In the shallows of the Chesapeake, Bay grasses sway in the aquatic breeze of the tides and currents. Also known as submerged aquatic vegetation or SAV, Bay grasses are an indicator of the health of the Chesapeake and its rivers.
Like all green plants, Bay grasses produce oxygen, a precious and often decreasing commodity in the Chesapeake. These plants also absorb nutrients like nitrogen and phosphorus. In the water, excess nutrients promote rapid growth of algae known as blooms. When these blooms die off and decompose, valuable oxygen is consumed. Algal blooms also reduce the amount of light reaching Bay grasses, which need it to survive.
Just like shoreline vegetation, Bay grasses trap excess sediment, which can cloud the water and bury bottom-dwelling animals. By reducing wave action, these grasses also help to protect the shoreline from erosion
In addition, the SAV provides food and habitat for invertebrates, fish and waterfowl. Microscopic zooplankton feed on decaying grasses and, in turn, become food for larger animals, such as fish and clams.
Barnacles and scallop larvae attach to the leaves and stems of eelgrass, a grass species found in saltier waters of the lower Bay. Fish, like bluegill and largemouth bass, live in the freshwater grasses of the upper Bay. Immature blue crabs, minnows and juvenile fish, like striped bass, hide from larger, hungrier mouths in the grass beds.
Bay grasses are a haven for vulnerable molting blue crabs, shielding them until their shells harden.
In the fall and winter, migrating waterfowl search the sediment for nutritious seeds, roots and tubers. Redhead grass and widgeon grass are favored foods of ducks of the same name, as well as many other waterfowl.
Bay grasses once formed immense underwater meadows, covering up to 200,000 acres in the Chesapeake Bay and its tributaries. With increasing development and nutrient pollution in the late 1960s and early 1970s, and Tropical Storm Agnes in 1972, the huge grass beds began to decline. Bay grasses continued to disappear, hitting a low of about 38,000 acres in 1984.
Factors that affect water clarity also affect the growth and survival of the Bay's grasses. Suspended sediment clouds the water, reducing the amount of sunlight reaching the plants. Often, sediment covers the plants completely.
Sources of sediment include runoff from farms; residential and commercial developments; and road construction. Shoreline erosion also adds sediment to the water.
Excess nutrients promote algae blooms that cloud the water, reducing sunlight, which the plants need to grow. Certain types of algae grow directly on the plants. Major sources of nutrients included sewage treatment plants, septic systems, agricultural fields and fertilized lawns.
Although Bay grasses have increased since the lows of the 1980s, efforts to improve the water quality must continue. Preliminary results in the Virginia Institute of Marine Science's "2011 Distribution of Submerged Aquatic Vegetation in the Chesapeake Bay and Tributaries and the Coastal Bays" show declines in all mapped areas. (Some portions of the Chesapeake Bay were not mapped because SAV signatures were masked by excess turbidity present months after the passage of Hurricane Irene and Tropical Storm Lee. All direct comparisons to previous years in this report are restricted to only those regions that were mapped in both years.)
In the Upper Bay, from the Susquehanna River to the Chester and Magothy rivers, Bay grasses decreased from 21,353 acres in 2010 to 13,287 in 2011.
In the Middle Bay, extending south from the Bay Bridge to the Rappahannock River and Pocomoke Sound, Bay grasses decreased from 30,053 acres in 2010 to 28,749 in 2011.
In the Lower Bay, south from the Rappahannock River and Pocomoke Sound to the mouth of the Chesapeake Bay, grasses decreased from 22,865 acres in 2010 to 15,645 in 2011.
And, in the Delmarva Peninsula and Coastal Bays, including Assawoman, Isle of Wight, Sinepuxent, Chincoteague, and southern Virginia coastal bays, grasses decreased from 18,095 acres in 2010 to 13,449 in 2011.
Here are actions that people can take to help Chesapeake Bay's underwater grasses:
- Reduce the amount of fertilizers applied to lawns.
- Replace some lawn grass with native vegetation.
- Those who own a septic system should make sure it is properly maintained
- Plant native vegetation along shorelines or streams to reduce erosion.
- Divert runoff from paved surfaces to vegetated areas.
- Avoid boating in shallow areas and Bay grass beds.
- Pump boat waste to an onshore facility.
For details about ongoing and past years' Bay grass surveys, go to http://web.vims.edu/bio/sav/