Swimmers and shoreline users are being warned to be careful along the Lake Mendota shore after toxic algae was discovered in the water. Toxic blue-green algae blooms have been spotted on Lake Mendota near the Memorial Union and Hoofers, according to a news release from UW-Madison. The cyanobacteria in the algae blooms can cause very serious health risks, said Public Health Madison and Dane County. Warning signs have been posted in the shore area.