Iowa DNR Issues Algal Bloom Alerts for Clear Lake, Six Others

The Iowa Department of Natural Resources has posted health advisories at seven state park beaches due to elevated cyanotoxin levels from blue-green algae blooms, including Clear Lake near Mason City, Black Hawk Lake in Sac County and Green Valley Lake in Union County. Microcystin concentrations exceeded the state's threshold of 8 micrograms per liter in composite samples, triggering yellow "WARNING" signs that read: "Concentrations of toxins produced by blue-green algae currently exceed acceptable guidelines for recreational use. Swimming is not recommended.". The advisories, first reported by the Globe Gazette, advise keeping children and pets away from discolored water and scum, and seeking medical help for sudden illnesses.

These blooms thrive on excess nutrients like phosphorus and nitrogen from farm runoff, tile drainage and manure, particularly in Iowa's Des Moines Lobe region where nitrate levels in lakes rank high nationally. Visible as paint-like scum, they have grown more severe, with historical data showing 25 microcystin advisories in one summer surpassing the 2013 record of 24. The DNR monitors weekly during recreation season per U.S. EPA guidelines, testing for microcystin—the most common cyanotoxin in Iowa waters—that can cause gastrointestinal distress, rashes or liver failure in high doses.

Clear Lake, drawing thousands of boaters and beachgoers yearly, joins others like Kent Park—where levels hit 12.5 µg/L, just below advisory but under watch—in highlighting statewide risks. Prevention through reduced nutrient pollution remains key, as blooms deplete oxygen, spur fish kills and impair recreation across Iowa's 12 million annual lake visits. The DNR's beach monitoring site provides real-time updates, emphasizing avoidance of algal mats even post-advisory.