Bay grass acreage in Chesapeake Bay, rivers decreases 7 percent in 2010

Underwater bay grasses covered 79,675 acres of the Chesapeake Bay and its tidal rivers in 2010, according to data from scientists with the Chesapeake Bay Program. This is a 7 percent decrease from 2009, when bay grasses covered 85,914 acres of the Bay’s shallows.

Despite the drop, the 2010 bay grass acreage estimate ranks as the third-highest Bay-wide acreage since 1984, when the annual survey began.

"Even with the decreases in the 2010 bay grass coverage, the patterns are similar to previous years,” said Lee Karrh, living resources assessment chief with the Maryland Department of Natural Resources and chair of the Bay Program's SAV Workgroup. “Many of the fresh and low salinity areas have very high abundances, including 16 that have reached their restoration targets. However, the saltier parts of the Bay continue to struggle, with most areas well below the restoration goals, with only the mouth of the James River exceeding the goal.”

Bay grass abundance is currently at 43 percent of the Bay Program’s 185,000-acre goal. This goal is based on approximate historic bay grass abundance from the 1930s to present.

“We were pleased that grasses remain healthy and abundant in two areas where nutrient pollution was reduced: the upper Potomac River and Susquehanna Flats,” said Bob Orth, scientist with the Virginia Institute of Marine Science and leader of the baywide annual survey. “However, the overall condition for bay grasses remains one of concern with many areas still having few, if any, grass beds.”

In the upper Bay (from the Susquehanna Flats to the Chesapeake Bay Bridge), bay grasses covered about 21,353 acres. This is a 10 percent decrease from 2009. Large increases were observed in the Chesapeake and Delaware Canal and part of the Sassafras River. However, these were offset by large decreases in local rivers, including the Bush, Bohemia and Magothy. The massive grass bed in the Susquehanna Flats continues to dominate this area.

In the middle Bay (from the Chesapeake Bay Bridge to the Potomac River and Pocomoke Sound), bay grass acreage decreased 11 percent to 35,446 acres. Most segments in this part of the Bay lost grasses. The largest percentage decreases occurred in the middle and lower central Bay, as well as the Choptank, Honga, Patuxent and Potomac rivers. Increases were seen in Tangier and Pocomoke sounds and the Manokin and Big Annemessex rivers, where eelgrass continued to come back following a 2005 die-off.

In the lower Bay (south of the Potomac River), scientists mapped 22,876 acres, a 1 percent increase from 2009. This is the fourth year that bay grasses in this part of the Bay have increased since 2005, when hot summer temperatures caused a dramatic large-scale eelgrass die-off. Most of the gains were in the upper Rappahannock, lower Piankatank, and the upper section and mouth of the James River. These gains offset losses in other areas.

“In 2010, our big concern arose in the lower Bay where eelgrass appeared to suffer another setback from the incredibly hot summertime temperatures,” said Orth. “Since we had mapped those beds prior to the heat wave, losses there are not reflected in our final figures. We believe the really hot summer temperatures in the early part of the growing season may have just cooked the grasses before we were able to map them, e.g. parts of the Honga River. The changes also occurred in areas dominated by just one species, widgeongrass, which has been shown to be a boom or bust species. 2010 may have been the hottest on record but it was those summer time temperatures in June that may have tipped the scale for SAV in some areas.”

Bay grasses – also known as submerged aquatic vegetation or SAV – are a critical part of the Bay ecosystem. They provide underwater life with food and habitat, absorb nutrients, trap sediments, reduce erosion, and add oxygen to the water.

Bay grasses are also an excellent measure of the Bay’s overall condition. The health of bay grasses is closely linked with Bay health. Annual bay grass acreage estimates are an indication of the Bay’s response to pollution control efforts.

Annual bay grass acreage is estimated through an aerial survey, which is conducted from late spring to early autumn. For more information about the aerial survey, and to view an interactive map of bay grass acreage throughout the Bay and its tidal rivers, visit VIMS’ website.