Will Oysters Improve Restoration Outcomes at a Little Edisto Island Salt Marsh?
Christopher Pettengill
Doctoral Student, Clemson University
Oysters are one of the most prevalent sessile organisms on the eastern seaboard of the United States. Oyster beds can be deployed in areas experiencing rapid shoreline erosion, but they are not a permanent fixture of coastal mitigation banks. Stakeholders are interested in the contributions of oysters to meeting restoration goals in coastal habitats, such as improvements in water quality and providing habitat structure for wildlife. Research is needed to understand better the impacts of incorporating oysters into salt marsh restoration. Christopher is investigating how the addition of oyster structures (wooden stakes, wireframes) to salt marsh restoration sites can enhance habitat quality and provide economic benefits to the site. Examples of these ecosystem services include providing habitat for fish and invertebrates (which provides food for aquatic birds) and biofiltration (for which they are interested in nitrate and phosphate reduction).
Christopher’s primary research objectives are:
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Calculate rates of biofiltration of oysters at the restoration site and compare it with rates observed at reference sites (those with oyster reefs and those without)
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Determine the spatial extent across which we observe the benefits of oyster ecosystem services (biofiltration and habitat provision)
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Record changes in community composition of fish and invertebrates at the restoration site following the addition of oysters to the site. Compare these communities with those seen at reference sites with varying oyster density.
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Determine how oyster abundance at the restoration site impacts habitat usage from waterfowl, shorebirds, and wading birds.
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Compare the recruitment and growth rates of oysters on different substrate types introduced to the restoration site.
Christopher’s hypotheses for each research objective are:
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The concentration of phosphorus and nitrate is lower at reference sites with a higher density of oysters. The concentration of phosphate and nitrate will decrease following the addition of oysters to the restoration site.
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The spatial extent of oyster ecosystem services at the restoration site will be much lower when compared to sites with well-established oyster reefs within the time frame of the study. If the site were monitored for a more extended period (ten years or more), we would likely see a more significant overall effect of oyster addition.
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Fish and invertebrate community composition at reference sites with high oyster density will be different from sites with low oyster density. The current community assemblage of the restoration site will be closer in composition to reference sites with low oyster density. As they continue to monitor the impoundments following the addition of new oyster cultch (shell, stone, or other structures that support oyster recruitment), Christopher expects that the biotic communities (invertebrates and fish) and water quality parameters (nitrate and phosphate concentration) will more closely resemble reference sites with oysters.
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Bird habitat usage will increase at the restoration site over time due to changes outside of oyster habitat addition, such as hydrologic alteration. The effects of increased oyster habitat may not be significant.
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No significant differences between oyster growth rates on tested oyster cultch materials (wooden stakes, wireframe reef structure) is expected. Community composition is also not expected to be significantly different between structure types.
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The restoration site where the study is being conducted is located at Little Edisto Island, South Carolina. Southeast Mitigation LLC (funding source and partner) is restoring natural tidal flux to a set of six saltwater impoundments at this location by removing sections of the surrounding berms.
Christopher is comparing the habitat conditions present at the restoration site currently with multiple reference sites also found on Little Edisto Island and Edisto Island, which he places into categories depending on the density of oysters and oyster beds present at the sites.
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Methods
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Each month, Christopher records water quality parameters (water temperature, salinity, conductivity, pH, dissolved oxygen concentration). He takes water samples, which he filters using a Millex brand 45-um filter unit, and processes in the lab to record concentrations of phosphate and nitrate. Christopher monitors bird activity at each of the impoundments during monthly surveys. For fish sampling, Christopher uses minnow traps (six per impoundment), crab traps (one per impoundment), a fifty-foot seine net (five sweeps per impoundment), and throwing an eight-foot diameter cast net (ten cast net throws per impoundment). Invertebrate surveys involve taking sediment core samples using a PVC pipe sediment trap, as well as a D-net. Christopher samples invertebrates along a transect from the most upland position in the impoundment (that is covered by water at high tide) to the deepest point (that is accessible) of the impoundment. The same survey methods are also used at each reference site where surveys of that type are possible. In a separate experiment taking place at the restoration site, Christopher will be comparing the growth and colonization rates of oysters on wireframes and wooden stakes. The goal is to determine which of the oyster structure types provides the greatest oyster growth and recruitment relative to the cost of deploying the structures. Stakeholders from Southeast Mitigation will be using the more efficient of these two methods to deploy oysters at a much larger restoration site, where they plan on creating one hundred acres of oyster habitat.
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Preliminary results – Prior to restoration actions
Pre-restoration conditions at the site show that wading birds, shorebirds, and wetland-associated songbirds use the impoundments at the Little Edisto Restoration site, mainly when large numbers of birds descend onto exposed mudflat habitat at low tide. Impoundments (and portions of impoundments) that have been unconnected to any tidal influence show few signs of bird activity outside of grassland songbirds. Waterfowl are currently essentially absent from the restoration site, outside of one male-female pair of hooded mergansers present in one of the impoundments with permanent water during a winter survey. Plant communities at the restoration site reflect what are traditionally considered salt marsh plant community assemblages, with Sporobolus alterniflorus, Salicornia depressa, and Distichlis spicata being the three dominant species. Water quality at the restoration site varies slightly across impoundments. Still, we are expecting to see a more significant effect of season on nitrate and phosphate concentrations, with higher concentrations being detected during summer than in winter.