(Iowa Capital Dispatch) – Labor Day weekend marks the unofficial close of lake-recreation season, and the official end of the Iowa Department of Natural Resources’ weekly beach monitoring program for summer 2025. This summer, 28 state park beaches, out of 41 monitored, had at least one weekend where swimming was not recommended due to high levels of E. coli or microcystins, a toxin released by certain types of algae.

For the final week of monitoring, swimming is not recommended at 15 state park beaches. The majority of these beaches have excessive levels of E. coli present, and one beach has excessive levels of algal toxins, according to DNR’s beach monitoring database. The “swimming not recommended” advisories are not a beach closure, but rather a warning to recreators that the beach may carry a higher risk of causing things like diarrhea, nausea or other acute symptoms than can result from exposure to E. coli and microcystins.

A single water sample taken at the beach must exceed E. coli measurements of 235 colony forming units, or CFU, per 100 milliliters, for a swimming advisory to be issued. An advisory is also triggered if the mean of five samples over a 30-day period exceeds 126 CFU/100 mL. At these levels, an estimated 36 out of 1,000 swimmers will experience minor illnesses. Single samplings at some state park beaches this summer had E. coli concentrations as high as 24,000 most probable number, or MPN, per 100 mL. MPN is a statistical calculation of the number of colonies rather than a direct count.

Microcystins are toxins produced by certain green-blue algae that typically have a paint-like or oily appearance on the water. The blooms occur in nutrient dense water. This summer five state beaches had microystin levels in excess of 8 micrograms per L, which is the level that triggers an advisory.

There are 15 weeks between Memorial Day weekend and Labor Day weekend. Three state beaches, at Backbone State Park, Beeds Lake State Park and Pine Lake State Park, had swimming advisories for all but three of the weeks. 13 beaches had no swimming advisories this summer due to water quality, this number includes the beach at Lake Keomah State Park, which was not monitored this summer due to ongoing renovations.

This means more beaches had adverse water conditions this summer than last summer. Iowa Environmental Council follows the DNR beach monitoring data each week and found that in 2024, 16 state park beaches did not have an advisory in place all summer.

IEC reported in 2024 that Green Valley Lake, Honey Creek, and Lake Anita were the only monitored state beaches that had never had an E. coli related advisory. This statistic holds true after this summer’s beach monitoring season, though the beach at Green Valley Lake had one advisory for microsystins.