General Algae Information

What are algae?

Algae are primitive, primarily aquatic, one-celled or multicellular plant-like organisms that lack true stems, roots, and leaves but usually contain chlorophyll. There are both marine and freshwater algae, and algae are found almost everywhere on earth. The Washington Department of Ecology's algae program focuses on algae problems in freshwater. Pond scum is algae (mostly), the green slime on rocks in a stream is also algae, as is the pea soup green seen in some nutrient-enriched lakes.

Why do algae grow in my lake?

Algae grow when they have the right conditions such as adequate nutrients (mostly phosphorus but nitrogen is important too), light levels, pH, temperature, etc. Generally the amount of phosphorus controls the amount of algae found in a freshwater lake or water body. The more nutrient-enriched a lake,Algae typically the more algae in the lake. As Washington lakes and their watersheds are developed, increasing problems are seen with algae.

Are there good algae?

Healthy lakes need algae. Algae are important to the productivity of a lake or water body. Algae are primary producers. They use sunlight (through photosynthesis) to produce carbohydrates and are eaten by grazers such as protozoa and zooplankton (little animals like water fleas and rotifers). The zooplankton are, in turn, grazed upon by fish, which are eaten by bigger fish, and on up the food chain. A productive lake produces large fish and good fishing for humans as well as supporting food and habitat for wildlife and waterfowl. In this context most algae are desirable for lakes.

Do some kinds of algae cause more problems than other algae?

Yes, filamentous green algae and blue-green algae (cyanobacteria) cause the greatest concern with Washington's freshwater body users. Another freshwater algae called Didymo is beginning to cause problems in some of Washington's streams and rivers.

Filamentous green algae

Filamentous green algae form billowing clouds of slimy or stringy algae in the water. These algae generally grow in nutrient-enriched lakes during the summer and early fall. Mats often form around the tops of aquatic plants. Filamentous green algae can interfere with boater access, recreation, and aesthetics, but are generally harmless. However, many lake groups are concerned with managing the growth of these nuisance algae. Unlike blue-green algae, filamentous algae can be physically removed from the water. However, techniques such as raking may not be effective when large areas of algae are involved. Mats can grow back quickly. Click here to see a photograph of filamentous algae.

Blue-green algae (cyanobacteria)

Nutrient-rich lakes or ponds may support rapid growth of blue-green algae (algae blooms). Blue-green algae are actually bacteria. They are called cyanobacteria after the blue-green pigments that they produce. Blue-green blooms can form surface scums up to several inches thick, especially along the shoreline. Cyanobacteria are of greater concern than true algae because some species can produce potent toxins. However, even known toxin-producing species may not produce toxins all the time. If a blue-green bloom is composed of toxin-producing species, it is not possible to determine whether it is producing toxins without special testing. Even then, if a bloom tests non-toxic, it has the potential to become toxic. Therefore people and their pets are warned to avoid contact with surface scums whenever a blue-green bloom is suspected. Even if blue-green blooms are not toxic or are composed of a species that does not produce toxins, they are unsightly and when they decompose often produce bad odors. Blue-greens are also not very edible. To view a close-up of the photograph (taken by Bijay Adams of Liberty Lake), please click on the photograph. Click here to learn more about cyanobacteria in Washington.

For additional information about cyanobacteria see this website.


Didymo (short for Didymosphenia geminata) has been showing up in some Washington rivers. Click here to see a photograph of Didymo. This diatom is normally found in alpine lakes but seems to be expanding its range to lowland rivers. Freshwater diatoms rarely cause problems, but Didymo has started to grow prolifically in some rivers in the United States and in other countries. It has been introduced into New Zealand where it is considered an invasive and noxious species. Didymo forms beige-to-brown mats that are attached by stalks to the river bottom. Another name for Didymo is rock snot and people have described its growth as looking like trailing toilet paper in the river. Biologists suspect that the presence of this algae has led to trout declines in a South Dakota river (story from the South Dakota Rapid City Journal). In addition to being unsightly, this species may cause habitat alterations. For more information about this emerging problem algae see this website: Didymosphenia Geminata

What can I do about algae in my pond or lake?

There are no simple answers. Algae may grow for a variety of reasons, but nutrients generally limit algae growth. Any long-term solution to algae management involves nutrient reduction. Taking steps to manage nutrient inputs to a water body is crucial, although many times nutrient management involves managing the entire watershed and not just the area immediately around the lake. Nutrients throughout the watershed may be contributing to the algae problem. Local jurisdictions must become involved with stormwater or agricultural issues or other external nutrient sources. In these cases, lake restoration techniques may be needed. Lake residents can employ best management practices to help reduce nutrients. In the short-term there are control options that may lead to a temporary reduction in the growth of algae.

What is that algae in my lake and how can I get it identified?

If you are a Washington state resident, the Washington Department of Ecology offers an algal identification service. If you think that your lake has an algae bloom and you want to have the algae identified, please see this site: Washington State Toxic Algae, and go to the “Report a bloom” tab and follow the instructions.

Join Ecology's Freshwater Algae Program List Serve

Ecology has set up a list serve for freshwater algae. The Freshwater Algae Program List serve will act as an information exchange and discussion forum about freshwater algae in Washington. A list serve is a program that allows people to send email to one address, which automatically resends that message to all of the other subscribers on the list. If you would like to join the list click here for directions on how to subscribe to the list.

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