Park officials in Montgomery County are keeping close watch on Lake Needwood after the water tested positive for toxic algae. It can cause severe liver damage if ingested.
The lake remains open. You can still go fishing - as long you cook your catch.
But the main concerns are dogs drinking the water and kids wanting to play in it.
At Lake Needwood in Rockville notices are up all over, warning visitors of toxic blue-green algae.
"Definitely don't want to drink it," says Raymond Dahl who likes to fish at the lake. "You don't want to get into it. If you get it on yourself, you want to wash it off."
"It's frustrating in that we don't know what to do about it," says Doug Redmond, Montgomery County Parks Natural Resources Manager.
He says the poisonous blooms multiply in warm water enriched with phosphorus or nitrogen.
They were first spotted here by a park patron in 2009. Since then, the bacteria has come and gone naturally.
"We will continue to test," Redmond says. "We will continue to post signs when appropriate. And hope that some of the research that's going on will eventually produce something that we can use to either eliminate or at least decrease the concentration of the algae and therefore the toxins in the water."
"Bring on the algae," says Shelly Levin, a Rockville resident. "It doesn't bother me at all."
Levin and many other park goers still soak up the beauty without testing the water.
John Byron of Aspen Hill walks his dogs here several times a week, but says not everyone exercises caution.
"You watch the other people, like I said, walk up to the signs and let their kids play in the water," Byron says. "People throwing the balls for the dogs to run out in the water. They stand right next to the sign. It makes you wonder what they're thinking."
Swimming is prohibited at all times in Lake Needwood. Dogs should always be walked on a leash.
And if you do fish, wash your hands before eating, and eat just the muscle meat. Toxins tend to accumulate in the internal organs.