Harmful algae blooms (toxic algae)


Blue-green algae are common

Blue-green algae are a kind of bacteria common in Colorado waters. Water nutrients such as nitrogen and phosphorus help algae grow and support fish and other aquatic life. But too much nitrogen and phosphorus in the water allows blue-green algae to grow quickly and form blooms and scums. Blooms can occur anytime but are most common during hot, sunny weather and in slow-moving water bodies such as lakes. Some algae blooms produce toxins (poisons) that can cause illness in humans, pets, waterfowl, and other animals that come in contact with the algae. These are called harmful algae blooms (also called HABs or toxic algae).

Harmful algae blooms in Colorado

Harmful algae blooms happen frequently in Colorado, particularly during warm months. They may occur anywhere but are less likely in high-elevation mountain lakes and reservoirs. The only way to know if an algae bloom is harmful is to have it tested. The Colorado state lab is one lab that can run these tests. 

Some bodies of water in Colorado are routinely monitored and tested for algal toxins to protect the health and safety of water users. Our toxic algae dashboard displays historic harmful algal bloom information from the state. This data do not represent current conditions of the water body so we recommend to always examine a body of water for warning signs before allowing children, pets, or yourself to come into contact with water. If you have questions about an area of water that isn't shown on this map, we recommend reaching out to the owner or manager of that water body. 

Harmful algae blooms
  • May look like thick pea soup or spilled paint on the water's surface.
  • Can create a thick mat of foam along the shoreline.
  • Usually are green or blue-green, although they can be brown, purple or white.
  • Sometimes are made up of small specks or blobs floating just at or below the water's surface.
Harmful algae blooms are not
  • Long, stringy bright green grass strands that feel either slimy or cottony.
  • Mustard yellow (this probably is pollen).

Health Information

Staying safe around harmful algae blooms
The only way to know if an algae bloom is harmful is to have it tested, and the best way to stay safe is to stay out of water with algae blooms. If you see algae blooms:
  • Stay out of the water.
  • Don\'t drink the water
  • Keep your pets and livestock away from the water.
  • Avoid boating near or through algae blooms.
  • Clean fish well and discard guts appropriately.
Health effects of harmful algae blooms

Health effects of harmful algae blooms vary according to the kind of bacteria in the algae and whether you touched it, swallowed it, or swam in it. Symptoms include:

  • Skin irritation or rashes.
  • Blisters around the mouth and nose.
  • Asthma.
  • Nausea and vomiting.
  • Stomach cramps.
  • Diarrhea.
  • Headache.
  • Sore throat.
  • Fever.
  • Muscle and joint pain.
  • Liver damage.
Contact your health care provider if you think algae has made you sick. Contact your veterinarian if you think algae has made a pet of livestock animal sick. If you have questions about the health effects of harmful algae blooms, contact the Poison Control at 1-800-222-2222.

Information for drinking water providers

 Colorado risk management toolkit for recreational waters
What problems do algae blooms cause for drinking water systems?

Algae in drinking water sources can cause the water to taste or smell bad. Taste and odor are not regulated but create customer concerns about water quality and safety. Most complaints water utilities receive are about taste and odor, and these issues can last for prolonged periods. In addition to taste and odor problems, toxins from harmful algae blooms can create a public health risk. Removing toxins in a safe and cost-effective way can be a challenge for treatment facilities, and not all are equipped to do so.

Are toxic algae in drinking water regulated?

No, and they are not routinely tested for either. After Toledo, Ohio experienced a severe problem with harmful algae blooms in 2014, the EPA developed health advisory guidance for water systems and suggested a multiple step process to identify and address problems. Following this guidance is not a regulatory requirement.

How can I tell if an algae bloom is harmful?

The only way to be certain if an algae bloom is harmful is to run specific water tests. The Colorado Laboratory Services Division is one lab that can complete these tests.

Could a harmful algae bloom affect a public water supply?

Yes. This happened in Toledo, Ohio in 2014. The economic consequences of such an event can be severe.

How we can help?
Drinking water providers can contact the Water Quality Control Division at 303-692-3500 with questions about algae blooms. We can help water utilities that experience taste and odor problems. This includes ideas about customer communication and steps utilities can take to manage algae blooms and best treat their drinking water.