Hot summer temperatures have arrived, which means there will be masses of people taking full advantage of the weather by hitting beaches throughout Wisconsin.

As much fun as that is, there are precautions beachgoers need to take. The most obvious include things like wearing sunscreen to prevent skin cancer and following basic water safety to avoid drowning.

However, there is also an environmental risk everyone needs to be aware of, especially those with children and pets, which can cause harmful effects, including serious illness. The Wisconsin Department of Natural Resources recently warned beachgoers of what to look for and the actions to take.

Blue-Green Algae Grows In All Wisconsin Waterbodies And Can Cause Illness

According to the DNR, blue-green algae grow in all Wisconsin waterbodies and it can cause serious illness.

When environmental conditions are just right, blue-green algae can increase in number to visible accumulations called “blooms.” Planktonic blue-green algae blooms float to the surface and form scums, paint-like conditions, or “pea soup” water containing billions of tiny particles.

Blue-green algae can produce several different toxins. Exposure to these toxins is possible through contact with the skin while swimming, through inhalation when doing recreation activities such as boating, jet skiing, or water skiing, or by swallowing contaminated water.

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Types of toxins and potential health effects include the following:

  • Dermatotoxins and Gastrointestinal Toxins: These toxins affect the skin and mucous membranes, and can cause allergy-type reactions such as rashes, eye/nose/throat irritation, and asthma, as well as headaches, fever, and gastroenteritis (nausea, stomach cramps, vomiting, diarrhea).
  • Hepatotoxins: These toxins affect the liver and other internal organs, and can cause gastroenteritis, tissue damage, muscle weakness, paralysis, and respiratory failure (with acute exposure), tumors, and possibly liver cancer (with long-term, chronic exposure).
  • Cytotoxins: These toxins also affect the liver and other organs (though through a different mode of action than hepatotoxins) and can cause malaise, headache, anorexia, vomiting, chromosome loss, DNA strand breakage, and damage to organs.
  • Neurotoxins: These toxins affect the central nervous system and can cause seizures, paralysis, respiratory failure, or cardiac arrest.

How To Spot Blue-Green Algae

Blue-green algae, or cyanobacteria, are photosynthetic bacteria and can appear in two distinct ways:

  1. Planktonic blue-green algae look like tiny particles, dust, or grass clippings floating in the water and are most often green, tan, reddish-purple, or blue.
  2. Bottom-dwelling mats of benthic blue-green algae can float to the surface, often brown, black, or turquoise.

In Wisconsin, blue-green algae blooms generally occur from mid-June to late September, with peak bloom season in July and August when waters are the warmest.

It's important to note that while waters with high nutrient levels like nitrogen and phosphorus are most susceptible to blooms, even lakes with excellent water quality may occasionally experience small blooms, especially after calm weather. Bottom-dwelling benthic mats are often seen in lakes with clear water since sunlight needs to reach the bottom.

Photo Credit: Wisconsin DNR
Photo Credit: Wisconsin DNR

When environmental conditions are just right, blue-green algae can increase in number to visible accumulations called “blooms.” Planktonic blue-green algae blooms float to the surface and form scums, paint-like conditions, or “pea soup” water containing billions of tiny particles.

How To Stay Safe On And Offshore And Symptoms Of Toxin Poisoning

Since most lakes and rivers in Wisconsin are not tested for blue-green algae, it is crucial to know what it looks like and to always assess conditions before swimming.

You can stay by following these best practices:

  • Do not swim in water that looks like "pea soup," spilled paint, or water that has a scum layer, mats, or puffy blobs floating on the surface. All of these may be blue-green algae, and the abundant particles can also host high bacteria levels.
  • Do not boat, water ski, etc., over such water to reduce inhalation exposure.
  • Do not let children play with scum layers, even from shore.
  • Always shower after contact with any surface water whether or not a blue-green algae bloom appears to be present since surface waters may contain other species of potentially harmful bacteria, viruses, or parasites.
  • Always avoid swallowing untreated surface water because it may contain bacteria, viruses, and parasites that could make you sick.
  • When in doubt, stay out.

Symptoms of blue-green algal toxin poisoning in people include vomiting and diarrhea from ingestion sore throat and wheezing from inhalation. Some people also experience skin irritation and rashes in areas where the algae are held against their skin under clothing.

How To Keep Pets Safe

If you have pets that love water, it's important to remember that blue-green algae can have harmful effects if ingested. Dogs are not deterred by surface scum and can swallow water while swimming. They may eat floating material in the water or mats washed up on shore or lick blue-green algae from their coats after swimming.

If the blue-green algae they ingest produces toxins, the animals can become ill and even die.

When looking for a safe place for pets to swim, choose the clearest water possible and keep pets out of areas with accumulations of blue-green algae or dense particulate matter. If there's any doubt about what is in the water, keep your pets out.

Other tips to keep pets safe include:

  • Do not let pets or livestock swim in or drink waters experiencing blue-green algae blooms.
  • Always offer fresh, clean water for pets to drink instead of lake water.
  • Give dogs plenty of breaks from swimming and retrieving in lakes.
  • Avoid having dogs bite at splashed water as a game, and use flat objects for retrieval instead of balls.
  • Wash pets off with clean water immediately after swimming so they don't lick any algae from their fur.
  • Supervise pets when outside so they don't eat algal scum accumulated on the shore or floating mats of algae or drink lake water.
  • If a pet eats grass, avoid using lake water for lawn irrigation if blooms are present.

SEE NOW: These Are Now The 5 Most Snake-Infested Lakes In Wisconsin

Symptoms of blue-green algal toxin poisoning in pets may range from lethargy and weakness to vomiting, diarrhea, difficulty breathing, loss of coordination, and seizures, requiring immediate veterinary care.

Report Blue-Green Algae Blooms

The Wisconsin DNR encourages the public to report significant blue-green algae blooms by sending an email to

Be sure to include the location of the bloom, name of the waterbody, nearest town, county, size, and duration of the bloom, and overall and close-up photographs for verification. The DNR is unable to test all reported blooms.

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