Madison's lakes are entering a productive season for blue-green algae blooms.
The blooms, also known as cyanobacteria, are caused by excess nutrients in lakes. They're often made worse by hot weather.
Earlier this spring, researchers predicted that the city's lakes would have an above normal year for these algae blooms, and beaches might be closed as a result.
Emily Stanley, a professor at the University of Wisconsin-Madison Center for Limnology and Department of Integrative Biology, said although they haven't yet seen large blooms she describes as "epic" in Madison's lakes, they are seeing frequent blooms. She said people should stay away from water that looks like it has white, blue or green foam floating on the top.
"If you see a gross, scummy bloom, don't go swimming and don't drink the water! That's the best advice I have," she said.
According to the state Department of Natural Resources, cyanobacteria can produce toxins that can be harmful if they are absorbed through the skin, inhaled through the air or swallowed.
It can be particularly harmful to children and pets, Stanley said.
"If you're running around the edge of the lake with your dog or your kid and you see that scum or those really conspicuous blue greens there, be cautious and probably don't go throwing tennis balls to the dog and have them swim through the water," she said.