• Original Reporting
  • Sources Cited
Original Reporting This article contains new, firsthand information uncovered by its reporter(s). This includes directly interviewing sources and research / analysis of primary source documents.
Sources Cited As a news piece, this article cites verifiable, third-party sources which have all been thoroughly fact-checked and deemed credible by the Newsroom in accordance with the Civil Constitution.
Algae growth at Washington Park's Grasmere Lake on Aug. 22, 2023. Not all algae growth is toxic, and this park lake is not meant for swimming or boat recreation. But residents can ask Colorado health officials to test the water. (Michael Booth, The Colorado Sun)

Colorado’s current heat blast is bound to grow more algae blooms in public lakes and reservoirs, state health and wildlife officials say. But the 10 warnings for toxic algae posted so far this year do not yet constitute a disaster compared to other years of blooms, looking at running lists of public access closures kept since 2019. 

Still, higher average temperatures for Colorado resulting from climate change will bring to state lakes more algae blooms that can be dangerous to human swimmers and pets. 

Here’s what you need to know about toxic algae in Colorado, based on conversations with Aimee Konowal, watershed section manager for the Colorado Department of Public Health and Environment, and department scientist Sarah Erickson: 

What’s the difference between “green floating stuff” and “toxic”?

Many bodies of water in Colorado can develop floating green clumps of algae; the season used to be called the “dog days” and was associated with August, but as summer temperatures rise algae is seen as soon as the 90s hit in June. 

Some of that green algae is just unsightly sludge. But certain strains can grow into toxins that are irritating to the skin and cause gastrointestinal symptoms if ingested. State health officials say the toxic strains tend to look like someone has poured paint or pea soup on the water, producing oily films that can be shades of red, orange, turquoise or other green-adjacent colors

Algae is in the news a lot, is it a big danger?

You don’t want to put this particular algae in a smoothie, but it’s easy to avoid if you’re paying attention, to liberally interpret the state health answer to that question. 

“So dogs are at risk especially because they have smaller body size and they tend to drink the water and sometimes eat that algal scum,” Erickson said. “It can make people sick. Kids are playing in the water and may be ingesting it. You can get gastrointestinal illness; you can get skin rashes.

A tennis ball floats in water
Smith Lake, seen at Denver’s Washington Park Aug. 21, is sustained by re-use water. The water contains high nutrient loads which contribute to productive algae and surrounding vegetation. (Olivia Sun, The Colorado Sun via Report for America)

“You can have respiratory symptoms. So, some various things that can be caused by algae. It’s not something you want to jump into.”

Is toxic algae getting worse in Colorado? 

Not really. 

The upward trend in average summer highs from climate change will slowly push up the number of toxic algae incidents in public lakes, experts say. But looking at the statistics of state warnings and beach closures does not show any alarming increase.

The health department coordinates with the Colorado Parks and Wildlife division to monitor, test and communicate with the public about bodies of water suspected of hosting algae blooms. Here are the number of cautions and warnings issued by the health department since they started keeping a dashboard in 2019: 

2019 – 6

2020 – 13

2021 – 20

2022 – 11

As of Friday – 10

“It’s certainly been a busy season so far, but also on par” with other years to date, Erickson said. 

More incidents, or more people watching? 

Toxic algae is a victim of its own skilled publicist. More and more people each year are using popular Front Range water bodies that are frequent hosts of algae blooms, such as birding-friendly Barr Lake State Park, or boating and swimming mecca Cherry Creek State Park. And more people are seeing the state’s education and warning notices on social media. 

“One thing to take into account is the public’s awareness about toxic algae is increasing, which means that people are spotting it more and reporting it more,” Erickson said. “So we may be seeing an increased number of reports” of blooms that were happening all along, unnoticed, she said. 

Is climate change a big algae boost? 

The answer is largely the same as with high-profile, intense weather incidents: While it may be impossible to attribute one particular bloom to climate change influence, the trend of such events is likely to increase as hotter conditions from global warming hit Colorado more days of summer. 

“It’s a little bit difficult to make specific conclusions,” Erickson said. “That being said, temperatures can definitely affect algae blooms, so that we can expect that warmer temperatures will likely see more blooms.”

As coastal waters off Florida hit 101 degrees this summer, threatening mass die-offs of important coral formations, threats to water bodies from climate change are gaining more prominence. 

The EPA’s warnings on algae and climate change note that warmer water is more stagnant, promoting toxic growth. The algae blooms absorb more sunlight than clear water, further increasing water temperatures. 

“Scientists predict that climate change will have many effects on freshwater and marine environments. These effects, along with nutrient pollution, might cause harmful algal blooms to occur more often, in more water bodies and to be more intense,” the EPA said. 

What can the public do to help? 

People should stay out of a lake when a warning is in effect, and keep their pets out as well. 

Lake fans who suspect an algae bloom has turned toxic can call the state hotline to prompt testing, if it’s not already underway.

Coloradans can contribute to slowing algae by avoiding over-fertilizing their lawns with phosphorus booster that runs off into water and feeds algae. Follow the directions. 

Clean up after dogs, and always follow backcountry advice about keeping human waste well away from bodies of water. It all contributes to prime algae conditions, even in high country lakes. 

Michael Booth is The Sun’s environment writer, and co-author of The Sun’s weekly climate and health newsletter The Temperature. He and John Ingold host the weekly Sun-Up podcast on The Temperature topics every Thursday. He is co-author with...