Algae: Cyanobacterial Harmful Algal Blooms

Cyanobacteria, also known as blue-green algae, are a type of photosynthetic bacteria. They are among the first organisms that learned how to make oxygen. They've played an important role in the creation of the Earth’s atmosphere.

Cyanobacteria are naturally found in all lakes, rivers, and ponds in Wisconsin and all over the world. Just like plants and algae, they perform photosynthesis and get their energy from the sun. Unlike true algae, cyanobacteria aren’t a key part of the food chain. Most organisms choose not to eat them.

Some species of cyanobacteria can create toxins that are harmful to human and animal health. They can also grow into cyanobacterial harmful algal blooms (cyanoHABs).

What are cyanoHABs, and why do they happen?

Under the right circumstances, cyanobacteria can grow quickly to form a bloom. A bloom is a sudden increase in cyanobacterial cells in a certain area of water.

If the cyanobacteria in a bloom are producing and releasing toxins, then it’s known as a cyanobacterial harmful algal bloom, or a cyanoHAB. CyanoHABs can make people and animals sick if they breathe in, swallow, or contact water or scum that contains toxins.

Sunlight, high temperatures, and lots of nutrients like phosphorous and nitrogen can increase the chance that a bloom will occur. Warm weather patterns and large rain events that wash agricultural and residential fertilizers (which contain phosphorus) into the water can also jump-start a bloom. In Wisconsin, blooms usually happen during warmer months, between mid-June and mid-September.

Blooms can take on many different appearances. They often form a thick scum layer or look like pea soup. Blooms can be blue, green, white, brown, or wine-colored. The water gets cloudy, and there also may be a strong, bad smell. The following images show blooms:

Submit photos

To submit photos of a bloom, email them to the Department of Natural Resources at dnrhabs@dnr.wi.gov. Include a description of bloom size, duration, and location with the lake, town, and county name.

Be sure to include photos of the bloom up close and farther away.

Learn more about CyanoHAB Health Concerns.

What precautions should I take if I see a bloom?

You can’t tell by looking at a bloom if it’s making toxins or not. The best approach is to treat every bloom like it’s harmful. When in doubt, stay out!

Follow these safety tips:

• Don’t swim or wade in blooms.
• Don’t boat, water ski, or jet ski through blooms.
• Don’t eat fish from lakes with blooms.
• Always shower with soap and water as soon as possible after swimming in any river, lake, or pond.
• Watch your pets:
  • Don’t let pets drink from water experiencing a bloom.
  • Don’t let pets eat cyanobacterial scum or lick it off their fur.
  • Wash your pet with clean water right away if they swim or wade in water during a bloom.
  • Seek care from a vet right away for pets with sudden, severe symptoms.

How to report a blue-green algae (cyanobacteria) illness

Members of the general public and veterinarians: should call 608-266-1120 or complete the Harmful Algae Bloom (HAB) Illness or Sighting Survey, F-02152 online form to report any blue-green algae blooms and related human or animal illnesses.