Study Shows Tenfold Increase in Native Bay Grasses in Lower Potomac River

A new study by the U.S. Geological Survey (USGS) suggests that underwater bay grasses in the Potomac River are increasing due to efforts to reduce nutrient pollution.

The study, which used data from 18 years of river monitoring, shows that fewer nutrients and clearer waters in the Potomac have increased the amount and different types of bay grasses growing in the river.

Bay grasses – also known as submerged aquatic vegetation or SAV – are critical to the ecosystem of the Chesapeake Bay and its tributaries. Bay grasses provide oxygen to the water and food and shelter for fish, crabs, waterfowl and other species.

Excess nutrient pollution in the water fuels the growth of algae blooms, which block sunlight that bay grasses need to grow and survive.

Between 1990 and 2007, the area covered by bay grasses in the lower Potomac River doubled, from 4,207 acres to 8,441 acres.

In addition, the area covered by native grasses has increased tenfold, from 288 acres in 1990 to 3,081 acres in 2007. Meanwhile, the proportion of non-native species to native species has declined. In 1990, more than 80 percent of the total amount of bay grasses in the lower Potomac was the non-native hydrilla; in 2007, hydrilla declined to 20 percent of all bay grasses.

The diversity of bay grass species in this reach of the Potomac has increased. More than a dozen species of bay grasses – including hydrilla – now co-exist in the lower Potomac.

These improvements have occurred “nearly in lock step with decreases in nutrients and sediment in the water” and reductions in nitrogen in treated wastewater from Washington, D.C., according to USGS scientist Dr. Nancy Rybicki.

“Upgrades to the wastewater treatment plant have benefited [bay grass] habitats 50 miles downstream. These findings underscore the benefits of nutrient reduction efforts on a major tributary to the Chesapeake Bay,” said Rybicki, who has been conducting research on the Potomac since 1979.

“People want to know that money spent on ecosystem restoration is having tangible results, but many feel that efforts to clean up Chesapeake Bay have so far had limited success,” said researcher Dr. Henry Ruhl of the National Oceanography Centre, which also contributed to the study. “Our results suggest that widespread recovery of submerged vegetation abundance and diversity can be achievable if restoration efforts are enhanced across the Bay.”

A multi-agency study released in July found similar results to the USGS study, correlating nutrient reductions with gains in bay grass abundance in some Bay tributaries, while noting a negative correlation between bay grasses and nitrogen.

For more information about the USGS study, visit the project website and the USGS Chesapeake Bay Activities page.