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Algae blooms being observed in Lake Stevens


July 3, 2012

An aerial view of the algae blooms on Lake Stevens.

The City of Lake Stevens and Snohomish County Surface Water Management (SWM) have been monitoring a series of algae blooms occurring this spring on Lake Stevens. Most of the observed algae has been harmless filamentous algae which appears as green and brown free-floating mats.

However, in mid-June, blooms of potentially toxic blue-green algae were also detected in isolated parts of the lake.

Also known as cyanobacteria, certain species of blue-green algae can produce toxins that affect the health of people and animals that recreate in lake water. Pets that drink lake water are of special concern. Blue-green algae look like blue, green, or even white paint floating on the surface of the water and will quickly dissipate if agitated.

Water samples were taken within hours of the initial confirmation of blue-green blooms. Since toxin testing takes several days, precautionary notifications were issued to nearby lakefront residents and CAUTION signs (see below) were posted at the public access location around the entire lake. The signs, warn people not to swim or ski in areas of scum, avoid drinking lake water, keep pets away from the water; clean fish well; and avoid areas of scum when boating.

Fortunately, the toxins of concern were found at levels below the recreational standards set by the Washington State Department of Health. The blue-green algae bloom has also since dissipated. Therefore, the CAUTION signs posted at all public access sites will be removed. The County and the City will continue to monitor the algae bloom. It is possible that blue-green algae blooms may re-occur this summer or fall. Citizens should exercise caution if blue-green algae scum is present.

Algae are microscopic organisms similar to plants that can be found in all freshwater lakes including Lake Stevens. Algae are a natural and essential component to the lake because they serve as the base of the aquatic food chain. However, excessive amounts of algae can occur in response to high levels of nutrients and favorable weather conditions. Typical nutrient sources are lawn fertilizers, runoff from roofs and driveways, and pet and animal wastes. Last year’s Eurasian water milfoil treatment may also be contributing to the growth. The decomposing plant matter can become a localized source of nutrients feeding algae. This is typical in the first year following a treatment.

To find out more information, track conditions at Lake Stevens, report blooms, or sign up for email toxic algae updates visit the County’s web site at:


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