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King County scientists see large scale algal bloom, known to be harmful to fish, in central Puget Sound


Scientists from King County’s Environmental Laboratory discovered a large harmful algal bloom in central Puget Sound – Heterosigma akashiwo – which is of particular concern to fish.


During routine water quality monitoring of Central Puget Sound late last week, field staff and scientists from King County’s Environmental Laboratory discovered a very large bloom of a common, yet harmful algal species frequently associated with fish kills.


Water sample analyses showed this alga at a concentration as high as 4 million cells per liter. The closest concentration in the last six years was half the size at roughly 2 million cells per liter at a single protected location.

“An algal bloom of this size is of concern to us because the species identified has been repeatedly associated with fish kills. The King County Environmental Lab is closely monitoring this situation” said Christie True, director of the King County Department of Natural Resources and Parks, where the Environmental Lab is operated.


The alga is a common and globally distributed coastal species called Heterosigma akashiwo, which frequently blooms in the shallow recesses of Puget Sound's shoreline. Rarely does a bloom appear in high numbers in the mixed waters of the sound’s three deeper basins.


Scientists believe that the bloom may have been caused by the recent period of rainy weather which established a stable surface layer of water that was low in salinity and nutrient-rich, where these cells could thrive and reproduce readily.


A relationship between Heterosigma blooms and rising water temperatures has been documented in field and laboratory studies elsewhere, suggesting that as the average global temperature rises, we could see an increase in the frequency of blooms of this toxic flagellate in Puget Sound waters.           


Since 2008, the King County Environmental Laboratory has monitored the phytoplankton community of the Central Basin of Puget Sound, using microscopy to document the enormous variety of phytoplankton that inhabit these waters.


King County shares data with state and local agencies and Tribes.






Logan Harris, 206-477-4516


About the King County Water and Land Resources Division

The Water and Land Resources Division works to protect the health and integrity of King County’s natural resources. Employees work to reduce flood risks, monitor water quality and restore wildlife habitat; manage, and reduce the harmful impacts from stormwater, noxious weeds and hazardous waste; create sustainable forestry and agriculture; and protect open space to support all of these efforts.