State officials cautioned Friday against swimming, water skiing or other activities in a portion of the Potomac River and one of its tributary because of a harmful algae bloom there.

Elevated levels of mycrocystis, a blue-green algae, have been reported in the Potomac south of Mattawoman Creek and in the mouth of the creek itself, according to state health and natural resources officials.

Blue-green algae, which often turn the water bright green, produce a toxin that can irritate the skin on contact or cause nausea and other ill health effects if swallowed. The Maryland Department of Health and Mental Hygiene warned people to stay out of the water in the general area, and to keep pets and livestock away.

State health officials urge anyone with possible health effects from algae blooms to report them by calling 1-866-703-3266. Anyone seeing dead or apparently sick fish should call the 24-hour bay health hotline at 1-877-224-7229.