Environmental factors associated with a toxic bloom of Microcystis aeruginosa

Environmental factors associated with the occurrence of toxic cyanobacteria l blooms and toxin production were investigated during the summers of 1994 and 1995 in Steilacoom Lake, Washington. A pronounced and prolonged toxic b loom of Microcystis aeruginosa occurred during summer 1994 but not during 1 995. Lake characteristics that were associated with the toxic bloom in 1994 were higher total phosphorus, decreased water transparency, high water col umn stability, high surface water temperature and pH, and decreased lake fl ushing. Decreased water transparency during 1994 may have been due to signi ficantly lower zooplankton abundance. We hypothesize that this decreased tr ansparency was caused by increased planktivory by higher numbers of coho sa lmon (Oncorhynchus kisutch) fingerlings during 1994 and (or) inhibition of zooplankton grazing by Microcystis. The success of Microcystis over other c yanobacteria was associated with low nitrogen to phosphorus ratios and low nitrate-nitrogen with sufficient ammonium-nitrogen concentrations. Toxin pr oduction (i.e., micrograms of microcystin per gram of plankton biomass) was not constant over the duration of detectable toxicity; hence, no relations hip was found between Microcystis abundance and microcystin concentration. However, microcystin concentration was positively correlated with increasin g soluble reactive phosphorus concentrations between 1 and 10 mu g.L-1, ind icating that toxin production may have been limited by phosphorus.