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Be Cautious of Algal Blooms in Lakes and Ponds This Summer

With summer approaching and temperatures rising, the City of Boulder and Boulder County remind community members to exercise caution around lakes and ponds where algae blooms may be present.

If community members see algae in lakes and ponds, they should take extra precautions to avoid contact with the water because some algae blooms can be harmful to people and pets.

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.

Algae has been observed at Wonderland Lake in north Boulder, where swimming, wading and boating are prohibited. The city reminds community members to keep dogs out of Wonderland Lake. 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.

The City of Boulder and Boulder County do not test for cyanobacteria at lakes and ponds because the production of cyanotoxins can be highly variable – with harmful toxins detectable at one time but not detectable hours later. Visitors and residents should exercise caution and keep children and pets out of the water in any areas where algae are observed.

Published: June 10, 2020

Media Contacts:

Phillip Yates, City of Boulder Open Space and Mountain Parks, 303-349-2438
Denise White, City of Boulder Parks and Recreation, 303-413-7258
Vivienne Jannatpour, Boulder County Parks & Open Space, 303-678-6277
Chana Goussetis, Boulder County Public Health, 303-441-1457