Crystal Lake in Newton, Massachusetts, is off limits because of a harmful cyanobacterial bloom on the water.
People and pets are urged to stay out of the lake.
"We walk by the lake every single day, and when the weather is warm, we let him dip his toes in there," said dog owner Sharan Leventhal.
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But there won't be any quick dip for her dog, Mr. Blue, as the blue-green bacteria is too unsafe for the 5-year-old Australian shepherd.
"He's a big wader and I'm sure he misses it because he's pulling to the water, toward the water, every time we go by," said Leventhal.
So far, five confirmed toxic blooms have popped up across the state. Besides Crystal Lake, state health officials have named Billington Sea in Plymouth, Devol Pond in Westport, Long Pond Marstons Mills in Barnstable and Shubael Pond in Barnstable.
A blue-green film on the surface of the water is an indication that there's a poisonous bacteria.
"Just be particularly careful with your dogs," said Newton Mayor Ruthanne Fuller. "Dogs are even more susceptible to the toxins than human beings. Everyone, stay out of the water until we get a clean bill of health."
The bacteria has become a national issue, as several dogs have now died or gotten sick after swimming in infected water in other states.