Signs warning against contact with the water are being posted at prominent areas, including Black Lake Grocery, Kenneydell County Park, the Black Lake boat launch and Columbus Park.
A sample of lake water was sent to a King County laboratory last week. Results showed concentrations of microcystin, a toxic species of algae, present at 8.2 parts per billion. The limit for lake recreational use is 6 ppb, according to the state Department of Ecology.
Another lake sample was taken Tuesday morning and forwarded to the King County lab for testing.