The Charles River Watershed Association discovered toxic algae in the Charles River and is advising residents to refrain from coming into contact with the water.
The association issued a public health advisory for the dangerous discovery on Wednesday. It said the toxic algae, known as cyanobacteria or blue-green algae, was found near the Boston University Bridge to the Museum of Science.
As a result of the finding, the Massachusetts Department of Public Health recommended signs be posted in the area to warn people. Dogs should be kept from swimming in the area, according to the association.
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Cyanobacteria can be caused by warm temperatures, sunlight and an excess in nutrients of nitrogen and phosphorus. In the Charles River, the algae can come about following a heavy rainstorm.