A state advisory for a harmful algal bloom in the Ohio River has been extended from the West Virginia border past Louisville to Cannelton Locks and Dam in Hancock County.

These algal blooms are made up of blue-green algae, or cyanobacteria, and produce toxins. The algae are naturally occurring in lakes and rivers, but when there are high concentrations of nutrients and warmer water, they flourish and create blooms.

hab advisoryCourtesy Kentucky DEP

In recent years, algal blooms have become common during the summer in Kentucky lakes. But those are lakes with still water. Blooms in a flowing river as large as the Ohio are unusual.

The state recreation advisory recommends people avoid contact with the water through swimming and boating. Water inadvertently swallowed could cause nausea or vomiting, and the water could also cause skin and eye irritation or breathing difficulties.

algalErica Peterson | wfpl.org

The Ohio River this week.

Getting rid of algal blooms can also be a challenge, especially when they’ve spread at such a magnitude. A Dartmouth College study released earlier this year found that cyanobacteria feed on nutrients like phosphorus and nitrogen, but once they establish a toehold in an ecosystem, the bloom can actually pull out nitrogen from other places in the environment and turn it into nitrogen it can use to feed itself.

Division of Water director Peter Goodmann said getting the blooms to go away depends on changing weather patterns.

“When it’s going to go away is when conditions in the river change significantly,” he said. “When we get increased flows, when we have drops in temperature, we get more cloud cover or the cycle of these things just dies out.”

Goodmann said he anticipates the algal bloom will persist until early to mid-October. That could prove problematic for Louisville’s Ironman competition on Oct. 11, which includes a portion of river swimming. Goodmann said he’s been in contact with race representatives, and they’re monitoring the situation.

About 3 million people get their drinking water from the Ohio River, including Louisvillians. The Louisville Water Company is using activated carbon and other methods to remove cyanobacteria and cyanotoxins from the water. Kelley Dearing Smith, a spokeswoman for the water company, said tests show the water is safe to drink. The extra monitoring and treatment is costing the water company at least an additional $7,000 a day, she said.

Erica Peterson is WFPL's News Director.