City of Seattle

Ed Murray, Mayor


SUBJECT:   Green Lake algae bloom only harmful where scum is found
9/12/2013  8:46:00 AM
Christina Hirsch  (206) 684-7241

Green Lake algae bloom only harmful where scum is found

While toxic algae has been found in accumulated scums along the shores of Green Lake, the lake remains open to most activities.

King County Natural Resources has been conducting testing on the lake for several weeks. After each test, the information has been reviewed by Public Health – Seattle & King County, and it has not recommended closure. Tests have revealed that the toxins are only found in the scummy algae that accumulate in some places along the lakeshore.

People and pets should not be in the lake where the scum has accumulated along the lake. Dog owners should be especially cautious not to allow animals to drink from the lake in these areas. If there is water contact for a pet, it is important to rinse well to remove all algae.

The lake remains open to fishing, boating, stand-up paddling boarding and other recreational activities. While Seattle Parks and Recreation’s lifeguarded beaches program is over for the summer, swimming is still allowed within 50 feet of shore.

A warm, dry summer has promoted the algae bloom, and a warm fall continues to promote it. Blooms have been known to last into November in particularly warm autumns, and typically disappear as the weather gets colder.

The King County Environmental Laboratory conducted the testing under the auspices of the Washington Department of Ecology’s algae program.

The lake was closed in 1999, 2002, 2003, and most recently in fall 2012 for toxic algae blooms. Intense blooms of blue-green algae have occurred in Green Lake since 1916.

Treating the lake with alum inactivates the phosphorus that is released from the bottom sediments and prevents stimulation of the algae growth. Green Lake was successfully treated with alum in 1991 and 2004. The water quality improved for several years following treatment on both occasions, and has been mostly good since 2004.

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Seattle Parks and Recreation