Blue-green algae is actually a form of cyanobacteria that can produce a variety of toxins, some of which are 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.
Blue-green algae is typically 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.
While a lab test is the most reliable way to determine if an algae bloom is toxic or not, the stick or jar test are easy ways to tell whether or not you're looking at a blue-green algae bloom. By simply sticking a stick into the water, blue-green algae will look like the stick was dipped in paint, and will slide off when lifted out of the water. Non-toxic green algae will often form slimy, stringy strands that will stick or cling to the end of the stick when lifted. Blue-green algae blooms typically look like pea soup or a mat of green paint. Check out a more detailed guide to the stick test and jar test here. Duckweed is commonly confused with algae, but is a native plant that forms floating colonies on the water's surface, indicated by separate tiny pieces.
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.
"When in doubt, keep out!"
Image: Minnehaha Creek Watershed District
Contact with blue-green algae blooms can be fatal to pets. In humans it can irritate the skin and eyes, make it difficult to breathe, and cause vomiting and diarrhea. Children are at higher risk than adults.
VLAWMO is working with partner agencies and residents to monitor locations where blooms have been reported. If you notice a suspicious algae bloom or have questions, please contact us at (651) 204-6070. Thank you for being vigilant and aware of our water resources!
Residents can help prevent these blooms by considering the tips on our water stewardship at home page.