Blue-green algae

How can you tell the difference before a blue-green algae bloom, which you should avoid, and regular algae blooms that are not a problem?


Blue-green algae

Regular algae

What it looks like.

There are two major groups of cyanobacteria blooms. One can look foamy, scummy, or thick like paint and is often blue-green, brownish red, pea green or white in color. The other looks like a dark green or black slimy mat that can have a smelly, offensive odor. Sometimes cells can be suspended in the water column, making it look bright green without a layer of visible scum.


These algae are often in suspension in the water column, affecting water clarity, and can also form thick floating foam or scum mats which may wash to the water’s edge leaving residue along the shoreline. These algae are not rooted to the substrate.

If you see leaves or roots, or distinguishable parts, it's likely a tiny (and harmless) aquatic plant like duckweed. Stringy, silky substances that can be draped over a stick are green algae. If it's yellow and almost “dusty” in texture, it might act like blue-green algae, but it's tree pollen.


In the pictures below you can see that the algae are attached to the ground, which is a clear indication that it is not blue-green algae.




The following are risks from recreating where there is a blue-green algae bloom:

  • Water contact can cause skin irritation or rash
  • Swallowing water can result in diarrhea, cramps, vomiting and dizziness
  • More severe reactions occur when large amounts of water are swallowed
  • Children and pets are at the greatest risk

Frequently Asked Questions from the Oregon Health Authority about Blue-Green Algae

What are cyanobacteria (harmful algae)?

Cyanobacteria are not algae at all. They are primitive bacteria found naturally in fresh and salt water all over the world. They are a beneficial bacteria that helped to create and sustain our oxygen atmosphere. In Warm weather, nutrients and low water flow can help these bacteria multiply quickly into what we call a bloom. Sometimes these blooms can produce cyanotoxins that can be harmful to people and pets.

Why is it called cyanobacteria?

The word "cyan" means blue-green, which is an appropriate name since most blooms are blue-green or green in color.

Why are these cyanobacteria blooms a health concern?

Not all blooms are harmful, but some cyanobacteria can produce cyanotoxins that can cause serious illness or death in pets, livestock and wildlife. These toxins can also make people sick, and in sensitive individuals also cause a red, raised rash or skin, ear and eye irritation.

What toxins are found in these blooms?

The most common toxins in harmful algal blooms in Oregon are microcystins and cylindrospermopsin

How can I be exposed? 

Exposure occurs when you swallow water with cyanotoxins during recreational activity, or when using affected water for drinking or cooking. Cyanotoxins are not absorbed through the skin, however, a red, raised rash or irritation of the skin and eyes can develop after contact with a bloom.

What are the symptoms of exposure? 

If enough water is swallowed you may experience one or more symptoms that mimic food poisoning; headaches, cramps, diarrhea, nausea and vomiting, numbness, dizziness, fever. Children and pets are at increased risk of exposure because of their size and level of activity. Symptoms generally begin within 24 hours and last 72 hours. .

When should I seek advice from a health provider or veterinarian? 

If you have severe diarrhea, vomiting, skin irritation or other related symptoms, or you experience these symptoms for more than 72 hours, you should seek medical attention to prevent severe dehydration or other problems. People with ongoing liver or kidney conditions should seek medical evaluation if they think their condition is worsening. Although it may be difficult to know if symptoms are related to cyanotoxin exposure, persistent symptoms should not be ignored. 

Dogs will exhibit symptoms after the first hour of exposure. Because dogs are susceptible to these toxins at extremely low levels it is very important to get your pet to a veterinarian as soon as possible if they show signs of diarrhea, vomiting, breathing problems, difficulty walking or standing or loss of appetite.