed with rich agricultural fields and urban development, the runoff from which often results in summer algal blooms. In 2014, a harmful algal bloom formed within the drinking water supply of Toledo, Ohio, affecting more than .

Until recently, cyanobacterial blooms were never recorded in Lake Superior.

Lake Superior is the largest, coldest and arguably the healthiest of the Great Lakes, owing to its . Cold water temperatures and low nutrient concentrations have historically inhibited the growth of algae.

At the same time, Lake Superior is one of the on the planet. In the past 150 years, Lake Superior has lost more than two months of ice cover. During the winter of 2024, only 12% of Lake Superior's surface was covered in ice, one-fifth of a typical winter.

Less ice cover has led to a , resulting in during the summer. Longer and warmer summers provide optimal conditions for algae to proliferate and for cyanobacteria to bloom.

along the southern shores of Lake Superior for the past decade. were first documented in 2012 and every year since 2016. In 2018, the largest bloom stretched over 100 kilometers, with reports that the waters turned an opaque green.

Evidence of cyanobacterial blooms along the northern shores of Lake Superior have been . Confirmed reports to the Ontario Ministry of the Environment, Conservation and Parks , northeast of Thunder Bay. Subsequent blooms were noted in July 2021 in Black Bay, a less developed area of western Lake Superior, again in July and September 2023 and , in the municipality of Shuniah.

While there is no evidence of toxicity in the blooms on the northern side of Lake Superior, sampling revealed that nearly all cyanobacteria species identified can generate toxins. However, it is not yet known under which environmental conditions cyanobacteria "switch" to become .

Changing conditions

Reports of and rivers suggest that climate change plays an important role in the . Cyanobacteria are more tolerant of and can make themselves more buoyant, allowing them to out-compete other algae for light.

In the , climate change is also contributing to more frequent and intense storms. Strong precipitation events lead to high rates of water runoff that mix nutrients from the watershed into local water bodies. For example, the in 2018 stemmed from .

The public health risks from cyanotoxins underscore the importance of addressing global environmental degradation to prevent the proliferation of these species. These occasional blooms highlight the need for continued monitoring and public awareness to protect the health of Lake Superior's northern nearshore regions.

Limiting the amount of nutrients delivered to water bodies is critical to . could include reducing the use of fertilizer, changing the timing of fertilizer application to limit the amount of nutrients entering lakes and tributaries, promoting infrastructure in urban environments to reduce storm-water runoff and conserving wetlands and riparian vegetation.

To date, there have been . However, algal blooms can be difficult to track, in part because of Lake Superior's size, as well as the transient nature of blooms.

Community scientists can become first responders to observations of algal blooms. As Canadians head out to enjoy our lakes, we urge community members to report sightings of algal blooms to their local provincial environment offices to further our understanding of algal blooms in one of Canada's most iconic lakes.

Kirill Shchapov from the Freshwater Ecosystems Group at the Cawthron Institute helped contribute to this article.