Toxic Algae (Cyanobacteria)

Current Toxic Algae and Surface Water Advisories

Toxic Algae

Toxic algae, also known as cyanobacteria or blue-green algae, can produce toxins. These toxins may kill pets, waterfowl, and other animals. They can also cause serious illness or even death in people. Toxic algae are naturally occurring but likely have increased due to impacts from human activities.

What Is A Toxic Algae Bloom?

Toxic algae can reproduce rapidly in fresh water when the amount of sunlight, temperature and nutrients are sufficient. Within a few days a "clear" lake, pond or ditch can become discolored with algae growth. This is called a bloom. Toxic algae blooms often float to the surface and can form a surface scum. This scum, or heavy concentration of algae, can look like green paint and be several inches thick near the shoreline (click here for an example). Wind and weather conditions can greatly change the amount and location of algae.

A health concern may exist whenever there is visible algae.

Toxic algae:

  • Often looks like green paint floating on the surface of the water.
  • Look like small dots in the water that are hard to pick up or hold.
  • Can be many shades of green, but can also be blue-green, brown or red.
  • Most commonly occurs in the summer and fall but can occur any time.
This slide show gives a general indication of what different amounts of algae look like in a lake.


Swallowing lake water containing toxic algae or prolonged skin contact may result in illness, such as muscle weakness, vomiting, diarrhea, and/or nausea. If you know or believe someone may have swallowed water containing large amounts of algae, it is extremely important to seek medical attention.

Toxic algae can cause a skin rash that develops after being in the water. This is a different rash then that caused by Swimmer's Itch.

The risk to pets is much greater than the risk to people, both due to the smaller body sizes of pets and to a pet being more likely to drink water containing a a large amount of algae. If a pet does ingest a large amount of algae and is showing signs such as vomiting, lethargy, disorientation, or seizures, it should be taken to a veterinarian as soon as possible.

Toxic Algae Advisory Explanations

Caution Advisory (Posted Sign): Lake may be unsafe for people and pets.
Warning Advisory (Posted Sign): Lake is unsafe for people and pets.

When a lake is posted with either a Caution or Warning Advisory please follow the precautions:

For more information, or to report a suspected algae bloom, please call the Health Department at (253) 798-6470 or email us at EHSurfaceWater/


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