Washington, D.C. –The algal blooms that were previously reported in the Potomac River have largely dissipated with a few remaining isolated patches according to regional experts monitoring the river. The earlier water contact advisories issued by the Maryland Department of Health and Mental Hygiene and the Stafford County, VA Department of Health are no longer in effect. 

Microcystis, a common species of cyanobacteria, also called blue-green algae, is often found in fresh and low saline waters during the summer, and first appeared during the second week in July in the Potomac River near Mattawoman Creek in Maryland and Aquia Creek in Virginia.  These blooms are caused by excess nutrients, nitrogen and phosphorus.  The nutrients enter the water in various ways, including agricultural runoff, urban stormwater, airborne sources, and discharges from sources like wastewater plants.

“A combination of cooler temperatures, increased wind, and above normal rainfall from Tropical Storm Lee and Hurricane Irene essentially caused the blooms to disappear,” said Stuart Freudberg, COG’s Director of Environmental Programs. “As we approach the fall season with cooler weather, the bloom is not expected to recur.” Regional experts noticed a drop in Microcystis levels even before Tropical Storm Lee brought record breaking floods to our region. The abundant amount of rainfall from the storm washed away the majority of blooms that remained. 

COG coordinated with officials from the State of Maryland, Commonwealth of Virginia, and District of Columbia as well as academics from George Mason University, the University of Maryland, and Virginia Tech to monitor the blooms throughout the summer months. Agencies will continue to monitor the Potomac River to determine water quality conditions and assess improvements related to restoration activities.  Moving forward, COG will also work with regional experts to understand the causes of this summer’s blooms, determine ways to forecast future ones, and study the water quality effects that Tropical Storm Lee is having now and could have on bloom developments next summer.