LOS BANOS (AP/CBS13) – A bloom of toxic blue-green algae in a Central California reservoir has reached the danger level, the state Department of Water Resources said Tuesday.

Lab results from this week’s tests showed an increase in toxin levels at San Luis Reservoir in Merced County, the department said.

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The San Luis Reservoir is California’s fifth largest water storage reservoir and the nation’s largest “offstream” lake, and is currently holding at less than 14% capacity as viewed on August 30, 2021, near Santa Nella, California. (Photo by George Rose/Getty Images)

Boating is allowed but people and pets should avoid physical contact with the water and algal scum. Fish and shellfish from the lake should also be avoided.

Toxic blue-green algae is also known as cyanobacteria. It can cause eye irritation, allergic skin rash, mouth ulcers, vomiting, diarrhea and cold- and flu-like symptoms.

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Blue-green algae is natural and the rapid growth that becomes a bloom typically occurs when water is warm and still and certain nutrients are present, according to the department.

In Southern California, a bloom of blue-green algae has reached the warning level at Silverwood Lake in San Bernardino County, the department said. Swimming beaches have been closed.

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Copyright 2021 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.