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's BestNovember 10

How To Honor Veterans Day in Madison

Wisconsin is the only U.S. state that doesn't recognize Veterans Day as a state holiday. Discover why it isn't an official state holiday,...

People in historical outfits parade on a street waving flags.
Madison NewsNovember 9

What Dems Want To Do With Their New Legislative Trifecta

With full control of the state government for the first time since 2010, Wisconsin Democrats prioritize lowering costs, increasing educat...

Bronze statue with one arm raised in front of the Wisconsin State Capitol
Food & DrinkNovember 5

The Best Breakfast Sandwiches in Madison

Madison restaurants have tons of options for a great breakfast sandwich in town, whether you want a quick grab-and-go sammie from Bagels...

An egg, bacon and spinach sandwich on a white table.
Madison NewsNovember 4

All the Latest Results From Election Day

Find Election Day results, vote counts, and race outcomes for key Madison races.

A man speaks at a microphone in front of a blue backdrop
Madison, ExplainedNovember 3

It’s Election Day! Here’s What You Should Know

Voting occurs on November 3, 2026. Find Madison polling locations and where to look for results.

Two photos of men in blue Milwaukee Brewers gear talking into microphones.
Madison's BestOctober 30

Our Favorite Photos of Your Pets in Their Halloween Costumes

Halloween is for everyone in Madison, that is why we are showing off our favorite pet halloween costumes.

A collage of pets in halloween costumes: Starting at the top right and going left to right across the top and then the bottom: a beagle in a ghost bandanna, a cavapoo as a pineapple and a hotdog, a cat dressed as a burglar, a bunny with red wings, a cat dressed as a rocker, a dog dressed as Waldo, a cat with a brewers helmet on, and a cat in a bee costume. All of these pets are very cute and against a blue background.
Madison's BestOctober 30

What To Do in Madison in November 2026

Discover the best things to do in Madison, WI this November - from cheering on your favorite sports team to celebrating your favorite pod...

A crowded bar with dim red lighting.
Food & DrinkOctober 29

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...

On the left a photo of the exterior of Waltzing Matilda’s, which is blue with a pink door, on the right a photo of the interior bar which is well lit with mosaic tiling, and plenty of paintings on the wall.