Officials hastily added "Frank" to a copy of a sign used at nearby Lake Needwood.
Elevated levels of a toxic substance have been found in a second lake in Montgomery County. Authorities are warning people to stay out of the water.
Microcystin is a toxic substance produced by blue-green algae. If ingested, it can do serious damage to the livers of both humans and dogs.
Park officials in Montgomery County have banned swimming at Lake Frank because of the microcystin. A similar restriction was placed on Lake Needwood just to the north for the same issue last month, though boating and fishing are still allowed there.
Signs line fences along Lake Frank warn passers-by to stay away, and pet owners are urged to keep their animals from playing in or drinking the lake water.
Both bodies of water are located within Rock Creek Regional Park.
These restrictions will stay in effect for the rest of the summer.