The Coralville Lake beach and boat dock near the Coralville Dam the evening of Friday, Aug. 14, 2015.
The Coralville Lake beach and boat launch near the Coralville Dam the evening of Friday, Aug. 14, 2015. Credit: Lyle Muller/IowaWatch

IOWA CITY — Johnson County beach-goers looking for a final summer swim before school starts should be aware of two unpleasant dangers — toxic algae and E. coli.

August is the peak month for the blue-green algae blooms that are an increasingly common threat to Iowa lakes during the summer. The blooms are caused in part by agricultural fertilizer, sewage and other sources that carry excess nitrogen and phosphorus into the water.

And they often carry microcystin — a toxin that can cause symptoms including skin irritation, rashes and vomiting. Exposure to higher concentrations of the toxin can cause serious liver damage and has been known to kill small animals, including dogs.

Local lakes and beaches in this eastern Iowa county including Lake Macbride, Coralville Lake, Terry Trueblood Recreation Area and F.W. Kent Park haven’t experienced blooms as frequently as other areas of the state, but the algae is still a threat, data show.

Kent Park has closed its beach twice this summer, for a week starting July 9 and for a week starting Aug. 6. Both times tests showed levels of microcystin exceeding the safety threshold of 20 micrograms per liter. During the week after July 31, the beach also issued a “Swimming Not Recommended” advisory for microcystin levels of 12.9 micrograms per liter.

Brad Freidhof, conservation program manager for the Johnson County Conservation Board, said when high levels are detected, it’s important to act quickly so the public is not exposed to danger.

“We try to get the signs up as quickly as possible,” he said.

Kent Park is more cautious than most, perhaps because they have had to issue warnings “more than a handful of times,” Freidhof said.

Typically, Iowa follows U.S. Environmental Protection Agency and World Health Organization standards in monitoring microcystin, which for recreational bodies of water places the safety threshold at 20 micrograms per liter. Most beaches won’t post warnings until toxins reach that level, and closures are rare.

Other local beaches have not experienced the same threat level from the algae this year.

“Microcystin levels have not reached levels of concern at Coralville Lake,” said Leo Keller, a hydrologist for the U.S. Army Corps of Engineers, which supervises testing for a number of Iowa lakes. The most recent test on July 27 showed microcystin levels at 0.1 micrograms per liter, well below the safety threshold.

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Likewise, Ron Puettmann, the Iowa Department of Natural Resources park manager for Lake Macbride, said no microcystin advisories have been issued for the lake this summer. A test on Aug. 4 showed toxin levels at 0.4 micrograms per liter.

The Iowa DNR tests for microcystin and for bacteria such as E. coli at state beaches, while local or private beaches can participate voluntarily. Testing for microcystin has been standard practice since 2005 and is generally done weekly during the peak season, from mid-April through October.


Coralville Lake is tested at Sandy Beach, West Overlook and Sugar Bottom, as well as in the center of the lake, because toxins might be stronger in certain areas, said Keller, who emphasized that toxins don’t just affect people swimming at beaches, but water skiers and boaters on the lake as well.

As a city park, Terry Trueblood Recreation Area is not required to test its waters for microcystin or other toxins, but Iowa City’s Superintendent of Parks and Forestry Zachary Hall said the park voluntarily participates in a weekly monitoring program with the DNR during the beach use season.

“We’ve always had the lowest, some of the lowest results in the state,” Hall said. The park did have an algae bloom in 2012 that caused microcystin to rise above 20 micrograms per liter — at which point the tests cannot measure toxin levels with the same degree of specificity — and data for other years wasn’t immediately available.

Lake Macbride. Photo taken Aug. 14, 2015.
Lake Macbride. Photo taken Aug. 14, 2015. Credit: Lyle Muller/

Macbride’s highest microcystin level was on July 5, 2011, when tests came back at 10.45 micrograms per liter — still below the advisory level — and by the next week, the level had dropped to 0.42.

However, warnings typically aren’t posted until after two rounds of testing — if the initial sample tests above the safety threshold then a re-sampling is done. If that sample is high, a warning will be issued. The turnaround time between the first test and a warning being posted is typically about three days.

“As the results come in, we are notifying the staff at the reservoir level so they can notify the public,” Keller said.

Dee Goldman, the U.S. Army Corps of Engineers operations manager at Coralville Lake, said officials post notices on beach bulletin boards, issue news releases and post updates on Facebook to keep the public informed. He said he would be prepared to close the beaches if the danger was great enough, although he hasn’t had to yet.

“If there’s a health issue or something like that, we would close it,” Goldman said, referring to the beaches on Coralville Lake.

However, the level of danger differs from person to person and can be higher for children, the elderly or animals.

“We’re concerned with people that may come out here that have health issues, that have young children,” Freidhof said.

Coralville Lake hasn’t had many algae blooms in part because the nutrients — such as nitrogen and phosphorous from agriculture, mowing and other sources — that help the algae form aren’t as much of an issue there.

“We don’t have a lot of heavy nitrates coming into it as much as some of the other lakes,” Goldman said.

Coralville Lake, in particular, might benefit because outflow from the dam means the lake doesn’t retain water long enough for harmful algae blooms (HAB) to form, officials say.

“HABs have just typically not been an issue at Coralville because of the retention time. It behaves more like a river,” Keller said.

Only certain strains of cyanobacteria can produce toxins. Read more about the science behind microcystin toxins.


Besides the threat from dangerous microcystin levels, some Johnson County beaches also have experienced high concentrations of E. coli.

The state procedure for E. coli is to issue a warning if the geometric mean — an average of the last five samples — is greater than 126 colonies per 100 milliliters. A one-time sample higher than 235 units also is considered unsafe. E. coli is often associated with fecal coliform, a bacteria that can be carried into lakes through animal manure, fertilizer runoff and sewage.

That kind of result is not unheard of at local beaches, and Puettmann said Lake Macbride has had consistently high bacteria levels this summer.

“We do have an E. coli, or bacterial, advisory on the lake and we’ve had one for about four or five weeks now,” Puettmann said. The most recent test, on Aug. 4, showed an E. coli concentration of 17,000 units and the beach’s geometric mean is 3,778 units.

Other lakes have had bacteria trouble as well.

“At Sugar Bottom there have been a couple readings that were over the state standard for E. coli,” Keller said. Those readings were on May 11, when the E. coli concentration reached 340 units per 100 ml, and May 26 and June 15, when bacteria at the beach reached 510 units per 100 ml.

Kent Park has issued two E. coli advisories this summer, once on June 24 when the geometric mean reached 134 units, and once on July 29 when a one-time test showed results of 560 units.

But that kind of spike can also be brief, officials say, and is often caused by heavy rain that deposits nutrients into lake water.

“Ninety percent of the time it’s related to a precipitation event,” Keller said, explaining that the rain brings a high volume of water containing nutrient runoff into the lake.

Ultimately, Freidhof said, posting warnings and closing beaches amounts to a band-aid solution, and more research into the cause of the toxins is needed. He said some study is underway into the importance of wetlands and watersheds in containing nutrients that often cause the toxins, but that the state needs to provide more guidance because the issue is so multi-faceted.

“Pinpointing the solution for that isn’t easy,” he said.

Reach Stephen Gruber-Miller at 319-887-5407 or

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