Oyster reefs, often referred to as oyster bars, are common submerged habitats in the southern United States. Oyster reefs in Florida are found in nearshore areas and estuaries of both coasts, but grow especially vigorously near estuarine river mouths where waters are brackish and less than 30 feet (10 meters) deep. For example, the Apalachicola River in northern Florida is a particularly productive area for oysters; the region supplies over 90 percent of the state's annual oyster catch.
Within the Indian River Lagoon, oyster reefs occur in the vicinity of spoil islands and impounded areas. In addition to being commercially valuable, oyster reefs serve a number of important ecological roles in coastal systems: providing important habitat for a large number of species; improving water quality; stabilizing bottom areas, and influencing water circulation patterns within estuaries.
Oyster reefs are built primarily by the eastern oyster, Crassostrea virginica, through successive reproduction and settlement of larvae onto existing reef structure. Oysters in Florida spawn from late spring through the fall. Oysters’ planktonic, free-swimming larvae require a hard surface to settle upon in order to complete development to the juvenile stage, with a strong preference for oyster shells over other materials.
Successive generations of oysters may form massive reefs with staggering numbers of individuals. An estimated 5,895 oysters, or the equivalent of 45 bushels, can be found within a single square yard of oyster reef.
Over time, reefs develop into highly complex structures, with many nooks and crannies that provide a wealth of microhabitats for many different species of animals. In North Carolina, one study found 303 different species utilizing oyster reef as habitat.
Common Indian River Lagoon species associated with oyster reefs include hard clam and bay scallop; space competitors such as the scorched mussel, ribbed mussel, the jingle shell, and Balanus barnacles; and gastropod mollusks including conchs and rocksnails. Sponges, crabs, whelks, flatworms and annelid worms are also common.
These abundant food sources draw a diversity of fish to oyster beds for feeding. In addition to preying upon the reef’s smaller residents, some utilize the oysters themselves for food, including the black drum and cow-nosed ray.
Oysters provide numerous benefits to their surrounding ecosystems, as well as to humans.
Oyster reefs are renowned for their ability to improve water quality in the areas where they occur. As filter feeders, oysters flow water over their gills to feed, straining out microalgae, suspended organic particles, and possibly dissolved organic matter from the water column. Under ideal temperature and salinity conditions, a single oyster may filter as much as 4 gallons (15 liters) of water per hour, up to 1,500 times its body volume. Spread over an entire reef, for an entire day, the potential for oysters to improve water clarity is immense.
Filter feeding also results in oysters accumulating several toxins and pollutants that may be found in the water column into their tissues. This makes them useful indicators of the environmental health of some areas.
Finally, the reefs themselves provide valuable shoreline stabilization and protection against erosion from wave action and storms. By dissipating incoming waves, reefs protect against sediment dislodging from shore, which diminishes water quality.
Oyster reef restoration has been a concern for resource managers all along the East Coast of the United States, but especially in areas where oyster harvesting has historically been commercially important. Over-harvesting, intensified coastal development, water pollution and persistent diseases such as MSX and Dermo have levied a devastating toll on many oyster populations along the east and Gulf coasts.
In the late 1800s, annual oyster harvests in the southeastern United States routinely topped 10 million pounds per year, and peaked in 1908 when the harvest was nearly 20 million pounds. Today, annual harvests for oysters in the Southeast averages approximately 3 million pounds per year.
In many areas, efforts are underway to revitalize depleted oyster reefs and encourage growth of new reefs. For example, the Florida Department of Agriculture has stockpiled calico scallop shells from processors and placed these on depleted oyster reefs from the spring through the fall spawning periods, when larvae are most abundant. Oyster larvae, having a preference for settling on shell material, then attach themselves onto the newly placed shells and metamorphose to the juvenile stage. These young oysters, under optimal conditions, will grow to marketable size in as little as 18 - 24 months.
Throughout the Indian River Lagoon, elsewhere in Florida, and along many areas of the Atlantic coastline, restoration efforts have also focused on installing artificial reef lines made of natural oyster shells to protect shorelines and provide areas for oyster larvae to colonize.
