The Department of Environmental Conservation says a potentially toxic form of algae is growing in Prospect Park Lake.
It's called blue-green algae and it looks like pea soup.
Experts say it can have harmful health effects for people and animals that come in contact with it. Symptoms include nausea, diarrhea, skin or throat irritation, and difficulty breathing.
DEC officials say that the blue-green algae was first spotted last Wednesday. It's urging everyone to avoid it.