Four beaches on Lake Monona and Tenney Beach on Lake Mendota were closed Wednesday because clouds of blue-green algae have left water unsafe for swimming.

The beaches closed on Lake Monona are BB Clarke, Olbrich, Hudson and Olin.

Jennifer Lavender Braun, a microbiologist with Public Health Madison and Dane County, said the toxic algaes persistence is similar to what she has been seeing all summer.

The week of June 16 saw the worst outbreak of algal blooms in decades, said Jake Vander Zanden, an aquatic biologist at the UW Center for Limnology, and Stephen Carpenter, limnology center director.

Braun said several days of rain which had persisted through the end of June can cause the algae to develop, and subsequent sunshine can allow it to thrive.

With all the rain weve had, that gets the nutrients in the water, Braun said. But the last couple of days with heat and sun creates conditions where algae takes over.

Braun said that although rain is what brings algae to the lakes in the first place, a good rain can also help the algae dissipate.

Despite the beach closures, there are still options for swimmers, she said. Several beaches, especially around Lake Mendota, remain open. Governor Nelson State Park, James Madison, Marshall, Memorial Union and Warner beaches are safe for swimming.

Public Health will be checking the beaches with blue-green algae every day for the rest of the week.

Blue-green algae can cause upset stomach, rashes and respiratory infection.

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5 Madison beaches close Wednesday due to blue-green algae - Madison.com