Phillip Westbrook, Leanna Heffner and Megan La Peyre

Well-loved for its culinary contributions to Louisiana culture, the eastern oyster also plays an important role in keeping coastal waters clean. With more than 60 percent of U.S. coastal rivers and bays moderately to severely degraded from excessive nutrient loading, also known as eutrophication, the eastern oyster is a valuable ally in offsetting declines in water quality.

As oysters feed, they filter nutrients, plankton and suspended particles in the water, using these materials to fuel their basic biological functions, such as growth and metabolism. As oysters grow, both nitrogen and carbon are assimilated into their tissue and shell, which is called bioassimilation. Harvesting oysters results in a net removal of these nutrients from the water.

Louisiana supports one of the most economically important and productive oyster fisheries in the United States, with harvest revenue exceeding $35 million in dockside sales every year. The potential removal of carbon and nitrogen in oyster tissue and shell through commercial and recreational harvest is high.

In a study conducted by LSU AgCenter researchers, oysters collected from reefs located in Sister Lake, also known as Caillou Lake, in Terrebonne Parish and Lake Fortuna in St. Bernard Parish were found to contain, on average, approximately 0.3 gram nitrogen and 10 grams carbon (tissue and shell combined). (Figure 1). Based on the 2013 harvest from these two regions, this translated to a removal of more than 10 tons of nitrogen and 375 tons of carbon from the system in one year. While shell may be sometimes returned to the system to provide for future reefs, more than 50 percent of nitrogen is found in the oyster tissue, indicating that harvesting and eating the oysters still provides a valuable contribution to nitrogen removal.

In the absence of commercial harvesting, oysters allowed to grow and build reefs may also be critical in removing nutrients from the water. Particles and nutrients not incorporated into their tissues and shells are excreted onto the surface of the sediment in packages called “pseudo-feces.” These pseudo-feces either can be buried in the reef, be re-suspended in the water or undergo chemical transformation to nitrogen gas through a process called denitrification. In the latter, deposited nitrogen diffuses from the sediment into the water and ultimately back to the atmosphere, providing a natural process for net removal of nitrogen from coastal waters. Denitrification is thought to be one of the most significant processes for removing excess nitrogen in estuaries; however, most work has focused on this process within the marsh landscape.

This study also examined denitrification simultaneously at two sites in coastal Louisiana to determine the potential contributions of oyster reefs to nitrogen removal. In the winter, and again in the summer, researchers measured gas flows from sediment at deep-water (greater than 6 feet) and shallow-water (less than 3 feet) oyster reefs as well as mud bottoms with no oysters as a reference. These measurements were used to estimate nitrogen removal via denitrification at oyster reefs and compare them to adjacent mud bottoms.

Nitrogen gas flows during the summer revealed some of the highest documented denitrification rates at oyster reefs in the United States (Figure 2). Because temperature is a critical control on denitrification, warm Gulf summer waters may contribute to these high numbers. Equally interesting and surprising is that the reference mud-bottom sites provided similar denitrification rates as the oyster reefs. This finding may be partially explained by the unique nature and history of Louisiana estuaries and reef complexes.

Many Gulf estuarine shallow-water bottoms are composed of an extensive complex of sediment and oysters, have a long history of active harvest and have extensive historic reef sites scattered throughout. For example, reefs in Sister Lake cover more than 30 percent of the water bottom, but these reefs are scattered across the area rather than formed into one or two large, distinct reefs. As a result, the effects of oysters on sediment characteristics and denitrification may extend beyond locations with discrete reefs because the scattered oysters and shells contribute equally to water filtration and denitrification.

Although Louisiana has no exact statewide estimate of oyster reef habitat acreage, more than 2 million acres of private and publically managed reefs support oyster harvest, and an equally large if not greater amount of nonmanaged reef areas may also contribute to removal of nitrogen from the water. Further investigation into the exact extent and distribution of these oysters will help create a fuller understanding of oyster reef contributions to nutrient mitigation.

The eastern oyster is often referred to as an “ecosystem engineer” because oysters and their reefs change the system in which they exist. Oyster reefs provide valuable microhabitats for invertebrates, crustaceans and finfish; promote shoreline stabilization by reducing wave energy; and contribute to mitigating nutrient loading.

This preliminary work provides evidence that oysters also permanently remove nutrients from the system through both bioassimilation and denitrification. Given the prolific annual oyster harvest and the extensive reef systems across coastal Louisiana, bioassimilation and denitrification may be important drivers of nutrient mitigation across the coastal region of the state. Ensuring the health and maintenance of Louisiana oyster reefs is a critical piece of coastal protection.

Phillip Westbrook is a former graduate student who received his master of science in December 2016 from the School of Renewable Natural Resources; Leanna Heffner is a post-doctoral research associate in the Department of Oceanography and Coastal Sciences; and Megan La Peyre is an adjunct professor with the U.S. Geological Survey, Louisiana Fish and Wildlife Cooperative Research Unit in the School of Renewable Natural Resources.

Acknowledgement: This work was possible through funding from Louisiana Sea Grant and Louisiana Coastal Protection and Restoration Authority.

(This article appears in the fall 2017 issue of Louisiana Agriculture.)

Jazmyn Bernard, an undergraduate student in the School of Renewable Natural Resources, dredges oysters in Sister Lake for studies. Photo by Megan La Peyre

Eastern oysters collected at Lake Fortuna to examine bioassimilation of carbon and nitrogen. Photo by Amy Smith-Kyle

Figure 1. Ratio of the total mass load of nitrogen and carbon in tissue and shell of oysters harvested from Sister Lake and Lake Fortuna. Oysters, on average, were found to contain about 0.3 gram nitrogen and 10 grams carbon.

Figure 2. Mean (± standard error) summer and winter nitrogen flows (denitrification) at deep-water reefs, shallow-water reefs and reference sites in Sister Lake and Lake Fortuna.