Blue-Green Algae

Be cautious of algal blooms in lakes and ponds. Boulder County does not test for toxic algae.

Harmful Algae Blooms

Blue-green algae has been confirmed in lakes and ponds in Colorado for the past few years, but Boulder County Public Health does not test for it in any parks and open space areas.

Not all algae is harmful to humans or pets, but the best way to keep your family safe during an algae bloom is to:

• Keep out of the water, including children and pets.
• Avoid drinking the water. Ingestion of blue-green algae can cause gastrointestinal distress in humans and is potentially lethal to dogs.
• Avoid boating near or through algae blooms.
• Clean fish well and discard the guts appropriately.
• When in doubt, just stay out!

Blue-Green Algae

Some algal blooms can contain cyanobacteria, often referred to as blue-green algae. While the vast majority of algae often seen in ponds and lakes during summer months are not toxic, cyanobacteria can produce toxins, which can be harmful to dogs and humans at elevated levels. Warmer temperatures, stagnant water, and nitrogen and phosphorus (nutrient) loading from fertilized lawns and other sources can facilitate algae and cyanobacteria growth.

According to the Colorado Department of Public Health and Environment, cyanobacteria blooms may look like thick pea soup or spilled bluish-green paint on the water’s surface and can also create a thick mat of foam along the shoreline. In general, the long, stringy, bright green strands that appear either slimy or cottony, or are mustard yellow in color are not the harmful type of algae.

At lakes and ponds where dogs are allowed to enter the water, practice caution if algae are present. Anglers should also exercise caution and follow state recommendations to clean any caught fish thoroughly and to discard guts appropriately.

Visitors and residents should exercise caution and keep children and pets out of the water in any areas where algae are observed. When in doubt, just stay out.