Temperatures are going up, and in Minnesota, many of us are cooling down at our favorite lake. But be aware that these high temperatures coupled with rainfall washing excess nutrients into the water can combine to create harmful blue-green algae in lakes. This type of algae can harm pets, livestock, and even people.
Not all blue-green algae are toxic, but there is no visual way to predict whether a blue-green algal bloom contains toxins and is harmful to humans or animals. Harmful blooms often 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. We tell people, if in doubt, stay out — if you are not sure, it’s best for people and pets to stay out of the water.”
Humans are rarely affected, probably because the unpleasant odor and appearance of a blue-green algal bloom tend to keep us out of the water. However, swimming in water with an algae bloom may cause skin rash, hives, runny nose, irritated eyes, and/or throat irritation. Swallowing water with an algae bloom may cause vomiting, diarrhea, or nausea; headache, throat irritation, or muscle pain.
Dogs are at particular risk as they wade in shoreline areas where algae may accumulate, and can suffer from seizures and convulsions from exposure to toxic blue-green algae.
There are currently no short-term solutions to fix a blue-green algal bloom. Once a bloom occurs, the only option is to wait for the weather to change — significant rainfall, wind shifts, or cooler temperatures — to disrupt the algae’s growth. According to Heiskary, the key to solving algae problems long term is to improve water quality by decreasing the amount of nutrients that runoff carries into lakes.
Some safety tips for you
Don’t swallow, swim, or wade in water with blue-green algae. If you do come in contact with blue-green algae or if it gets on your skin, wash it off thoroughly, paying special attention to the swimsuit area.
For more information, including who to contact if you suspect an algal bloom, visit
Blue-green algae and harmful algal blooms page.