Oysters thrive under brackish conditions, and now a University of Florida (UF) study reveals that the bivalves can actually help create the mix of fresh water and brine they crave. While evaluating a new method of restoring degraded oyster reefs, researchers confirmed an observation that Cedar Key oystermen have made for years – some oyster reefs act as natural dams, impounding fresh water that flows seaward from nearby creeks and rivers. The result – large areas of reduced-salinity water that help maintain near-shore estuarine habitats supporting oysters, sea grasses, juvenile game fish, and invertebrates important to the marine food chain as well as seafood production and recreational opportunities for people. “This finding could aid ecological and fishery restoration projects along Florida’s Big Bend Coast,” said project leader Peter Frederick, a professor with UF IFAS Department of Wildlife Ecology and Conservation.
The Big Bend is one of the nation’s few coastal areas featuring numerous oyster reefs that run parallel to shore and stand above the water’s surface at low tide. The study site, off the Levy County coast, is a chain of oyster reefs punctuated by a few openings that allow seawater to mix with fresh water that the reef holds back as it empties into the Gulf of Mexico from the Suwannee River. The team investigated the area’s salinity levels after local oystermen told Frederick the reef held back fresh water. In response, Frederick recruited David Kaplan with the UF College of Engineering. The pair checked salinity in multiple locations for 18 months and their findings confirmed the oystermen’s observation – on average, the salt content of water on the near-shore side was nearly 20 percent lower than it was on the ocean side, and up to 90 percent lower on some days at some locations. Furthermore, computer modeling showed that if restoration increased the elevation of the degraded reef to expected levels, the reef would probably hold much more fresh water near the shore.
Meanwhile, the team accomplished its primary mission – demonstrating that both lime rock boulders and polyester mesh bags filled with clam shells can be used to fortify degraded oyster reefs, said Leslie Sturmer, a UF/IFAS aquaculture extension agent based in Cedar Key. The bags, weighing from 100 to 300 pounds, are expected to remain in place long enough to provide suitable habitat for free-floating oyster larvae to settle and grow, rebuilding the reef. Initial results showed that young oysters were colonizing the mesh bags at impressive rates. Within 18 months, oyster densities on the UF restoration sites were higher than the densities found at most other restored oyster reefs in the Gulf of Mexico. The UF control sites showed no improvement, indicating that the lime rock and bags of clam shells made the difference. The reef’s long-term response is potentially more important, Frederick said, because scientists believe that successfully restored sites will be more resilient to environmental change and weather events.
Support for the project was provided by grants from The Nature Conservancy and National Oceanic and Atmospheric Administration, Florida Sea Grant, and National Fish and Wildlife Foundation. Government agencies and local organizations, including the Oystermen’s Association and Aquaculture Association, assisted as well.
The complete UF IFAS news article can be viewed here.
The complete project report (pdf file) can be accessed here.
