About Toxic Algae

Image of toxic algae bloom in Harts lake
Toxic algae bloom in Harts lake

Cyanobacteria, also known as blue-green algae, are bacteria which have some of the characteristics of plants. Cyanobacteria are found throughout the world on land and in lakes, rivers, and ponds, and in estuaries and seawater (oceans).

Cyanobacteria often form blooms when the conditions are favorable. Blooms are often seen in standing water in lakes, ponds, ditches, lagoons, or embayments of rivers. Because many cyanobacteria species can regulate their buoyancy, they often rise to the surface of the water to form a surface scum. A scum is a thin oily-looking film that can become several inches thick (and can sometimes look like spilled paint). When conditions are good for a bloom, a lake or pond can change from clear to turbid within just a few days.

Image for toxic algae bloom in shadow lake
Toxic algae bloom in Shadow Lake

Most cyanobacterial blooms occur during warm summer and early fall months but in Washington, toxic blooms also occur during colder winter months. For example, American Lake in Pierce County has a history of toxic episodes during the winter at low water temperatures (7-8°C). It is possible that a bloom can be found somewhere in Washington nearly every month.


Most of the above information is from "Toxic Cyanobacterial Blooms - A Field/Laboratory Guide". Dr. M. A. Crayton, Pacific Lutheran University, Tacoma, WashingtonWashington Department of Health. Used with permission.

For more information please see:
EPA Harmful Algal Blooms
WA State Freshwater Algae Control Program
WA State Department of Health

Photo credit for Maple Leaf on algae scum (banner photo) belongs to Bruce Andre Photography

View our gallery


These photos provide an introduction to the types of algae we test for. Both macro and microscopic images.

Freshwater algae recreational guideline levels

These levels are provisional recreational guidelines developed by the Washington Department of Health (DOH) to protect human health. The numbers were set to protect human health when toxins are below the recreational guidance values. When toxic algae are present in a lake, the lake may be unsafe for people and animals. DOH recommends that people and pets stay out of areas where algae scum is present.

Microcystin: >= 8 µg/L
Anatoxin-a: >= 1 µg/L
Cylindrospermopsin: >= 15 µg/L
Saxitoxin: >= 75 µg/L

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