In the shallow waters of Chesapeake Bay, underwater grasses sway in the aquatic breeze of the current. Known as submerged aquatic vegetation or SAV, these amazing plant communities provide food and shelter for waterfowl, fish, shellfish and invertebrates. Like other green plants, SAV produces oxygen, a precious and sometimes lacking commodity in the Bay.
These underwater plants filter sediment that can cloud the water and bury bottom-dwelling organisms like oysters. As waves roll into SAV beds, the movement is slowed and energy is dispelled, protecting shorelines from erosion. During the growing season, SAV takes up and retains nitrogen and phosphorus, removing excess nutrients that could fuel unwanted growth of algae in the surrounding waters.
An SAV bed serves as habitat for many aquatic animals. Microscopic zooplankton feed on decaying SAV and, in turn, are food for larger Bay organisms. Minnows dart between the plants and graze on tiny organisms that grow on the stems and leaves. Small fish seek refuge from larger and hungrier mouths. Shedding blue crabs conceal themselves in the vegetation until their new shells have hardened.
SAV is a valuable source of food, especially for waterfowl. In the fall and winter, migrating waterfowl search the sediment for nutritious seeds, roots and tubers. Resident waterfowl may feed on SAV year-round.
There are approximately 13 species of SAV commonly found in the Bay or nearby rivers. SAV beds range from the low tide line to a depth of about 2 meters, or the point where there is not enough light for plant growth. Some freshwater species thrive up to 3 meters deep. Salinity, water depth and bottom sediment determine where each species can grow. Survival of SAV is affected most by the amount of light that reaches the plants. Poor water quality resulting in less light penetration is the primary problem for SAV in the Bay.
Factors that affect water clarity, therefore, also affect the growth of SAV. Suspended sediment and other solids cloud the water, blocking precious sunlight from the grasses. Excessive amounts of suspended sediment may cover the plants completely. Sources of suspended sediments include runoff from farmland, building sites and highway construction. Shoreline erosion also adds sediment to the Bay. Land development, boat traffic and loss of shoreline vegetation can accelerate natural erosion.
Nutrients, although vital to all ecosystems, can create problems when present in excessive amounts. Major sources of nutrients include sewage treatment plants, acid rain, agricultural fields and fertilized lawns. High levels of nutrients stimulate the rapid growth of algae, known as blooms. Algae blooms cloud the water and reduce the amount of sunlight reaching SAV. Certain types of algae grow directly on the plants, further reducing available sunlight.
Historically, more than 200,000 acres of SAV grew along the shoreline of Chesapeake Bay. By 1978, a survey of SAV documented only 41,000 acres. Declining water quality, disturbance of SAV beds and the alteration of shallow water habitat all contributed to the Bay-wide decline. The absence of SAV translates into a loss of food and habitat for many Chesapeake Bay species.
Water quality is the key to restoring grasses to the Bay. Scientists have identified the water quality conditions and requirements necessary for the survival of five SAV species, wild celery found in fresh waters; sago pondweed, redhead grass and widgeon grass, found in more estuarine water; and eelgrass found in the saltier water of the lower Bay.
Submerged aquatic vegetation is making a comeback. Water quality is beginning to improve because of the ban of phosphates in detergents, reduction of fertilizer use by farmers and homeowners, protection of shallow water habitat and the reduction of nutrients in sewage effluent. In 1984, only 38,000 acres of SAV were reported in the Bay and its tidal tributaries. By 1993, more than 73,000 acres of SAV were reported, almost doubling the amount over nine years.
To monitor the status of SAV species in the Chesapeake Bay, an annual SAV survey is conducted. Researchers interpret aerial photography to identify the location and size of SAV beds in the shallow water habitats of the Chesapeake Bay and tidal portions of major Bay tributaries. Because different SAV species emerge at different times during the spring and summer, some beds may not be apparent in the photographs. Very small beds may also be missed.
In order to ensure the accuracy of the aerial SAV survey, volunteers check or "groundtruth" the beds identified in previous surveys, identify any new beds and, if possible, identify the SAV species growing in each bed.
To hunt for SAV, one must have access to the Bay or tidal rivers during the summer months. Because it grows in shallow water areas, SAV is usually located by wading into the water or using a shallow draft boat.