The conventional wisdom around harmful algal blooms is that they like it hot, ostensibly explaining why big blooms tend to show up in warmer months. However, a new study led by Wisconsin’s Lake Superior Reserve’s research coordinator, Dr. Kait Reinl, has shown that some of the cyanobacteria that fuel these blooms like it cold, persisting in temperatures below 15 degrees Celsuis—and even under ice.

“It’s really counterintuitive to what we’ve understood about these blooms,” says Reinl, who together with 27 scientists and the Global Lake Ecological Observatory Network authored the paper, which was published in the journal Limnology and Oceanography Letters. 

Reinl provided leadership for this collaboration, which included researchers from 3 continents, 9 countries, and 27 institutions. “This type of collaborative work helps to elevate the profile and expand the network of the Lake Superior Reserve and the Reserve System on a national and international level,” says Reinl.

Harmful algal blooms (HABs) have become more common around the globe in recent years. They often affect ecological and public health by blocking the light other aquatic plants and animals need to survive, depleting oxygen, changing food webs, and producing unpleasant odors and tastes, and straining local water treatment systems.

The study challenges assumptions of what is behind these sometimes toxic blooms, which typically were associated with warm water temperatures and high nutrient conditions. The study documented more than three dozen cases of harmful algae that occurred in water temperatures below 60 degrees Fahrenheit—and even under ice.

“These findings don’t contradict the fact that algae blooms like it hot,” says Reinl. “Rather, it’s to challenge researchers to consider that some blooms also don’t seem to mind the cold.”

Reinl points out that most monitoring occurs when it’s warmer and easier to gather data, making it likely that cold water cyanobacterial blooms are significantly undercounted. 

“Assumptions that blooms only happen when you have high temperatures is a potential blind spot in our ability to manage blooms and steward our lakes,” she says. “Without a foundational understanding of how blooms occur, we cannot identify management levers to control them or the most effective strategies for intervention, mitigation, and prevention.”

The study proposes several ways in which algae blooms can form in frigid waters. For example, cyanobacteria may have adapted to conditions with very low light, temperatures, and nutrient levels (also known as winter!). Blooms can also form when nutrients deep in lakes are brought to the surface when the water is mixed by big storms or underwater currents such as upwelling. And some algae may form when the water is warm, and then persist in the lake after the water cools.

“Those are some of the things we hypothesize,” said Reinl. “Our next steps are to improve monitoring to better document these blooms, test these ideas in the lab, and collect monitoring data.”

You can find the full open access article here.