In the summer of 1996, off-flavor episodes involving 2-methylisoborneol (MIB) occurred in two southern California reservoirs: San Vicente Reservoir, in the San Diego area, and Lake Skinner, in western Riverside County. These events resulted in many consumer complaints from the respective communities served. During the San Diego episode, moderate numbers of Pseudanabaena and other plankton were present in the water, and MIB was detected in one sample at 23 ng/l. Two strains of Pseudanabaena that produced MIB were isolated; cultures of this organism yielded 22 and 23 μg/l, respectively. Lake Skinner water also contained a moderate number of Pseudanabaena and other plankton when MIB levels began to rise, with MIB reaching 10 ng/l. Liquid and agar cultures derived from Lake Skinner water developed a strong MIB odor, and eventually three MIB-producing isolates were obtained. MIB levels in the cultures were all less than 1 μg/l. This was the first known MIB episode of planktonic origin in Lake Skinner, and it was the first off-flavor event in either reservoir in which Pseudanabaena was implicated. These two events underscore the potential importance of an organism not generally recognized as a cause of off-flavor in water supplies.

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