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These large green clumps are part of the algae bloom in Lake Sammamish.
On Friday September 19, 1997, King County Water and Land Resources staff responded to a citizen's call about an algae bloom on Lake Sammamish. Not all of the lake was surveyed on September 19, but an extensive algae bloom that extended along much of the south shore of the lake was observed and a large bloom was reported on the west side extending about two-thirds of the way up the lake. The swimming area of Lake Sammamish State Park had sufficient algae to tint the water a bright green and extensive surface film was present at the beach. A grab sample of this surface bloom was collected from within the swimming area of the park and on Saturday was carried to Dr. Michele Crayton, an algal toxicology expert at Pacific Lutheran University. The cyanobacteria blooming in the lake was primarily Microcystis aeruginosa and tested positive for toxicity.
This cyanobacteria bloom was tested for toxicity using a mouse bioassay similar to the one used for detecting "red tide" toxins found in marine environments. Using this test, the relative toxicity of the Microcystis bloom in Lake Sammamish was considered to be high. The greatest risk from toxic algae is to small children and pets, which require a smaller dose of the toxin to be harmful. Lake residents and visitors were cautioned to keep children and pets out of any area containing a bloom until the bloom had disappeared completely.
Toxicity levels were still high throughout the lake on October 1, 1997 even though the bloom appeared to be diminishing. Water samples taken on October 9, 1997 showed that the lake water was no longer toxic. Typically the toxin remains in the water for about a week after the bloom disappears.
Algae bloom in a swimming area in Lake Sammamish State Park.
The 1997 Microcystis aeruginosa bloom began around Thursday September 18 and continued through the first week of October. Whole lake surveys and sample collections were conducted on September 23, 1997, October 1, 1997, and October 9, 1997. Plankton samples of the blue-green algae were collected at the boat ramp in the State Park at the southeast corner of the lake, off the mouth of Lewis Creek, in the middle of the lake, and at the mouth of the Sammamish Slough at the extreme north end of the lake where the lake empties into the Sammamish River at each site. Samples were collected at these geographically separated locations to determine the extent of the bloom and if all of the bloom was toxic.
On September 23, winds blowing out of the north concentrated the surface accumulations of cyanobacteria, or blue-green algae, downwind in the southern portion of the lake. The algae bloom was very heavy in the southern two-thirds of the lake, and extended all the way across the lake with particularly heavy accumulations of algae in bays and inlets along the south shore. Surface accumulations were also observed in the middle of the lake. The northern part of the lake near Marymoor Park was clear, although low concentrations of algae could be seen in the water. By September 24, the bloom and the toxin were found throughout the lake with highest toxicity (based on the mouse bioassay) in the south and lowest concentrations at the north end near the entrance to the Sammamish Slough. A distinctive smell could be detected over the southern two-thirds of the lake where the bloom was most concentrated. Advisory signs were posted at Lake Sammamish State Park, Idylwood Park and at Marymoor Park to warn people of the toxic bloom, and several stories in the local news media provided the public with information on the bloom. 
Cyanobacteria along the shore of Lake Sammamish.
On October 1, sampling was conducted in heavy rain and fairly strong winds that prevented the formation of surface accumulations that had been observed previously. High concentrations of algae were collected from the near surface waters using a plankton net. All of the algae samples tested positive for toxicity, with the samples collected at the mouth of the Sammamish Slough in the north end of the lake and off Lewis Creek in the southwest tested the most toxic.
Samples collected on October 9 were much less concentrated and would not be characterized as a bloom. Samples showed a species shift from Microcystis aeruginosa to non-toxic Coelosphaerium. The bloom occurring during this same period in Seattle's Green Lake was also non-toxic Coelosphaerium.
An extensive study was conducted in Lake Sammamish in the summer and fall of 1999. This study revealed a toxin-producing bloom during late August and early September despite the absence of visible cyanobacterial biomass (B. Kenworthy, masters thesis University of Washington, 2000). We do not know how many future blooms of these cyanobacteria in Lake Sammamish will be toxic, or what the long-term effects might be. What we do know is that if nutrients entering the lake continue to increase, blooms will occur more frequently; and based only on probability, toxic blooms would occur more frequently as well.
For more information about current monitoring for toxic cyanobacteria in King County visit the King County Toxic Algae Web page.
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