Possibly harmful and toxic blue-green algae, or cynobacteria, have bloomed in three East End water bodies, according to Suffolk County and the New York State Department of Conservation.
The blooms have occurred in Lake Maratooka in Mattituck, Lake Agawam in Southampton and Big Reed Pond in Montauk.
Health officials are asking residents not to swim or wade in these waters and to keep their children and pets away.
Residents who have been exposed to cynobacteria and exhibit symptoms such as nausea; skin, eye or throat irritation; or breathing difficulties are advised to seek medical attention.
According to the county, cynobacteria are naturally present in lakes, but when they become abundant blooms in shakes of green, blue-green, yellow, brown or red appear, and they may produce floating scums on the surface or cause the water to take on paint-like appearance. Those who come in contact with water that is scummy or discolored are advised to rinse off with clean water immediately.