The cyanobacterium has blossomed during the state's drought. It occurs naturally and can cause gastrointestinal problems and allergic reactions in people, and sickness and death among pets and wildlife.
The microbes are causing more trouble because nutrients that built up during the drought were washed off hills during this year's rains.
Officials say there is no quick solution to the problem, and the best thing to do is to stay out of the water until the bloom is over.
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