Harmful Algal Blooms

Harmful Algal Blooms (HABs)


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

How harmful algal blooms form:

In Wisconsin, blooms typically occur during the warm weather months between mid-June and mid-September. Lakes and rivers in Wisconsin can become cloudy with rapidly reproducing algae. Blue-green algae will follow sunlight and nutrients by floating to the surface, where they can form thick scum layers or mats and may look bubbly or frothy.  Algal scums can be pushed to different locations by wind or wave action.

Harmful algal blooms can seem to appear overnight. This is because many species of blue-green algae have developed the ability to control their buoyancy as the availability of light and nutrients changes with various weather conditions. 

At night, when there is no light, the cyanobacteria cells are not able to adjust their buoyancy and will float to the surface, forming a scum on top of the water that is visible in the morning, seemingly appearing overnight.

What Do Harmful Algal Blooms Look and Smell Like?

When harmful algal blooms are present, the algal scum can be a variety of colors such as fluorescent blue, green, white, red, or brown. Blooms can have more than one color present and may look like thick paint floating on the water.  Algal blooms can give off a foul odor, which is particularly offensive in the warm summer months.

Harmful algal blooms can often be mistaken for pollen, duckweed, and filamentous green algae in waterways.  These organisms are part of the ecosystem and can serve as primary producers in their food web.

Human Exposure and Symptoms:

Humans can be exposed to harmful algal blooms in a variety of ways, including:

  • Accidentally drinking water that comes from a lake or reservoir with a harmful algal bloom present.
  • Drinking untreated water.
  • Being in the water with a harmful algal bloom.
  • Inhaling aerosols (spray) from water-related activities such as jet skiing or boating.
  • Inhaling aerosols when watering lawns or irrigating golf courses with pond water.
  • If you wade into water up to your knees, and cannot see your feet, the amount of algae could be unsafe. 


What to Do if You Experience Symptoms

If you think you are experiencing symptoms from exposure to harmful algal blooms, contact your doctor or the Poison Information Hotline (800-222-1222) right away.

For more information on HABs, please click one of the links below:

Health Concerns Related to Blue-Green Algae

CDC Harmful Algal Bloom (HAB)-Associated Illness