The toxin if ingested in high concentrations can harm the liver, the agency said.
It said it was particularly concerned about dogs that were off leash and may swim in or drink from the lake, despite regulations.
The agency called on visitors to Lake Needwood and Lake Frank to take these precautions:
•Avoid direct contact with water while boating or fishing.
•No swimming. (It is prohibited in the lakes at all times, the agency said.)
•Keep dogs on leash (as is regularly required at the park), and do not allow them to drink or be in contact with the water.
•Wash hands thoroughly before eating, drinking or smoking if someone comes in contact with the water.
•Only eat properly cooked muscle meat of fish that are caught in the lake.
Recreational activities including fishing and boating will continue, the park agency said. Officials said parts of both Lake Needwood and Lake Frank may be contaminated for the rest of the season. Conditions will be monitored and posted warning signs will be updated if warranted, officials said.