City Cast Madison logo

🌊 The Muck in Our Lakes

Posted on August 16, 2023   |   Updated on September 30, 2025
Hayley Sperling

Hayley Sperling

Blue-green algae is a type of bacteria that grows toxic blooms in our lakes. (Photo by City of Madison)

Blue-green algae is a type of bacteria that grows toxic blooms in our lakes. (Photo by City of Madison)

Being on an isthmus, Madisonians are surrounded by water. But unfortunately, because of the literally freezing-cold winters and other environmental factors, our time on the water is limited. This time of year, one of the biggest threats to our beaches is blue-green algae.

What Is It?

Despite its name, blue-green algae is a type of bacteria. It can grow quickly forming “blooms” full of toxins that can be harmful to people and animals. In humans, symptoms can include stomach upset, rashes, and respiratory irritation.

Blue-green algae may not always be blue-green. Sometimes blooms appear as reddish-purple or brown. Oftentimes, blue-green algae will look like spilled paint on top of the water.

Why Is It Here?

Blue-green algae thrives in warm, nutrient-rich water. In the case of Madison’s lakes, phosphorus runoff from fertilizer has contributed to the growth (and stench) of algal blooms.

The Madison Metropolitan Sewerage District is tasked with cleaning up the phosphorus in our city’s water but it’s not a simple job.

How To Avoid It.

Public Health Madison and Dane County tests the waters at Madison’s beaches from Memorial Day to Labor Day and lists the open status of Madison beaches on its website.

And in general, always look at the water around you before you get in and don’t swim in water that looks like "pea soup." Don’t let your pets swim in it either.

If you see a blue-green algae bloom, PHMDC wants to hear from you.

Share article

Hey Madison

Sign up for your free daily toolkit to get the most of the city you love.

Can't subscribe? Turn off your ad blocker and try again.

Urban Almanac

See All
Urban AlmanacJune 11, 2025

When Is It Safe to Swim in Madison’s Lakes?

Summer is here and it's time to take a swim! But there are a few things to keep in mind before diving into one of Madison's lakes.

Two people swimming in a lake during sunrise.
Urban AlmanacMay 28, 2025

How To Keep Madison’s Wild Baby Bunnies Safe

The Dane County Humane Society’s Wildlife Center is receiving a lot of calls about Eastern Cottontail rabbits as people are finding more...

Gray Eastern Cottontail bunnies in a nest.
Urban AlmanacMarch 31, 2025

How To Take Care of Indoor Plants This Spring

Are you wondering how to take care of your indoor plants this Spring? Or thinking of getting some new ones? We dive into plant ideas and...

Collection of monstera and other indoor plants under a warm lamp taken from above
Urban AlmanacOctober 9, 2024

How a New Tool Helps Reunite Pets With Their Owners

A new Madison Police Department microchip scanner will help pet owners reunite with their lost dogs and cats.

Two women hold a bulldog and an electronic device.
Urban AlmanacOctober 2, 2024

Take a Break From the Rake!

Fall is here! Soon, you’ll be serenaded by the sound of your neighbor’s leaf blower, and wonder if it’s time to get out your own, or a ra...

The feet of a woman in a blue coat kick leaves in the forest.
Urban AlmanacSeptember 25, 2024

Why the Cows Come Home To the World Dairy Expo

The cows come home to Madison next week as the World Dairy Expo returns to the Alliant Energy Center Oct. 1 to Oct. 4. You might have hea...

A group of cows line up in a stadium on a green floor.
Urban AlmanacSeptember 18, 2024

Hang Out At Madison’s New Lakefront Porch Park

A lot of people passed by the Madison Water Utility reservoir at 201 S. Hancock St. and just saw concrete. But others saw an opportunity...

Tables and chairs sit on a wooden outdoor platform.
Urban AlmanacSeptember 11, 2024

A Sheep Way To Help Solar Projects

It’s a farm friendship as unlikely and fruitful as Charlotte the spider and Wilbur the pig – sheep and solar panels. Solar array projects...

Sheep graze in a field next to solar panels.

The latest in Madison

Madison, ExplainedSeptember 28

What To Know About The Proposed BECI Transmission Line

The 190-mile BECI high-voltage transmission line aims to secure Wisconsin’s grid, but locals and the Dane County Board are demanding dela...

Two large electric towers stand next to a white house.
Food & DrinkSeptember 24

The Best New Restaurants in Madison

This month’s list of the area’s best new restaurants includes a new outpost for a wildly popular local pizza chain, a new sushi spot near...

People eating at booths in a restaurant with a glassed-in kitchen at the rear.
Madison's BestSeptember 23

‘Park People’ Will Satirize Madison’s Protest Culture

Madison filmmaker Corey Holl launches a crowdfunding campaign for his new political satire, "The Park People." Explore the proof-of-conce...

Two production staff members on the left hold equipment and a microphone for two actors sitting on a park bench being filmed.
Madison, ExplainedSeptember 22

Know the Constitutional Amendments On Your Fall Ballot

Wisconsin's Nov. 3 election features key governor and legislative races—plus three Republican-backed constitutional amendments. Learn wha...

Multiple red, white, and blue "I Voted" stickers are scattered on a white table.
Madison, ExplainedSeptember 21

Big Changes Planned For Madison’s Breese Stevens Field

As Breese Stevens enters its second century, its owners plan to increase the number of concerts and events. But some neighbors of the fie...

A band playing on an outdoor stage in front of an audience.
Madison, ExplainedSeptember 15

The House on the Rock Is Even Weirder Than You Think

Located about an hour west of Madison, The House on the Rock is one of Wisconsin's most infamous, and certainly oddest, tourist attractio...

People in a narrow room covered in glass.
Madison, ExplainedSeptember 15

How To Guard Against Ticks in Wisconsin This Fall

Tick season isn't over yet. Learn what to expect from fall tick activity in Dane County and get expert tips on how to protect yourself an...

A close-up photo of a small eight-legged insect on a human arm.
Madison NewsSeptember 14

How the Historic Meta Settlement Will Impact Wisconsin

Get the details on Wisconsin’s $200M share of the historic $17B Meta settlement as Attorney General Josh Kaul joins City Cast Madison to...

Wisconsin Attorney General Josh Kaul stands at a podium