Hot, humid weather causing unhealthy algal blooms in Ohio ponds, lakes


COLUMBUS, Ohio — Harmful algal blooms, like those affecting Grand Lake St. Marys, are popping up in ponds and small lakes throughout Ohio, and Ohio State University Extension educators are encouraging landowners to protect their family, pets and livestock.

“We are seeing the same harmful algal blooms in landowner ponds and lakes as we are seeing in Grand Lake St. Marys,” said Bill Lynch, an OSU Extension associate in aquatic ecosystem management with the School of Environment and Natural Resources.

“The development of harmful algal blooms is a relatively uncommon event, but it is increasing every year and can be a health risk to humans and animals.”


Several harmful algal bloom-forming organisms (cyanobacteria) are native to Ohio, but only cause problems when environmental conditions (hot, humid weather), coupled with excessive nutrient inputs, favor them. When stressed, the algae can release toxins that can cause a variety of health problems ranging from minor skin irritation to liver or nerve damage.

“The culprits that can contribute to excessive nutrients include Canada geese, fertilizer applications around ponds, septic leaching, ag run-off, and manure from small farm animals like goats and alpacas,” said Lynch. “In a normal system, the nitrogen to phosphorus ratio should be around 20-to-1 or higher. Once that ratio shifts lower, then that’s when the harmful algal blooms develop.”

Problem areas

Lynch said that older ponds are the ones that are being affected the most because they lose more oxygen at deeper depths and the result is the release of more phosphorus from mud along the bottom.

“Add an external nutrient input to the mix and you’ve got a potential problem on your hands,” said Lynch.

Lynch is encouraging landowners to keep pets, livestock and children away from ponds or lakes suspected of being contaminated with harmful algal blooms. Scum color varies and includes white, brown, purple, blue-green and black. Some appear as green paint spills, green globs or dots in the water. Sometimes there is a foul odor.

“Humans or animals that drink the water can become ill. In some small animals, such as goats or rabbits, death can occur,” said Lynch. “Our goal is to make the ag community and local veterinarians aware of the situation, so they know what to look for if there is a problem. Just because an algal bloom is present doesn’t mean toxins have been released, but you shouldn’t take any chances.”


There are a number of ways to prevent harmful algal blooms, but Lynch said that the No. 1 recommendation is to install a bottom aeration system to keep oxygen plentiful and prevent the release of phosphorus from bottom mud into the water.

“We also recommend practicing watershed best management practices to manage external nutrient inputs,” said Lynch. “Another key recommendation is to have a back-up water source, like a tanker or a well, if you use the pond as a primary water source. Your pond could be unusable for weeks in an algal bloom outbreak, and your animals need water.”

Once an algal bloom outbreak has occurred, Lynch said that it might be best to just let it run its course. Cooler temperatures in the fall will eventually break up the algae and clear the pond or lake.

“Treating the lake or pond with an algaecide can do more harm than good,” said Lynch. “You’ll kill the algae, but release the toxins at the same time, and there’s no telling how long it takes for the toxins to dissipate.”


A number of fact sheets are available that provide detailed information on the types of harmful algal blooms, how to manage them, safety procedures and recognizing symptoms. They include:

Ohio Environmental Protection Agency’s Harmful Algal Blooms — Protect Your Pets and Livestock, and Harmful Algal Blooms Can Be Deadly to Pets and Livestock, at

Ohio Sea Grant’s Harmful Algal Blooms in Ohio Waters, at

Other areas

Ohio isn’t the only state experiencing problems with harmful algal blooms. The problem extends across the Midwest and into southern Canada, Lynch said.


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  1. I have an adjoining pond owner who is complaining of excessive phosphorus levels in his pond water. This particular pond is a much area that was damned of several years ago, about 8 feet deep at it’s deepest, and is used as a storm water retention for the village of Montpelier. He has supposedly had someone from the state test his water as he has had high algae blooms this year and the water tested high in Phosphorus. The adjoining cropland is about 7 acres and was in corn the last 2 years with only starter fertilizer in the row as a source of P. The village dumps around 40 to 50 acres of residential storm water in this pond also. He seems to think that we need to put in a waterway in our low ground to alleviate the problem. There is already a filter strip about 50 feet wide between the field and pond. Any comments or suggestions?


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