Harmful Algal Blooms

With hotter temperatures and increased nutrient runoff in summer months, Harmful Algal Blooms can take over a creek, pond, lake or other slow-moving waterways to cause serious problems. This overgrowth reduces oxygen levels in the water system, which can cause the death of large numbers of fish, shellfish and plants. In addition, the bacteria can be toxic to people, pets, and livestock that enter or drink the water.

How to Spot an Algal Bloom

The blooms are visible as a thick foam or scum on the top of the water — looking much like someone spilled paint across the surface. Colors can vary from bright green and blue-green to white, red or yellow-brown. As bacteria begin to die-off, they can release an unpleasant odor that smells like rotting plants.

While not all blooms are harmful, it is difficult to tell which ones are by simply looking at them. It’s best to avoid waterways that don’t look or smell safe, to not fish in affected areas, and to keep pets away as well — following the mantra “When in Doubt, Stay Out”.

To report an environmental concern in St. Charles County, call the Division of Environmental Health and Protection at 636-949-1800.

Missouri Department of Natural Resources photo

Example of excessive algae growth in a Missouri pond

Harms to Humans, Pets and Livestock

Humans experience health risks by swallowing, inhaling or coming into contact with contaminated water. Symptoms of exposure can include skin rash or irritation, diarrhea and stomach cramps, vomiting, dizziness, and numbness or temporary paralysis. Inhaling water vapors can create allergy-like symptoms. If you suspect exposure to contaminated water, seek medical support and call the Poison Help line at 800-222-1222.

Pets and livestock experience serious problems with these exposures, as they are more likely to swim in the water and drink larger proportions relative to their body size. Symptoms in animals typically begin soon after exposure and include: drooling, vomiting, diarrhea, difficulty standing or breathing, and convulsions. Heavy exposure can cause the death of the animal. If you suspect exposure to pets or livestock, contact your veterinarian.

Additional Information

Missouri Department of Health and Senior Services
U.S. Environmental Protection Agency
Centers for Disease Control and Prevention