What does an algae bloom look like?

How do you know if your lake is experiencing an algae bloom? The photographs on this page may help you decide if the changes that you are seeing in your lake or river are from algae or are caused by something else. For example, sometimes tree pollen can look like algae. Click on each photograph to see a larger version of the photograph.

TIP: Often sunny periods following heavy rainfall trigger algae blooms. The rain brings in nutrients and the sunlight helps the algae grow quickly.

Some of the most obvious blooms are caused by blue-green algae (also known as cyanobacteria). Many species of blue-green algae are buoyant and will form scums on the water's surface when the wind is not blowing.


This is a blue-green algae bloom in Lake Steilacoom in Pierce County. If you enlarge the photograph, you can see that this algae bloom has floated to the water's surface where it formed a surface scum. Blooms in Lake Steilacoom often produce toxins. Unfortunately there is no way to tell if a bloom is producing toxins from its appearance. Blooms that are not producing toxins one day may start producing toxins the next day. It's best to avoid contact when you see concentrated algae scums on the water. DO NOT let your kids or pets play in the water when you see algae scums! Click on the photograph to enlarge it. Back to the algae monitoring page

This is a blue-green algae bloom washed up along the shoreline in Long Lake, Thurston County. Note the bluish color. This means that some of the algae are starting to decompose leaving this blue pigment. Click on the photograph to see a larger version.
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This blue-green bloom is well on its way to decomposing. Note the pink and blue pigments mixed in with the green. People often mistake blue-green blooms for paint spills. In the later stages of decomposition the bloom can look white, too. When a declining bloom looks like this, it may also produce nasty odors. To see a larger version of this photograph, click on the photo.
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This aerial photograph shows a blue-green bloom on Liberty Lake in Spokane County, Washington. Blue-green blooms can be wide-spread throughout an entire lake or localized along shorelines.
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Bijay Adams, Liberty Lake Manager, took this photograph of blue-green algae growing under the ice in Liberty Lake, Spokane County, Washington. People often think of algae blooms occurring during warm weather, but blue-green blooms can happen at any time of the year - even under ice! Click on the photograph to enlarge it.
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A fish's perspective on what an algae bloom looks like. Jenifer Parsons took this underwater photograph of a bloom in Battle Ground Lake in Clark County, Washington. You can see how a bloom decreases water clarity. To see a larger version of the photograph, please click on the photograph.
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If your lake looks like lakes in these photographs, you may have algae problems. If your lake is in Washington State and you would like to send in a sample for identification, please see this site https://www.nwtoxicalgae.org/Default.aspx , go to the “Report a bloom” tab and follow the instructions.

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