Joe Moore and Summit Lake reservoirs have tested positive for toxic levels of blue-green algae, according to Colorado Parks and Wildlife.
Staff members discovered algal blooms at the lakes last week, said Jim White, CPW aquatic biologist.
Algae samples from both reservoirs tested above the limits for recreation set by the Colorado Department of Health and Environment.
The threshold for microcystin algae is 8 micrograms per liter, and samples in both lakes were at 60 micrograms per liter, nearly eight times above the safe threshold. Follow-up testing is planned for next week.
Warning signs have been posted at Joe Moore and Summit reservoirs to inform lake visitors about the dangers of blue-green algae blooms. The toxins can affect the liver, skin or neurological systems.
“These blooms are temporary; once testing shows a return to normal, the signs are taken down,” White said.
Algae is common and natural to Colorado lakes and rivers, but when it forms blooms or scums, it creates toxins that can harm people, animals and the local environment.
Lake users are advised to stay out of the water, to not drink the water, to keep pets and livestock away from the water, to avoid boating near or through algae blooms, and to clean fish well with clean water.
Toxic algae or harmful algae blooms are made up of what many people call blue-green algae. Technically, these organisms are a special type of bacteria called cyanobacteria.
Although these organisms naturally occur in Colorado waters, they become a problem when they multiply rapidly, resulting in a dense cyanobacteria concentration or “bloom” that produces toxins.
Blooms tend to occur when the ecosystem gets out of balance and the cyanobacteria outcompete other phytoplankton.
There is a distinction between green algae, and blue-green algae, White said. Green algae by itself is not normally a problem and is a food source for aquatic organisms.
“Blue-green algae is different. When there is an abundance, it can put off toxins in the water, and that is a concern,” he said.
Toxic algae might resemble thick pea soup and can create a mat of foam along the shoreline. Lake organisms won’t feed on it.
Warm and low water levels, long daylight hours and high ambient temperatures can spur a blue-green algae bloom.
The conditions have existed this summer at Joe Moore and Summit. The irrigation reservoirs are lower than normal for this time of year because of below-average snowpack last winter, ongoing drought and irrigation demand.
CPW has been testing lakes more often for blue-green algae hazards, White said.
“Pay attention to the warning signs,” he said.
The blooms can lead to fish kills, although none have been reported at the reservoirs. When blue-green algae decays, it draws oxygen away from the water, which can starve fish, especially in low water.
Recent rain and cloud cover can help diminish the conditions needed for an algae bloom.
For more information, visit www.cdphe.colorado.gov/toxic-algae.