Quantifying the Distributions and Ecosystem Services of Oyster Reefs within Virginia's Coastal Bays

Within Virginia’s coastal bays, the eastern oyster, Crassostrea virginica, is a native and primarily intertidal species. Although historically abundant, oyster populations sharply declined beginning in the mid-19th century from overharvesting and were further reduced in the 20th century with added pressures from disease and potentially poor water quality. Restoration efforts have enabled oysters to increase population size, although they still comprise only a few percent of what once existed. While restoring population size is the primary goal of most restoration initiatives, more recently efforts have aimed to enhance the health of the broader coastal bay system through ecosystem services attributed to oysters. These services include coastal protection by buffering wave energy, increasing habitat for flora and fauna, and altering sediment composition.
As restoration efforts continue within Virginia’s bays, the focus of this research is to provide new methods for monitoring and quantifying where oyster populations exist and the ecosystem services they provide in order to guide successful restoration efforts. This research was composed of four distinct research projects. First, I utilized remotely-sensed airborne light detecting and ranging (LiDAR) data to develop a methodology to identify and map intertidal oyster reefs within the Virginia Coast Reserve (VCR). From this dataset of oyster locations, combined with the physical characteristics of elevation, wave fetch, and water residence time, I developed a physical habitat suitability model to determine suitable and unsuitable locations for oyster growth. Second, I used the LiDAR dataset to derive a method using slope statistics to locate marsh edges and quantify marsh edge morphology. This method was used to determine if oyster reef-adjacent edges experience protection from edge erosion by examining differences in morphology and retreat in reef-lined and control marsh edges. Third, utilizing a specific restoration site constructed in 2017 composed of constructed reefs varying in elevation and width, I quantified which designs best enhance oyster populations and coastal protection. At this site I also determined differences in infauna communities and sediment composition before and after restoration across a 4-year time-period. Fourth, with an understanding of the effects that oyster reefs can have on intertidal communities, I used the mapped oyster reef dataset combined with field sampling to analyze if distance to oysters influences infauna and sediment distributions.
The results of these four studies are evidence that remotely-sensed airborne LiDAR data can be used in intertidal environments to monitor changes in oyster distributions. Data from mapped oyster patches indicate that oysters exist in a narrow band of elevation (-0.81 to -0.18 m NAVD88, the North American Vertical Datum of 1988) and approximately 12 % of the VCR bay and intertidal region is suitable for oyster habitat in terms of elevation, water residence time, and fetch. The results also advocate that LiDAR data can be a useful tool to remotely locate and quantify marsh edge morphology. Marsh edges adjacent to reefs were found to be more gently sloping, and this measurable difference in morphology is likely a precursor to changes in retreat. Measurements were analyzed from the top of marsh platforms and indicate no difference in retreat thus far, while it is likely that the lower toe edge, protected by the reef, is eroding more slowly. The long-term monitoring of constructed reefs provided evidence that reef elevation was important to design and reefs higher in elevation, relative to the neighboring marsh edge, better foster oyster growth and wave attenuation. Trends in increased infauna diversity and sediment organic matter were also observed after restoration. Additionally, data found that distance to oyster reefs is affects distributions of infauna taxa with large differences in presence apparent at approximately 40 m from reefs. Meanwhile local flow velocity is likely the primary driver of sediment distributions on intertidal mudflats.
Overall, these findings add to understanding of oysters as an important engineering estuarine species and contributes innovative methods to guide the success of continued monitoring and restoration efforts in Virginia’s coastal bays to support oyster populations, enhance community composition, and protect shorelines.