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Blue-Green Algae

Algae blooms naturally occur in aquatic ecosystems and can appear rapidly during hot weather. They also tend to appear in slow-moving water bodies, such as lakes and ponds. Some algae blooms can contain cyanobacteria, often referred to as blue-green algae. While most algae often seen in ponds and lakes during summer months are not toxic, some cyanobacteria species can produce cyanotoxins during algal blooms which can be harmful at elevated levels if ingested by dogs, wildlife, and humans, or during wading and other recreational contact with water.

If you are in doubt about whether or not an algae bloom is harmful, keep all kids, adults, and pets out of the water. When in doubt, stay out!

About Blue-Green Algae

What causes blue-green algae blooms?

Algae blooms favor slow-moving water bodies, such as lakes and ponds, and are less likely to occur in flowing streams (e.g., Boulder, St. Vrain, Left Hand, Coal, and Rock Creek.). However, as water levels decrease following spring runoff, stagnant pools that form along the banks of flowing streams can experience algae blooms. Warmer temperatures, stagnant water, and high levels of nutrients (nitrogen and phosphorus) produce favorable conditions for algae growth, including blue-green algae. Common sources of nutrients include fertilized lawns and pet waste.

Blue-green algae blooms may look like:

  • Thick pea soup
  • Spilled bluish-green paint on the water’s surface
  • A thick mat of foam along the shoreline

What you can do

Blue-green algae play an important role in our aquatic ecosystems. However, there are some important actions that we can take to help reduce the frequency and intensity of blue-green algae blooms. Since we can’t control the water temperature, the best thing we can do is to reduce the amount of nutrients entering our lakes and streams.

Learn more about what you can do at home to reduce the amount of nutrients entering our waterways.

Water quality monitoring and blue-green algae

While local governments do water quality testing along some waterways in their communities, they generally do not test for cyanobacteria in lakes and ponds. Production of cyanotoxins can be highly variable, with harmful toxins detectable at one time but not detectable hours later. For these reasons, it is important to stay aware and take precautions when recreating in waterways when any algae is present.


How can I recreate safely this summer?

Not all algae are 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.
  • Be mindful of posted signage/warnings at water bodies.
  • When in doubt, just stay out!

Avoid entering or playing in bodies of water that:

  • Smell bad
  • Look discolored
  • Have foam, scum, or algal mats on the surface
  • Contain or are near dead fish or other dead animals (for example, do not enter a body of water if dead fish have washed up on its shore or beach)

Recreating safely with dogs

Blue-green algae can be toxic to dogs, and they can die within hours of ingestion. Take these precautions to keep your pet safe during algae blooms:

  • Do not let pets eat algae, get in the water, or go on the beach or shoreline.
  • Rinse pets off with tap water after they have been in a lake, river, or pond; do not let them lick their fur until they have been rinsed.
  • Seek veterinary care immediately if your pet has consumed or licked algae on its fur after swimming or playing in water that has an algae bloom.