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by: Bernadette Rae Kenworthy

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Engineering

University of Washington
Program Authorized to Offer Degree: Department of Civil and Environmental Engineering

This study was funded by a grant from the National Science Foundation Professional Opportunities for Women in Research and Education Program (NSF POWRE DEB-9805585) to Dr. Jean Jacoby (Department of Civil and Environmental Engineering, Seattle University, Seattle, WA), who served as chairperson of the graduate committee. Other members of the graduate committee were: Dr. Jonathan Frodge (King County Water and Lands Resources Division, Seattle, WA) and Drs. Michael Brett and Eugene Welch (Department of Civil and Environmental Engineering, University of Washington, Seattle, WA).


In fall 1997, a toxic bloom of the cyanobacterium Microcystis aeruginosa was documented in Lake Sammamish (western Washington) for the first time. Ingestion of the toxic material was implicated in the death of a pet dog and the illness of children who swam in the lake. Cyanobacterial activity and environmental conditions that may promote toxic cyanobacterial blooms were investigated during summer and fall 1999. Bloom proliferation was hypothesized to be due to runoff of nutrients from the watershed (external loading hypothesis) or from migration of dormant cyanobacteria from the nutrient-rich sediments into the water column (cyanobacterial migration hypothesis). Analysis of microcystins using an enzyme-linked immunosorbent assay revealed a toxin-producing bloom during late August and early September 1999 despite the absence of visible cyanobacterial biomass. Microcystin concentrations ranged between 0.19-3.8 µg L-1 throughout the lake and at all depths with the exception of the boat launch where concentrations reached 43 µg L-1. Comparison of the conditions associated with both the 1997 and 1999 blooms indicate that blooms of toxin-producing cyanobacteria appear to be associated with a stable water column (relative thermal resistance to mixing = 255), increased surface total phosphorus concentrations (> 10 µg*L-1), surface temperatures greater than 22 degrees C, high total nitrogen to phosphorus ratios (> 30), and increased water column transparency (up to ~5.5 m). Migration of the cyanobacteria, Microcystis and Anabaena, occurred in both the deep and shallow portions of the lake. Microcystis dominated (89-99%) the migrating cyanobacteria with greater migration from the shallow station. External loading of nutrients due to the large rainfall preceding the 1997 toxin-producing bloom may have provided the nutrients needed to fuel that bloom. However, toxin-producing Microcystis occurred in 1999 despite the lack of rain and subsequent external runoff. The migration of cyanobacteria from the nutrient-rich sediments may have been the inoculum for cyanobacterial presence and subsequent toxin production in 1999.

A complete copy of this paper may be obtained from the University of Washington Engineering Library.

For questions about Lake Sammamish, please contact Debra Bouchard, Water Quality Planner or Curtis DeGasperi, Lead Hydrologist, King County Science and Technical Support Section.