Toxic blue-green algae prompted Madison and Dane County Public Health officials to close James Madison, Tenny and Warner beaches, all along Lake Mendota, Monday.
"While we see this algae in the lake every year, it's unusually abundant this year," Center for Limnology Director Steve Carpenter said. "So we are seeing an unusual, a somewhat unusual event this year."
Health officials said the algae has to be considered contaminated because it's likely phosphate pollution coming from farm manure and soil runoff.
"Now with this bloom under way, the main thing is for people to be careful about their health," Carpenter said. "Don't let your pets drink the stuff, if you happen to come in contact with the lake wash your hands."
Carpenter said compounding the problem is what is being called an invasive species killing most of the tiny algae-eating water fleas. So there's virtually nothing on the water to naturally bring the algae under control.