ANNAPOLIS, MD — The Maryland Departments of Natural Resources, Health and Mental Hygiene, and Environment today advised Marylanders to take precautions when swimming, boating or recreating in the Charles County and Prince Georges-Charles County border waters of the Potomac River and its tributaries. Current surveys show that blue-green algal blooms of Microcystis are present from Piscataway Creek to Smith Point. The last major Microcystis bloom event on the Potomac River was in 2004.

Blue-green algae naturally occur in tidal freshwater portions of the Chesapeake Bay and its tributaries. However, heavily affected bloom waters may appear as if bright green paint is floating on the surface of the water as cells accumulate in scums. Tides and weather (e.g., wind, rain and the resulting runoff) can affect the location and intensity of the bloom on the river.

Since 2000, 100% of laboratory tests of Microcystis blooms occurring in Maryland tidewaters have detected a liver toxin (microcystin). One third of the tests showed levels considered cautionary for human contact or consumption. There is no way to tell if a bloom is toxic or the level of any toxins present just by looking at the water. Four samples were sent out for toxin testing; results are pending.

Although there have been no confirmed cases this year of human illness, pet or livestock deaths linked to the bloom, people should take common precautions to reduce the risk of illness or discomfort if they come in contact with algae blooms:

Algae are a natural and critical part of our Chesapeake and Coastal Bays ecosystems. However, excess nutrients, food web imbalances and poor flushing are factors that can contribute to excess algal growths expressed as ‘blooms’. Algae may become harmful if they occur in an unnaturally high abundance or if they produce a toxin. Some algal species can also produce chemicals that are toxic to humans and aquatic life. Fortunately, of the more than 1400 species of algae in Chesapeake Bay, less than 2% of them are believed to have the ability to produce toxic substances.

To report human illness from bloom water contact or consumption, a fish kill or fish health-related event, call Maryland’s Bay Safety and Environmental Health Emergency Hotline at (877) 224-7229, 24 hours a day.

The Maryland Departments of Natural Resources, Health and Mental Hygiene, and Environment will continue to monitor bloom conditions on the Potomac River. Algal Bloom news is reported at the DNR website: http://www.dnr.state.md.us/bay/hab/index.html.

The next monitoring cruise on the Potomac River is scheduled for September 5, 2006.