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Mild weather leads to late-season cyanobacteria bloom in Lake Washington


Environmental investigators have found toxic algae blooms in some parks of Lake Washington. People and dogs are advised to avoid swimming, wading, or playing in the lake if algae are present.


Generally mild weather this fall has extended the algae-growing season on lakes across King County. Toxic cyanobacteria (also known as blue-green algae) are currently blooming along some shorelines on Lake Washington. Some cyanobacteria present are toxic; avoid all contact to be safe.

The King County Environmental Laboratory found concentrations above the state recreational guideline that are potentially dangerous for human health. "Scums" or accumulation of toxic algae were found at Arrowhead Point in Kenmore, Mercer Island, Magnuson Park and Gene Coulon Park in Renton, but they may be found in other areas because the blooms float and are moved easily by the wind.

Advice for people and dogs:

People and dogs should not swim, wade, or play in lake if a blue-green algae or scum is present. Dog owners should not allow their dogs to have any contact with the water if there is an algae bloom or scum nearby. If a dog has contact with an algae bloom, do not let them lick their fur, rinse the dog with clean water, and then rinse your hands and any exposed skin.

Warning signs:

Visitors to the lake are advised to watch for warning signs, which will be updated as conditions change. However, because wind can blow scums rapidly from one part of the lake to another, people should avoid water that appears to have algae even if there are no warning signs present.

Local and state staff will continue to sample for toxicity until the bloom ends. "This is a natural occurrence that will end at some point, but unfortunately there is no way to predict when that will be. Lakes usually follow seasonal cycles, and this particular bloom is out of the ordinary," said Sally Abella, a senior engineer with King County's Lakes Stewardship Program.

Potential health impact:

Toxic blue-green algae can affect humans and make animals very ill, even proving fatal to dogs if ingested in large quantities. If swallowed, the mycrocystin algae toxin may cause abdominal pain, diarrhea, vomiting, and in some cases, severe liver toxicity. Some people or dogs may be sensitive to touching the toxins and have mouth or nose irritation and skin rash.

King County works with the Washington State Department of Ecology (Ecology) on an algae identification and testing program for lakes with frequent algae blooms. To report a bloom or view statewide data and more information about toxic cyanobacteria can be found at:

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