Don’t let its murky water or overabundance of weeds and algae fool you. Lake Mendota is completely safe for swimmers, according to the Madison Department of Public Health.

During the summer months, Mendota’s seven beaches are tested weekly, largely by Department of Health officials, to ensure bacteria levels are not potentially harmful to health.

Lake Mendota is frequented by University of Wisconsin students and Madison residents in the summer months.

Some UW students are turned off from participating in these activities while they sweat and sublet in the summer because of what they say is the murky water, unpleasant smell and large amount of algae in the lake.

Kirsti Sorsa, an environmental technical supervisor for the Public Health Department, said there are only about two times per summer when a beach has to be closed due to high levels of bacteria.

“It is usually very isolated,” Sorsa said.

During the period when a beach is closed, Sorsa said the department posts signs on the beach and informs media outlets of its closure. The department also posts test results on their website.

According to Sorsa, the most common cause of high levels of bacteria is after heavy rain when contaminates are washed into the lake.

Director of Environmental Health for Dane County James Clark said an increase in fecal coliform and E.coli, the most commonly found bacteria in the lake, usually occurs between two and five days after rainfall.

Clark said ingesting Mendota’s water during times of high level bacteria counts could cause flu-like symptoms such as headache or nausea, but is unlikely to cause serious health problems.

Swimmers itch, caused by a parasite, is much more common in lakes but does not pose a health risk, according to Clark.

The bacteria originate in the intestines of warm-blooded animals, and rain washes the sewage and other runoff from farms into the lake, Clark explained.

Similarly, the increase in algae is caused by nutrients, like nitrogen and phosphorus, which are washed into the lake from farms and other property as well as creeks and streams that feed into the lake.

The nutrients, which are most often found in fertilizers, cause more algae and weeds to grow around the shoreline. Also, the higher temperature of the shallow water makes the environment right for growth to “explode.”

“When you have that kind of organic matter growing, it takes up a lot of oxygen,” Clark said.

When weeds and algae die and sink to the bottom of the lake, they use oxygen when they decompose, a maturation process of the lake known as eutrophication.

Sue Jones, Dane County watershed management coordinator, said several government agencies are working to improve the runoff quality of storm water.

Currently, the public-health department is working on a two-year project funded by the Environmental Protection Agency to find more risk factors to water quality and to create a predictive model for beach conditions.

Jones said there are also city and county ordinances aimed at protecting Madison’s lakes. The city’s spills ordinance, for example, makes it illegal to dump materials such as paint or oil down storm drains.