When underwater grasses, also called "submerged aquatic vegetation" or SAV, began to disappear from many of the Chesapeake Bay's shallow reaches in the 1960s and 1970s, we realized that we had taken them for granted. In some areas grasses were even mowed to clear boat channels. Few could predict that in many places they would simply vanish.
After the great die-off began, scientists identified the disappearance of sea grasses as part of a disturbing shift in the Bay's ecology. The Bay, following World War II and especially after Tropical Storm Agnes in 1972, had changed from a system largely driven by life on the bottom (the benthos) to life floating in the water. Among other changes, this shift has meant fewer underwater grasses rooted to the bottom and more floating algal blooms that block sunlight and then deplete oxygen levels as they die and decompose.
Exactly what killed the Bay's underwater grasses? That was the question posed by Maryland Sea Grant's award-winning film, Chesapeake: The Twilight Estuary. While many suspected big industry or the herbicides used to kill weeds on farm fields, this 40-minute documentary followed researchers as they arrived at a surprising answer. Though industrial pollutants and weed killers can stress the grasses, they were not the cause of the great sea grass die-off. Instead, scientists discovered that nutrients such as nitrogen and phosphorus, along with sediment, were killing the grasses by robbing them of light. Nutrients fuel large blooms of algae that, along with sediment, shadow grasses growing on the bottom. Nutrients also feed the growth of small plants called epiphytes that can cover the blades of underwater grasses, further blocking out light. Without sunlight to stimulate photosynthesis, the plants wither and die.
Today, a region-wide restoration effort, led by the U.S. Environmental Protection Agency and the states of the Chesapeake Bay watershed, is struggling to bring the grasses back. The multi-jurisdictional Chesapeake Bay Program has set its Bay-wide goal at 185,000 acres of submerged aquatic vegetation (SAV) by the year 2010. This is approximately the area of grass beds thought to have covered the Bay in the first half of the 20th century.
Guide to Underwater Grasses
To help citizen volunteers,students, and others interested in learning more about underwater grasses, Maryland Sea Grant has produced a new guide in collaboration with the National Oceanic and Atmospheric Administration's Chesapeake Bay Office, the Alliance for the Chesapeake Bay, and the Maryland Department of Natural Resources. The 80-page full-color guide, Underwater Grasses in Chesapeake Bay and Mid-Atlantic Coastal Waters, includes detailed photographs, an identification key and maps that show potential distribution.
Chesapeake Bay Program
Maryland Department of Natural Resources
NOAA Chesapeake Bay Office
U.S. Fish & Wildlife Chesapeake Bay Field Office
Virginia Institute of Marine Science
Integration & Application Network
Bay Grass Restoration in Chesapeake Bay
August 2005 Newsletter (pdf)