|Submerged Aquatic Vegetation|
Grasses that grow to the surface of—but do not emerge from—shallow water are called submerged aquatic vegetation (SAV). One of the most important ecosystem services of the shallow waters that fringe the Chesapeake Bay is to provide refuge in which small fish and shellfish can hide from larger finfish predators. The habitat value of shallow water is greatly enhanced when it has structure that provides additional cover, and the structure provided by SAV provides some of the greatest habitat value for many species.
SAV provides important ecological services in the Chesapeake Bay by:
SAV also is valuable to people because it provides good places for fishing, crabbing, waterfowl hunting, wildlife study, and bird watching and because it protects shorelines from erosion.
Submerged grasses can grow only in shallow water because almost all of them are rooted, and their leaves don’t float on the surface; therefore, they are limited to waters that are shallow enough to allow sufficient light to reach them. They rarely grow taller than five or six feet. They lack the waxy cuticle that keeps terrestrial plants and the upper surfaces of the leaves of floating plants from drying out, so these grasses have to be submerged almost all the time. Special air cells allow SAV to float upright in the water column.
Where the water is murky, as it is in much of the Chesapeake Bay, underwater grasses are limited to water that is only three feet deep or less; however, they can’t grow right next to the shore because they would be exposed at low tide and would dry out, and they could be uprooted by wave action. As the water gets murkier, therefore, the depth range over which SAV can grow shrinks, and eventually there is no suitable depth range left, and thus no SAV.
In addition to murky water, rising water temperatures (one of the consequences of climate change) pose a threat to eelgrass (Zostera marina) in the Chesapeake Bay. This species can tolerate very cold water, growing as far north as the Arctic, but it cannot tolerate prolonged summer water temperatures above 25° C. The Chesapeake Bay, where summer water temperatures can reach 32° C or more, is near the southern limit of its range on the East Coast of the United States, evidenced by its bimodal growth pattern here (it grows actively in spring and fall when the water is cooler, with less growth in the summer). Eelgrass grows in coastal lagoons in North Carolina, south of the Chesapeake Bay, but these are closer to cooler ocean water than is most of the Chesapeake Bay. Unusually warm water in summer 2005 in the Virginia portion of the Chesapeake Bay apparently caused temperature-related diebacks of eelgrass in six lower Bay segments, documented in the 2006 VIMS SAV survey report since the photos of eelgrass for that survey are taken in the spring.
To support the health of SAV in the Chesapeake Bay, NOAA has been active in restoring SAV in rivers throughout the region. To monitor progress of restoration efforts and to keep tabs on how SAV is doing in the Bay, surveys map SAV around the Chesapeake and guide protection and restoration efforts.