An Underwater Forest

Like weeping willows swaying in a warm summer breeze, underwater grasses gently wave in the Chesapeake’s shallow waters.

Fringing the shoreline of the Bay and its tributaries, shallow waters house an underwater forest of submerged aquatic vegetation (SAV) that supports a diverse community of fish, shellfish, invertebrates and waterfowl.

What is SAV?

Widgeon Grass. USFWS photo
Widgeon Grass. USFWS photo

SAV, or bay grasses, grow completely underwater. The roots, stems, and leaves of SAV are specially adapted for an underwater environment. They lack the structural support that trees or flowers have, because water provides support and buoyancy. And unlike leaves on a tree, SAV leaves do not have a waxy coat covering their surface. SAV leaves are thin and porous – to exchange water, nutrients and gases with the water.

There are 15 species of SAV in the Chesapeake Bay. The most common types of SAV are wild celery, sago pondweed, redhead grass, widgeon grass, and eelgrass.

The Role of SAV in the Chesapeake Bay

Doubler crabs. USFWS photo
Doubler crabs. USFWS photo

Microscopic plankton and bacteria feed on dead SAV (detritus) and become food for larger organisms, such as fish and clams. Shedding crabs, called “softshell” crabs, seek refuge from predators in SAV beds until their shells have hardened. Small fish like minnow, killifish and striped bass hide in SAV to escape from predators.

SAV is also a critical food source for many species of waterfowl. In fact, redhead grass gets its name from the species that consumes it - redhead ducks. Canvasbacks eat wild celery and widgeons eat widgeon grass. In addition, waterbirds such as herons, pelicans, and osprey depend on the fish and invertebrates that live in SAV beds for food.

Pair of American Widgeons. USFWS photo.
Pair of American Widgeons. USFWS photo

SAV beds slow down and reduce the impact of water currents on the shore and filter suspended sediments in the shallow waters. Suspended sediment not only makes the water appear cloudy, but it prevents light from reaching SAV plants and buries bottom-dwelling animals, such as oysters. By absorbing wave energy, SAV also slows down water movement, allowing suspended sediments to fall the Bay’s bottom, improving water clarity and impeding shoreline erosion.

SAV Decline

Eastern Oyster, photo courtesy MD Dept. of Natural Resources
Eastern Oyster. MD DNR photo

In the past, SAV was viewed as a nuisance. Boaters and fisherman complained about grasses tangling their motors and fishing gear. Others didn’t like the texture of it on their feet when they were swimming. It’s estimated that the Chesapeake Bay once supported more than 200,000 acres of SAV. Today, 90,000 acres remain.

The biggest threat to SAV is poor water quality. Suspended sediment and other solids cloud the water, blocking sunlight from reaching the grasses. Excessive amounts of sediment may cover the plants completely. Sources of sediment include runoff from farms, building sites, and highway construction. Shoreline development, dredging, boat traffic and loss of stabilizing vegetation also add sediment to Bay water.

Because SAV can only thrive where water quality is good, it serves as an indicator of health for the Chesapeake’s shallow waters – like the canary in a coal mine. Scientists from federal, state, and local agencies have identified the water quality conditions necessary to restore SAV beds successfully. By banning phosphates in detergents, applying less lawn fertilizer, using less pesticide on farm land, and reducing sewage effluent, we can protect shallow water habitats and restore the Chesapeake Bay.

For more information on Submerged Aquatic Vegetation:

Chesapeake Bay Program Bay Grasses

Maryland Department of Natural Resources and Chesapeake Bay Foundation
Bay Grasses in Classes