(Jared Strong/Iowa Capital Dispatch)

The water at a Storm Lake beach this week had about 50 times the amount of bacteria that triggers a swim warning from the state, according to the Iowa Department of Natural Resources.

The cause? Geese feces, said Kim Woltman, the city’s public services program manager.

“We do have a heavy goose population here, and I’ve noticed (the parks are) pretty thick lately with them,” she said. “Their babies have all started coming, and they’re hanging around pretty close.”

Sampling at the beach at Frank Starr Park on the lake’s northwest side on Tuesday revealed more than 11,000 viable bacteria in less than a half cup of lake water. A one-time test that measures 235 can result in a state swim warning.

Animal feces can be washed into a lake by rainfall and contaminate the water, and bacteria concentrations can rise and fall significantly over the course of several days.

An example: Beach water at Twin Lakes in western Iowa had a concentration of 24,000 bacteria per 100 milliliters last week, which plummeted to 430 this week.

Because the Storm Lake beaches are not managed by the state, the DNR does not issue swimming warnings for them. Woltman said the city’s beaches have signs posted year-round that warn about potentially elevated bacteria concentrations, which can cause illnesses and infections.

Two other beaches on the south shoreline of Storm Lake — Casino and Bel Air — also had elevated bacteria concentrations but were far less than Frank Starr.

The DNR tests the state’s lakes once each week for elevated bacteria and their toxins. There are three where swimming is not recommended this weekend:

• Backbone State Park in Delaware County.

• Beeds Lake State Park in Franklin County.

• Prairie Rose State Park in Shelby County.

There were eight state beaches with swim warnings last week, including Backbone and Beeds.

This article first appeared in the Iowa Capital Dispatch, a sister site of the Nebraska Examiner in the States Newsroom network.

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