Climate change is causing algal blooms in Lake Superior for the first time in history

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.

Cyanobacteria 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 microcystins and other cyanotoxins, that can adversely affect humans and the environment.

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 proven fatal to pets and livestock.

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 plagued by harmful algal blooms.

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 500,000 residents.

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 northern location and remote population. Cold water temperatures and low nutrient concentrations have historically inhibited the growth of algae.

At the same time, Lake Superior is one of the fastest-warming lakes 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 per cent of Lake Superior’s surface was covered in ice, one-fifth of a typical winter.

Less ice cover has led to a longer open-water season, resulting in warmer water temperatures and less oxygen during the summer. Longer and warmer summers provide optimal conditions for algae to proliferate and for cyanobacteria to bloom.

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

Evidence of cyanobacterial blooms along the northern shores of Lake Superior have been far more limited. Confirmed reports to the Ontario Ministry of the Environment, Conservation and Parks began in September 2019, 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 twice in August 2024, east of Thunder Bay, 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 toxin-producing.

Changing conditions

Reports of cyanobacterial blooms in remote, clear and healthy lakes and rivers suggest that climate change plays an important role in the proliferation of cyanobacterial blooms. Cyanobacteria are more tolerant of warmer water temperatures and can make themselves more buoyant, allowing them to out-compete other algae for light.

In the Great Lakes region, 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 large bloom in southern Lake Superior in 2018 stemmed from heavy rainfall and flooding.

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 reducing the likelihood of harmful algal blooms. Effective solutions to curtail harmful algal blooms 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.

This article is part of The Conversation's series Our lakes: their secrets and challenges. This summer, The Conversation and La Conversation invite you to take a fascinating dip in our lakes. With magnifying glasses, microscopes and diving goggles, our scientists scrutinize the biodiversity of our lakes and the processes that unfold in them, and tell us about the challenges they face. Don’t miss our articles on these incredibly rich bodies of water!

By Sapna SharmaProfessor, York University Research Chair in Global Change Biology and director of the United Nations Institute for Training and Research Global Water Academy, and Joshua Culpepper, a postdoctoral researcher in York's Department of Biology

This article is republished from The Conversation Canada.