The scientists from the Chesapeake Bay Program, a regional partnership, view the grasses as a key measure of bay health because they provide shelter for fish and crabs, protect the shoreline and keep the water clear. The program measures the grasses annually from the Susquehanna Flats to the mouth of the bay, as well as those in the system's rivers.
The scientists attributed the decline to unusually hot weather and heavy rains in the last two years, though those conditions actually helped some areas of the bay.
"The summer of 2010 was unusually hot, causing eelgrass beds in the lower bay to severely die back after they had been surveyed earlier in the year," said Robert J. Orth, a professor at the Virginia Institute of Marine Science. "Then in spring of 2011, during the growth season, heavy rains and freshwater runoff created very muddy conditions for many low-salinity species, in the upper bay and tributaries. This was followed in September and October by Hurricane Irene and Tropical Storm Lee that again muddied the water."
Last year marked a shift from past trends, said Orth and Lee Karrh, a biologist at the Maryland Department of Natural Resources. In the past decade, the Upper Bay grasses had been increasing but had major losses last year. The grasses in the brackish part of the Middle Bay did the opposite, increasing last year after decreasing for years before.
The pair did note a big positive: Grasses on the Susquehanna Flats maintained through the heavy storms. The grass acreage there had tripled in size since 1991 and surpassed restoration goals from 2008 through 2010.
Other gains were made in acreage in the Eastern Bay, Choptank River, Little Choptank and Honga rivers. Scientists say this may have been because of those storms, which lowered the salinity and stimulated germination of dormant seeds, including widgeon grass.
The scientists measure the grasses through an aerial survey from late spring to early fall and view the results as an indication of how well pollution control measures are working.
"Our hope is that the implementation of the bay 'pollution diet' will help increase [submerged aquatic vegetation] acreages in areas that currently are below our restoration goals and reduce the impacts of extreme weather events in the future," Karrh said.