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Submerged Aquatic Vegetation: Where Have All the Grasses Gone?

These small and modest grasses are known as submerged aquatic vegetation or SAV. More than a dozen species of SAV are native to the Chesapeake Bay. Salinity, water depth, and bottom sediment are factors which determine where each species can grow. However, the survival of all SAV depends on the amount of sunlight reaching the plants.

These amazing plants provide food and shelter for diverse communities of waterfowl, fish, shellfish, and invertebrates. Like all green plants, SAV produces oxygen, a precious and ever-decreasing commodity in the Chesapeake Bay. SAV filters and traps sediment, which can cloud the water and bury bottom dwelling organisms like oysters. SAV also absorbs nutrients like nitrogen and phosphorus.

Microscopic zooplankton feed on the decaying SAV and, in turn, are food for larger Bay organisms, such as fish and clams. Thus, SAV is a key contributor to the energy cycling in the Bay. 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 different species of SAV year-round.

Like a forest, field or wetlands a SAV bed serves as habitat for a multitude of aquatic animals. Barnacles and scallop larvae attach to the leaves and stems of eelgrass in the salty waters of the lower Bay. Fish, like bluegill and largemouth bass, live in the freshwater grasses of the upper Bay. Minnows, small anadromous fish, like juvenile striped bass, and blue crabs seek protection as well as food in the SAV beds.

The Decline

Since the 1960s, well over half of the SAV has disappeared from the Bay waters. Declining water quality, disturbance of SAV beds, and alteration of shallow water habitat all contributed to the decline. The absence of SAV translates into a loss of food and habitat for many Chesapeake Bay species.. Today, all areas of the Bay have experienced the decline of these important grasses.

The extensive loss of SAV has forced some species of waterfowl to migrate to other wintering areas or to change their feeding habits. Canvasbacks that continue to winter on the Bay now rely mainly on the Baltic clam as a primary food source. Other waterfowl, like redhead ducks, have all but abandoned the Chesapeake. Survival of SAV is affected most by the amount of light that reaches the plants. Environmental factors that affect water clarity also affect SAV growth.

Suspended sediment and other solids cloud the water, blocking precious sunlight from the grasses. Excessive amounts of sediment may cover the plants completely. Sources of sediment include runoff from farms, building sites, and highway construction. Shoreline erosion also adds sediment to Bay water. Land development, boat traffic and loss of shoreline vegetation accelerate natural erosion.

Nutrients, although vital to all ecosystems in natural levels, create problems when present in excess amounts. 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.

Nutrients come from three major sources, sewage treatment plants, agricultural fields and fertilized lawns. Every day, more than one billion gallons of treated sewage effluent enters the Bay from treatment plants. This translates into 87 million pounds of nitrogen and 9 million pounds of phosphorus a year!

Runoff from farm fields and lawns dumps tons of nutrient-rich fertilizers into the Chesapeake Bay each year. In addition to fertilizers, oils and other pollutants, runoff may also contain herbicides and pesticides that are toxic to aquatic organisms.

After Reading:

  1. Why are there different kinds of SAV growing throughout the Bay?
  1. What do all the bay grasses have in common?
  1. Explain how bay grasses are important to the following organisms:
  1. Oysters-
  2. Waterfowl-
  3. Blue crabs-
  1. What effect is the decline of bay grasses having on waterfowl? Give two specific examples.
  1. List three sources of excess nutrients that end up in the Bay and the total amount.
  1. Explain how excess nutrients and sediment contribute to the decline of bay grasses.