The sometimes toxic cyanobacteria also known as blue-green algae has covered the lake, prompting officials to prohibit boating, fishing and hiking until further notice.
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Experts assured the public that the quality of the district's treated drinking water has not been affected.
"This is a recreation issue, not a drinking water issue," Metropolitan Water Quality Manager Dr. Mic Stewart said. "We don't want folks coming into contact with or fishing in this water."
Caused by naturally occurring organisms that have produced large areas of green water, the algae bloom is among the largest ever seen at the lake since it opened to the public for recreation in 2003.
Blue-green algae blooms are common this time of year because of the warm weather. Officials are continuing to test the toxicity levels of the bloom and will lift the suspension when conditions improve.