Wintertime toxic blooms of the blue-green alga Anabaena flos-aquae have been documented since 1989 in American Lake, a 446-ha lake near Tacoma, Washington. The toxic episodes were unusual in that algal toxicity occurred during the winter and this lake is considered to be only moderately productive. A year-long study was conducted to determine the environmental factors associated with toxic conditions. Toxic blue-green algal blooms in American Lake were associated with increased nutrient (especially phosphorus) availability following winter turnover. Phosphorus released from lake sediments under conditions of low oxygen during lake stratification fueled winter blooms of algae, including the toxic Anabaena flos-aquae strain. The lake's extremely low iron content was insufficient to completely remove this released phosphorus from the water column during lake turnover.

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