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Blue-Green Algae FAQ

Blue-green algae bloom

What Is It?

Blue-green algae is "algae" in name only – it is actually a "cyanobacteria" that can produce toxins harmful to people and animals. When water conditions are right, blue-green algae can bloom, covering areas of the water in a thick, dense mat. It is found in lakes, ponds, and slow-moving streams. 

What Causes It?

Typically blue-green algae can become most severe when there is little wind, abundant sunlight, warm water, and a high amount of nutrients (phosphorus or nitrogen). Excessive nutrients can come from polluted stormwater runoff, runoff from agriculture, fertilizer, lawn waste, or other material that washes into the lake and decomposes. Toxic blooms are more common in the late summer and early fall but can happen at any time during the season. 

How Can I Tell if There is Blue-Green Algae in the Lake?

Blue-green algae blooms typically look like pea soup or green paint. On the other hand, harmless green algae will come in slimy, stringy strands and can look like floating mats. Duckweed is a native plant that can look like algae film but is actually a very small, floating plant.

Though not all blue-green algae is toxic, it is best to stay out of the water if you see it. In addition, people and pets should avoid contact with discolored water or areas with visible algae, never drink untreated surface water, and rinse yourself and/or pets off after swimming in any pond, lake or stream. 

What Harm Can It Cause?

Contact with blue-green algae blooms can be fatal to pets. In humans it can irritate skin and eyes, make it difficult to breathe, and cause vomiting and diarrhea. Children are at higher risk than adults.

Showing blue-green algae on left and green algae on the right

Learn more

Blue-green algae and harmful algal blooms - MPCA

Recognize blue-green algae - Travel OK

From the Wisconsin Department of Natural Resources: