Ocracoke nature, flora & fauna

North Carolina’s Underwater Gardens: Submerged aquatic vegetation

August 2013

by Pat Garber

A gusty winter wind tugged at my jacket as I scrambled along the shore of Pamlico Sound, gath­ering up handfuls of dead, grayish seagrass and stuffing them into a brown paper sack. I filled four bags before turn­ing back and heading home, pleased with myself and day-dreaming about a spring gar­den. The eelgrass I carried would soon be tucked around my dormant plants, serving as mulch that would protect them and eventually break down into a nutritious fertilizer. I have been mulching my flow­ers and vegetables with sea­grass, washed up on sound-side beaches, since I moved to Ocracoke years ago. Its value as a garden-enhancer, howev­er, is only a tiny proportion of its immense importance to our coastal environment.

Looking out across the great expanse of Pamlico Sound, one may see buffleheads swim­ming and diving, least terns plummeting into the dark wa­ters, or the head of an occasion­al diamond-back terrapin pop­ping above the water’s surface. There is little indication from above, however, of the wealth of life that may lie below. Only a few feet below the surface, expansive beds of eelgrass and shoal grass form underwater gardens where life flourishes. The estuaries of coastal North Carolina have about 200,000 acres of these aquatic plants. Known as aquatic submerged vegetation (SAV), they play an important role in safeguarding the entire coastal ecosystem.

More than 150 species of fish and marine invertebrates use SAV as adults or juveniles, thirty of which are important com­mercial fisheries. SAV beds are nurseries for blue crabs, pink shrimp, and spotted sea trout, and provide habitat for juve­nile fish and small species such as mummichogs and pipefish. They form hunting grounds for larger predators such as flounders, red drum, and rays. Post- larval shellfish such as bay scallops attach to the surface of these seagrasses before reach­ing adulthood. Waterfowl such as brant and widgeons depend on eelgrass as an important food source.

Submerged aquatic vegeta­tion also enhances the health of other marine habitats by providing oxygen for coastal waters, reducing turbidity, and lessening the effects of tur­bulence. Their roots provide sediment stabilization and the grasses themselves reduce storm damage to shoreline by lessening wave action. Ac­cording to Joann Burkholder, eelgrass is a “highly efficient biological filter that removes harmful pollutants from the water.” Changes in SAV cover­age can be a sensitive indicator of water quality and overall es­tuary health, and Mark Fonse­ca, of NOAA’s Beaufort marine lab, calls them “the canaries of the estuaries.”

There are at least 50 spe­cies of seagrasses, but here in North Carolina’s estuaries the main ones are Zostera marina , or eelgrass, and to a lesser de­gree Halodule wrightii, or shoal grass. Coastal North Carolina is a unique blend, for it is on the southern-most boundary of the eelgrasses, and the northern-most boundary of shoal grass. Most beds in the state occur in waters less than six feet deep, because of their requirements for light. They are dependent on the clarity of the water column for their survival.

Seagrasses are not true grasses, being more closely re­lated to lilies. They are flower­ing plants. The flowers of eel­grass are enclosed in sheathes at the leaves’ bases, and their fruits are bladder-like and will float. Shoal grass is among the most grass-like of the differ­ent species. Sea Grasses live in a harsh environment, with their roots fixed in a bottom of sulfide-ridden sediments toxic to most plants, forming under­water meadows.

During the 1930s there was a massive die-off of eel­grass in northern Atlantic waters. Ninety percent of the beds were lost to what was called the “wasting disease,” its causes still not fully under­stood. There has been some recovery since then, but sea­grasses are still a matter for concern. Here in North Caro­lina, the more eastern beds of SAV, which have high salinity, seem to be stable (with pos­sible exceptions in southern estuaries). Beds nearer the coast, however, in the western parts of Pamlico, Albemarle, and Currituck Sounds, have been in decline since the 1970s, with losses of fifty percent or more in these low salinity ar­eas. Studies show that SAV is declining on a national and global level.

While the reasons for the decline are not fully under­stood, it is believed that run­off from the mainland, which causes nutrient and sediment overload, excessive amounts of nitrates in the water, and a reduction in light, is a primary cause. Warmer water tem­peratures are also believed to contribute, causing reduced grass shoot density, a decrease in leaf and root development, and alterations in internal carbon and nitrogen composi­tions. Disturbance by channel dredging, filling submerged bottoms, and trawling in ar­eas of grass cause further de­cline, and the effects of climate change and sea level rise are now being assessed.

According to Patricia Smith, Public Information Officer for DMF in Morehead City, the distribution, abundance, and density of SAV varies season­ally and from year to year, making it more difficult to monitor and protect. Mount­ing concern over the health and well-being of SAV beds and the life that depends on them has led to new efforts to protect them. State agencies have been mapping the beds since 1981, and recent map­ping efforts using sophisticat­ed GPS devices have identified at least 139,000 acres of SAV along 75,000 miles of coastal shoreline.

In 2006 a Memorandum of Agreement was signed by 25 state agencies, universities, and conservation groups, cre­ating the NC SAV Partnership, dedicated to promoting con­servation efforts to protect the state’s submerged grass beds.

The North Carolina Habi­tat Protection Plan, which was updated in 2010, identifies the protection of SAV as a prior­ity. It has determined that “the monetary value of the ecosys­tem services provided by SAV, such as waste management, food production, and climate regulation are very high.” The plan contains new information on the ecological understand­ing of eelgrass and shoal grass, including the light and water quality conditions needed for healthy grass beds.

Plans to protect and im­prove SAV habitat include the adoption of coastal storm-wa­ter rules by EMC, a modified SAV definition by MFC, and revised dock rules by the C R C. Certain kinds of fishing gear are prohibited in SAV, includ­ing trawlers, oyster and clam dredges, hand tongs, and clam rakes more than a foot wide. DMF does not permit shellfish leases or the seeding of oyster cultch in SAV, and CRC has rules prohibiting new dredg­ing and filling in areas with grass beds.

Seagrasses are also pro­tected by the U.S. Corps of En­gineers, which in 2012 imple­mented new regulations which forbid the disturbance of SAV beds.

With the current levels of concern and the actions be­ing taken for their protection, it is hoped that North Caro­lina’s underwater gardens of eelgrass and shoal grass will remain stable or increase, thus ensuring the many benefits they  provide.