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Understanding Blue-Green Algae

What are blue-green algae?

Many species of algae are found in Wisconsin lakes and rivers. Algae act like other plants and convert sunlight into energy, forming the base of a lake's food chain. Algae are eaten by zooplankton, which are in turn eaten by small fish, then larger fish, and eventually the larger fish are eaten by birds, shore animals, and people. True algae are a vital part of lake systems; however, blue-green algae are not true algae. Blue-green algae are photosynthetic bacteria known as cyanobacteria, which can cause illness and death in humans and animals. While blue-green algae can convert sunlight into energy, they are not an important part of the food chain because most organisms prefer not to eat them.

Blue-green algae are a natural part of lake ecosystems and algal blooms have occurred for many centuries. While scientists are learning more and more about blue-green algae, researchers are only beginning to understand the health risk that blue-green algae pose to humans and animals.

What are algal blooms and why do they occur?

When environmental conditions are right, the algal population can grow quickly and a bloom can occur. A bloom is a sudden increase in algae cells in a certain area of water. Little wind, warm water, sunlight, and plentiful nutrients - especially phosphorus - all 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 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 the surface may look bubbly or frothy. Algal scums can be pushed to different locations by wind or tide. When blue-green algae 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.

What should I do if I see a bloom?

People should use common sense when dealing with algae. It is impossible to tell from a visual inspection whether an algal bloom is toxic. The safest thing to do is to treat every algal bloom as if it could be dangerous.

Guidance for people

  • Do not swim or wade through algal scums.
  • Do not boat, water ski, or jet ski through algal blooms.
  • Do not fish in lakes where algal scum is present.
  • Always shower off with soap and water after swimming in a lake.

Guidance for pets

  • Do not let dogs drink lake water during an algal bloom.
  • Do not let dogs eat algal scum, or lick it off their fur.
  • Wash your dog off with clean water immediately if your dog swims or wades in water during an algal bloom. 

For members of the general public and veterinarians: 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 to the Wisconsin Harmful Algal Blooms Program.

Last revised August 7, 2023