A visual assessment of Lake Mendota’s waters at those beaches on Tuesday showed blue-green algae blooms, Public Health spokesperson Morgan Finke said. The algae — photosynthetic bacteria that grow in warm, nutrient-rich lakes, ponds and streams — make the waters unsafe for both humans and animals, Finke said in a phone interview.

Madison’s 18 other beaches that are open remain safe for swimming, according to the department’s website.

Memorial Union and Spring Harbor’s closings mark the second and third beaches to be closed for bacterial reasons this year: Frost Woods Beach closed on June 2 after a sample showed elevated levels of E.coli, Finke said.

The closings are not necessarily abnormal, Finke said: Rain followed by heat can create a “perfect storm” for algae blooms. Showers on Sunday were followed by warm conditions early this week, with temperatures reaching 76 degrees on Tuesday. Algae blooms typically occur between mid-June and late September, according to the Wisconsin Department of Natural Resources.

Last year, Madison’s first blue-green algae bloom came in early June.

The algae blooms present a heightened risk for dogs, Finke said. “They’re most likely to be the ones to swallow a lot of water, their bodies are smaller,” she said. “Even a little bit of blue-green algae in their system can create a dangerous situation for them.” In a statement last week, the department encouraged swimmers to stay away from water if they notice algae blooms, which make the lake look like “pea soup” or create a scum layer on the surface.

“Your eyes are your first indicator that you need to be careful,” Finke noted.

While the concrete beach at the union was closed, the pier appeared open Wednesday. Josh Buhr, a 2022 UW-Madison graduate fishing on a nearby pier, said in days past he had pulled in a number of small fish such as perch and bass from the same spot but had no luck on Wednesday. The day before, he said, the water went from “clear to disgusting — something flipped.”

Lukas Schorr, 18, of Verona, who was also fishing on the nearby pier Wednesday afternoon said the fishing was poor. “But I don’t know if it’s the blue-green algae,” he noted. “It may be user error.”

The presence of the algae likely stems from increased concentrations of nutrients such as phosphorus and nitrogen, according to the DNR.

The chemicals can seep into the water from agricultural activities, untreated sewage and phosphorous-based fertilizers and detergents.

Depending on the weather, algae levels can take between a day and a week to normalize, Finke said. Wednesday’s heavy rain will likely wash away the bloom from the shoreline, but Public Health will test for toxin levels Thursday if a bloom is still visible, she said.