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Toxic Algal Blooms Persist In Ohio River, But They’re In Decline

U.S. Environmental Protection Agency

Hundreds of miles of the Ohio River are still contaminated with unsafe levels of toxic blue-green algae, though seasonal changes have helped to improve conditions over the last week or so.

Drought conditions across the Ohio Valley in September helped fuel the growth of harmful algal blooms. The algae led organizers to cancel the Great Ohio River Swim in Cincinnati and the swimming portion of Louisville’s Iron Man competition earlier this month.

The blooms are appearing on about 265 miles of river from Louisville to Greenup, Kentucky. This is only the second time the river has had a large algal growth in the last two decades, said Sam Dinkins, technical program manager at the Ohio River Valley Water Sanitation Commission.

Back in 2015, an algal bloom expanded to cover more than 600 miles along the Ohio River, he said.

“This is certainly a noteworthy event, but it’s certainly not the worst we’ve seen along the Ohio,” Dinkins said.


Blue-green algae can produce a toxin known as microcystin that’s harmful to the liver. When ingested or touched, the toxin can cause stomach pain, nausea, vomiting, numbness and other health effects. Pets are particularly vulnerable.

The algal blooms grow throughout the river, often appearing as green, paint-like scum on the water’s surface. The highest sample seen this year was greater than 5,000 micrograms per liter, taken on October 9 in Madison, Indiana.

That’s 625 times higher than the 8 microgram-per-liter recreational advisory threshold in Kentucky and Indiana. However, just a week later, levels at that same site were back below the advisory limit.

The improvements over the last week are driven largely by the changes in season. Shorter daylight hours, cooler temperatures and more rain have helped to restore the river, Dinkins said.

“A number of locations have dropped below the advisory levels now,” he said.

In other parts of the country, high levels of nutrients like phosphorous and nitrogen in the watershed have helped to fuel the growth of toxic algae, but so far it’s unclear how nutrient loading has affected algal growth along the Ohio River, Dinkins said.

Climate scientists say the frequency of extreme weather events like droughts will become more common as the temperature rises. That in turn, could increase the frequency of harmful algal blooms.

Despite recent improvements, Dinkins recommended people and pets should avoid contact with water that has obvious algal blooms.

Read more about recreational advisories here.

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