While commonly associated with the summer, algal blooms can also persist into the fall. As of late October 2024, blooms were still present along the southern shores of Lake Erie.

In the warm weather, people flock to Ontario’s small lakes to swim, canoe or simply enjoy the tranquil shores. Every year, lakeside areas become bustling economic hubs, home to cottages, waterfront ice cream shops, cafes and restaurants that benefit from the scenic views they offer. However, these idyllic settings can become spoiled when smelly green algae wash up on the shore. Whether in the summer or fall, algal blooms interfere with our ability to fully enjoy time near or on the water.

Some blooms can even become toxic, which are harmful to people and even pets.

Our new research from the University of Waterloo suggests that a changing climate could be contributing to these declining water quality trends, and things may get worse in the future. This could result in the increased presence of unsightly and potentially harmful algal blooms in lakes across Ontario.

While algal blooms are an annual occurrence in places like the western basin of Lake Erie and parts of Lake Ontario, research suggests that they will appear more frequently in both small and large lakes across the province. Muskoka and the Kawarthas, known for their thriving cottage cultures, are at particular risk.

This is concerning because algal blooms can negatively impact the environment, human health and the economy. It can also require expensive water treatment. Blooms limit the use and enjoyment of lakes and rivers for swimming, boating and fishing. These blooms can, in turn, result in fewer visitors and loss of economic benefits for local industries.

Canada must get serious about tackling the causes of algal blooms in a warming world.

What causes algal blooms?

Algae are a natural and essential part of aquatic food webs. However, increased levels of nutrients such as nitrogen and phosphorus can create an imbalance in the lake that causes algae to grow very quickly into an algal bloom.

Nutrients make their way into water bodies from multiple sources across the watershed, including agricultural run-off, sewage and stormwater discharge, as well as phosphorus left over from past land use practices.

Algal blooms tend to happen later in summer because just like any plant, factors such as temperature and light are also important in determining how quickly algae can grow.

Eventually, algal blooms collapse when the conditions become unfavourable. The decaying algae then serve as a food source for bacteria. This is when you may notice a rotten egg smell. The multiplying bacteria also suck oxygen out of the water which can suffocate fish and other aquatic organisms.

It is well known that climate change is exacerbating the intensity and frequency of algal blooms in water bodies worldwide. Scientists commonly understand that a warming climate results in reduced ice cover, rising water temperatures and an increase in nutrient runoff due to more frequent and severe storms.

What is less well known is how climate change affects winter conditions, which can lead to more algal blooms in the warmer months. Our research aimed to shed further light on this question.

Increasing levels of phosphorus

Our research analyzed 300 streams in the Great Lakes basin. We found dissolved phosphorus concentrations to be increasing in every river studied, with concentrations almost doubling in some rivers over the past decade.

Dissolved phosphorus is particularly concerning because in this form, it is most accessible to algae. While we anticipated increases in agricultural areas, we observed increasing concentrations in urban and forested watersheds as well. Interestingly, the highest increases we saw came from forested areas.

Our models also revealed that the rate of increase was higher further north, likely because the climate is warming faster in these regions.

When climate change results in warmer winters, the soil thaws earlier, releasing phosphorus (or nitrogen) that would otherwise remain bound until spring and summer. But the plants are still dormant during the winter so the phosphorus released runs off into streams and lakes, where it sits until temperatures rise, providing ready fuel for the algal blooms. This differs from the past when phosphorus wasn’t released until spring and summer, allowing plants to absorb it for growth and preventing it from running off into rivers.

This means that not only will we see algal blooms occur earlier in the season and more frequently, but also in lakes across the Great Lakes basin, including previously unaffected water bodies such as reservoirs and small pristine lakes.

Blooms are already emerging in unexpected places like Lake Superior, where they were once unheard of. This is troubling news for Ontario’s beloved cottage communities.

To limit algal blooms, we must not only reduce agricultural fertilizer runoff but also cut greenhouse gas emissions to protect the water quality of Ontario’s thousands of small lakes. Proactive measures are essential.

By adopting innovative management strategies and committing to reducing carbon emissions, we can preserve the beauty and health of our lakes and beaches for future generations.