Weather Forecast


High temperatures can bring harmful algal blooms

When the summer sun shines and temperatures climb, conditions are ripe for Minnesota lakes to produce harmful algal blooms. The Minnesota Pollution Control Agency (MPCA) is reminding people that some types of algae can harm pets, livestock and even humans.

Algae, microscopic aquatic plants, are a natural part of any aquatic ecosystem. Most algae are harmless, but under the right conditions certain types of algae can pose health risks. People or animals may become sick if they touch or ingest affected water. In extreme cases, dogs and other animals have died after exposure to lake water containing toxic blue-green algae.

"High rainfall, which has been common throughout much of Minnesota this spring, results in nutrient-rich runoff entering our lakes, fueling algae growth," MPCA lakes expert Steve Heiskary said. "While spring and early summer temperatures were cooler than normal, lake temperatures have warmed rapidly. Given these conditions, we are likely to see blue-green algae blooms on many of our lakes."

There are many types of blue-green algae. They are found throughout Minnesota, but thrive particularly in warm, shallow, nutrient-rich lakes. Often blown toward downwind shorelines, it is under these conditions that humans and animals most often come in contact with blue-green algae and where the risk of algal toxins is greatest.

Not all blue-green algae are toxic, and there's no visual way to predict whether a blue-green algal bloom contains toxins and is harmful to humans or animals. But harmful blooms are sometimes said to look like pea soup, green paint, or floating mats of scum, and they often have a bad odor.

"You don't have to be an expert to recognize an algae bloom that might be harmful," Heiskary said. "If it looks bad and smells bad, don't take a chance. Stay out and keep children and pets away from the water until the bloom subsides."

An animal that has ingested toxins from an algal bloom can show a variety of symptoms, ranging from skin irritation, vomiting, severe disorders involving the circulatory, nervous and digestive systems, and severe skin lesions. In worst cases, the animal may suffer convulsions and die.

Humans are rarely affected, probably because the unpleasant odor and appearance of a blue-green algal bloom tend to keep people out of the water. But human health effects can include irritation of skin, eyes and nasal passages, and nausea and vomiting. Extreme cases can produce paralysis and respiratory failure.

These factors make it important to take a preventive approach to dealing with potentially harmful algal blooms. There are currently no short-term solutions to correct a blue-green algal bloom. Once a bloom occurs, the only option available is to wait for weather changes, such as significant rainfall, wind shifts or cooler temperatures, to disrupt the algae's growth.

Over the long term, the key to solving algae problems is to reduce the amount of phosphorus that is allowed to flow into lakes through runoff. Some sources of phosphorus include pet waste, lawn clippings and yard debris, and certain fertilizers. Phosphorus-containing fertilizers are now banned in Minnesota under most circumstances.

For more information, go to the MPCA's Blue-green Algae and Harmful Algal Blooms webpage ( or call 651-296-6300 or 800-657-3864.