Recent surveys by the Maryland Department of Natural Resources indicate that densities of Microcystis in the Upper Chesapeake Bay and tributaries have declined somewhat from concentrations reported during the week of August 5. Maryland DNR conducted field surveys of the Sassafras, Bohemia, Elk, and Northeast rivers on August 15. Small, scattered surface blooms were evident on portions of the Bohemia, Elk, and Northeast rivers. No surface blooms were observed on the Sassafras River, although some Microcystis cells were observed in the water column. DNR has also received reports of surface blooms on the Back and Gunpowder rivers on the western shore. Samples collected by DNR on Back River (August 16) showed densities of 164,376 cells/ml, considered to be a moderate bloom. Samples of the algae were collected on August 9 and 15 and submitted to laboratories for toxin analysis. Results of these samples are pending.

Although the surface blooms appear to have declined in recent days, Microcystis cells are still present in the water and may serve to initiate future blooms if conditions turn favorable. Furthermore, there may be isolated blooms in small tributaries that have goen unreported. Maryland Department of Health and Mental Hygiene advises against swimming in waters that have visible blooms. Microcystis blooms may be identified by the presence of small, blue-green flakes (often described as appearing like thumbnail size flecks of green paint) floating on or near the water surface. Microcystis blooms may be confused with watermeal (Wolffia), a harmless, bright green duck-weed like plant that has recently been observed on the Sassafras River near Georgetown. Watermeal has a course, granular feel when rubbed between the fingers which can be used to differentiate it from Microcystis.

Maryland DNR will continue to monitor Microcystis and periodically publish updates on this web site.

An interagency press release was issued on Aug. 9
to alert the public to this situation.