Brookline - Environmental groups are urging residents to keep dogs and children away from the Muddy River after a hazardous bacteria was detected in the Back Bay Fens yesterday.

The bacteria, known as blue green algae, was spotted by environmental crews near the Agassiz Road bridge in Boston. An initial test showed the water contained a potentially hazard level of the bacteria, which can cause rashes and throat irritation in humans and has been known to kill dogs in some cases.

The crewmembers of the Charles River Watershed Association reported seeing signs of the bacteria down stream from the bridge, but not in the water flowing down from Leverett Pond in Brookline. The association plans to conduct further tests next week.

The association notified community members and public health officials this morning. The Emerald Necklace Conservancy plans to post warning signs early next week.

Blue green algae are toxic bacteria that feed off pollutants and decaying organic matter.

Kate Bowditch, director of programs for the Watershed Association, said the algae bloom was not likely caused by a recent mineral oil spill in Leverett Pond, but could be related to droppings from geese. Residents have recently reported unusually large goose populations in Brookline and town officials are taking some steps to push them out of sensitive parks.

Yesterday’s finding was a fortunate accident. The association frequently checks the Charles River for blue green algae, but crewmembers just happened to notice the bacteria in the Muddy River as they were passing by yesterday.

“There isn’t anyone I’m aware of who is monitoring blue green algae levels on the Muddy River,” Bowditch said. “That may need to change.”