Evaluating success of oyster reef restoration
Blomberg, Brittany Nicole
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Oyster reefs are the most degraded marine habitat type, with estimated global losses of 85% from historic abundances. Restoration efforts have increased to restore important ecosystem functions and services associated with oyster reefs. The objective of this study was to evaluate the success of oyster reef restoration projects from a variety of perspectives. The importance of oyster reefs, causes of degradation, and methods of restoration were reviewed (Chapter I). Oyster reef restoration projects across the United States were analyzed to examine temporal trends and influences of national policies (Chapter II). Oyster reef habitat was restored in Copano Bay, Texas and monitored for two years to examine habitat value (Chapter III), oyster diet composition (Chapter IV), and nutrient regulation functions (Chapter V). Finally, the results and implications of each chapter are summarized (Chapter VI). In Chapter II, data were compiled from the National Estuaries Restoration Inventory to analyze oyster reef restoration projects. Over $45 million has been invested for the restoration of more than 150 ha of oyster reef habitat. Trends over time indicate projects are being implemented at larger scales with improved cost efficiency. However, this analysis revealed a lack of available monitoring data or project assessments. In Chapter III, an oyster reef complex was constructed in Copano Bay, Texas. The restored reef and natural reference habitats were monitored for two years to examine oyster and nekton communities. The restored reef had substantial oyster recruitment and growth, with oyster abundance and size comparable to reference conditions within the first year. Fishes and crustaceans recruited to the restored reef within six months post-construction, and abundance and diversity were comparable to reference habitats. High densities of oysters and nekton relative to other studies indicate this restored reef complex was successful in providing important ecological functions associated with habitat provision and oyster production. In Chapter IV, a dual stable isotope (δ13C and δ15N) approach was employed to assess oyster diet composition. Oysters and potential composite food sources — water column and sediment surface organic matter — were sampled from restored and reference habitats. Oyster diet composition was similar among habitats, but changed over time with increasing contributions of sediment organic matter. Results demonstrate that benthic food resources are important components of oyster diets, and that oysters may enhance the development of benthic algal food resources. In Chapter V, nitrogen removal attributed to the burial and mineralization of oyster deposits was quantified at the restored and natural oyster reefs through emergy analysis. Emergy evaluations can represent environmental and economic values of a system in equivalent units based on solar energy, and thus present an ecological approach to quantify ecosystem services. Results demonstrate that the restored reef was providing a greater function per unit area compared to the natural reef within the second year post-restoration. This study provides insight into the effects of national policies on restoration trends, and stresses the importance of project assessments and data sharing to ensure future restoration projects make meaningful scientific contributions. The restored reef in Copano Bay was successful in supporting high densities of oysters, fishes and crustaceans. Much of this success could be attributed to the design of the reef complex, and thus will support the planning of future restoration efforts. Benthic organic matter was determined to be an important food resource for oysters, which will improve the development of food web and population dynamic models. This work also demonstrates the application of emergy analysis to quantify ecosystem functions and services, which can complement traditional economic valuation methods. Overall, the work presented here makes novel contributions to the broader knowledge of oyster reef restoration practice and theory.