MADISON — Receding floodwaters, sunshine and warming temperatures has created a resurgence of bacterial algae blooms on Madison's lakes which can be hazardous to health.
The Madison and Dane County Public Health Department observed bacterial algae blooms Monday at James Madison Park on Lake Mendota and B.B. Clarke Beach on Lake Monona, said Kirsti Sorsa, the department's program manager.
The hazardous blooms are caused by an overabundance of the nutrient phosphorus. They often occur at this time of year, when water begins to cool and sink. Nutrients that have built up near lake bottoms may rise to the surface and create cyanobacteria, also known as blue-green algae. Phosphorus is also often carried off farm fields and city streets by rain.
Monday's blooms were likely the result of storm runoff following heavy rains that fell across the area's drainage basins Aug. 20, she said. The algal scum could spread considering this week's forecast of higher temperatures and calm winds, the Wisconsin State Journal reported.
"We often see cyanobacteria blooms after storms transport nutrients into the lakes," Sorsa said.
It would be difficult to differentiate between bacterial colonies washed in from recent rain or bacteria that had been in the lake all along, said Richard Lathrop, a University of Wisconsin-Madison expert on freshwater lakes.
"There's a lot of nutrients in the bottoms of the lakes already," Lathrop said.
Bacterial outbreaks caused a record number of beach closings in Dane County this summer.
Residents shouldn't go into water with suspected algae outbreaks because the toxins can cause digestive, respiratory and neurological sickness, as well as skin rashes, health officials said.