Study: Harmful algae blooms like it hot, but can occur in cold water

A bright algae bloom along a lake shore
A neon blue algae bloom is seen on Burnt Lake in the Boundary Waters Canoe Area Wilderness on Sept. 28.
Courtesy of Lienne Sethna

Harmful algae blooms, those thick, blue-green, oily layers of scum that have become more common on Minnesota lakes in recent years, are typically seen when water temperatures warm.

But a new paper challenges assumptions of what causes these sometimes toxic blooms. It documents more than three dozen cases of harmful algae in relatively cold water, including when there’s snow on the ground, and even, under ice.

“It's really counterintuitive to what we've understood about blooms in the past,” said Kaitlin Reinl, Research Coordinator at the Lake Superior National Estuarine Research Reserve in Superior, Wis. She collaborated with 27 other scientists through the Global Lake Ecological Observatory Network to author the paper published in the journal Limnology and Oceanography Letters.

New discoveries

In recent years, cyanobacteria, the stuff that forms harmful algae blooms, has been blamed for the deaths of several dogs in Minnesota. In 2014 it temporarily forced the shutdown of the public water supply in Toledo, Ohio.

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The algae is most commonly seen on lakes surrounded by homes and agricultural land, because the blooms are fueled by nutrients that run off into the lakes from lawns and farm fields. They often occur in calm conditions in mid-to-late summer when water temperatures spike.

Lately, scientists have been shocked to find them in lakes with very cold, very clean water: Lake Superior, and in the Boundary Waters Canoe Area Wilderness.

This new paper, called “Blooms also like it cold,” collected reports of 37 blooms in scientific journals, media articles, and personal accounts that occurred when water temperatures were below 15 degrees Celsius, which is about 60 degrees Fahrenheit.

A few of the blooms were in the Upper Midwest, including one in Minnesota, in Lake Itasca. Others were documented across North America and Europe.

Likely undercount

Reinl said researchers didn’t try to count every single instance of a reported cold-water bloom. And she said there’s definitely an “observer bias” in where the blooms were recorded, based on where researchers are located.

Scientists, for the most part, also aren’t actively looking for algae blooms in cold-water conditions. Most monitoring programs occur when it’s warmer and easier to gather data. So there’s likely a significant undercount of cold water cyanobacteria.

The point of the article, Reinl said, isn’t to challenge the fact that algae blooms “like it hot,” which is the name of an oft-cited study published over a decade ago.

Rather, it’s to challenge researchers to consider that some blooms also don’t seem to mind it when it’s cold.

“We don't want to create a blind spot with bloom ecology and our ability to manage blooms and steward our lakes, just because we have these preset assumptions that blooms only happen when you have high temperatures,” Reinl said.


In the paper, researchers propose several ways in which algae blooms can form even when the water is relatively frigid.

For example, cyanobacteria has developed adaptations in which they can form even in conditions with very low light, temperatures and nutrient levels. This is helpful in the winter, when there’s not a consistent influx of nutrients, the water is colder and ice cover can block sunlight.

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 the things we hypothesize,” said Reinl. “And so some of the next steps are to test those hypotheses in the lab and by collecting monitoring data.”

Bob Sterner, director of the Large Lakes Observatory at the University of Minnesota Duluth, says recent blooms in Lake Superior have been clearly linked to warmer temperatures.

“When it’s a warmer year we’re much more likely to see a bloom happen in Lake Superior,” Sterner said. “We tend to think that it’s a climate change driven problem that we’re just beginning to experience.”

Still, Sterner said the more scientists learn about cyanobacteria, it’s clear there are many different conditions in which harmful algae blooms can thrive, including when lakes are cool.

“So it’s really good to have this paper come out and help us appreciate the diversity of organisms out there.”