Literature Review

Prepared by Cali Mitchum and Lauren McElfresh

Virginia Beach Environmental Studies Program at the Brock Environmental Center in partnership with the Institute for Coastal Adaptation and Resilience at Old Dominion University

March 31, 2024

The City of Virginia Beach is conducting a marsh restoration project in Back Bay National Wildlife Refuge in pursuit of more valuable marsh acreage that will provide habitat, ecosystem improvements, and flood risk reduction. To develop a holistic view of the importance of this project and scan for potential design fallacies, the City should consider a historical perspective of Back Bay’s wave climate and ecosystem dynamics. This literature review will examine the history of Back Bay’s marsh extent and the ecosystem services offered by submerged aquatic vegetation and marshes to provide support for the project. This project will calculate wave metrics and compare wave climates in Back Bay over time to provide context for and analyze Virginia Beach’s restoration initiative. 

Back Bay’s Historic Marsh Extent

In a report released in 1989 by the Virginia Institute of Marine Science, the history of Back Bay and its development over time is explained. It posits that in the late 15th century, the Atlantic Ocean was fully accessible from the Old Currituck Inlet, which meant that the inlet had higher salinity levels than what can be observed in Back Bay today (Priest et al., 1989). The connection to the ocean also resulted in the inlet having discernable tides. During this period, marshes consisted of black needlerush and big cordgrass. 

During the 1930s, the closure of the Canal and the establishment of dunes on the eastern side of the inlet by Roosevelt’s Civilian Conservation Corps prevented saltwater from entering Back Bay (US. Fish and Wildlife Service). As a result, Back Bay consisted of mostly fresh water until a massive storm, the Ash Wednesday Storm, in 1962 washed saltwater from the ocean into Back Bay, causing a significant increase in the salinity. After a study administered by the Virginia Commission of Game and Inland Fisheries, the North Carolina Wildlife Resources Commission, and the U.S. Fish and Wildlife Service, it was determined that an increase in salinity would result in flocculation in Back Bay. Flocculation occurs when sediments like clay or silt solidify and settle to the bottom of a body of water. Those who conducted the study argued that this would decrease turbidity in Back Bay, thus facilitating the growth of SAV.  In parallel with that study, the City of Virginia Beach attempted multiple saltwater pumping efforts between 1965 and 1973 in an attempt to decrease the turbidity in the bay to encourage SAV recovery. The city also conducted wildlife surveys to determine the effect of the pumping on freshwater fisheries, finding that the largemouth bass population dwindled as a result of decreased SAV and increased fishing. Saltwater pumping ceased in 1987 because SAV populations did not respond; gradually the salinity decreased and stabilized at the oligohaline levels they are today (Pomposini, 2017). 

The VIMS report also analyzes how the pumping brought to light differences in the resilience of freshwater and brackish plant species when the salinity fluctuates: brackish plant species are better able to adjust to changes in salinity compared to freshwater plant species in Back Bay. This resulted in brackish plants dominating the Back Bay ecosystem. In 1977, the most common SAV species included water arrowhead (Sagittaria subulata), muskgrasses (Chara spp. and Nitella spp.), redheadgrass (Potamogeton perfoliatus), bushy pondweed (Najas quadalupensis), widgeongrass (Ruppia maritima), wild celery (Vallisneria americana), sago pondweed (Potamogeton pectinatus), and Eurasian watermilfoil (Myriophyllum spicatum) (Priest et al., 1989).

Wave Attenuation

Saltwater marshes are integral in reducing wave energy by absorbing it and reducing fetch length. In saltwater marsh environments, marsh vegetation (density, size, and biomass production) is positively correlated with shoreline stabilization (accretion and erosion reduction), meaning that marshes are a valuable resource for coastal communities and shorelines (Shepard et al, 2011). A study modeling wave climates on marsh terraces in the northern Gulf of Mexico found that marsh terraces are effective at reducing wave energy; the model used was corroborated with observed data to conclude that areas with marsh terracing create wave climates with small, high frequency waves (Osorio et al, 2022). 

The wave energy reduction capability of marshes and living shorelines can be optimized by organizing types of vegetation. According to a 2021 study conducted by Xiaoxia Zhang and Heidi Nepf, living shorelines are most effectively designed with highly flexible vegetation lining the shoreline closest to the water and stiffer vegetation at a greater distance away. This is because wave energy is the highest when it hits the first layer of vegetation, so these grasses must be flexible enough to withstand the force of the waves. However, as the distance from the waterline progresses, wave energy slightly dissipates as it passes through the flexible vegetation, enabling stiffer vegetation to survive the wave energy and absorb the energy more effectively than flexible grasses.

Marshes along the coastline not only reduce wave energy, which is responsible for shoreline erosion, but they are also effective in trapping sediment that is deposited due to water turbulence. As a result, shoreline elevation could even increase due to the buildup of sediment along the shore, and may counteract the effects of sea level rise (Chandler, 2021).

The necessity for marsh recuperation in Back Bay is further underscored by the fact that reduced wave energy can allow for the more efficient growth of submerged aquatic vegetation. This is because high wave energy typically forces SAVs deeper in the water column and creates more water and sediment movement, leading to higher levels of suspended solids and a more turbid environment (Eismann et al, 2021).

Role of Submerged Aquatic Vegetation in Reducing Wave Energy

Waves building over flexible submerged vegetation will experience a dissipation of energy, and various models appropriately assess the level of dissipation to aid assessments of shoreline protection by wetlands (Henderson, 2019). Wave attenuation by submerged aquatic vegetation is impacted by the properties of the blades of grass; specifically, attenuation is positively correlated with blade stiffness and dependent on a combination of leaf length and shoot density (Paul et al, 2012). The dominant submerged aquatic vegetation species in 1977 (when they were still prominent in the environment) included Eurasian watermilfoil, Sago pondweed, Wild celery, Widgeongrass, Bushy pondweed, Redheadgrass, Muskgrasses, and Water arrowhead (Priest et al, 1989). Eurasian watermilfoil, the most common species (Pomposini 2017), is an invasive species of SAV with feathery leaves that can form dense mats on the surface and take over littoral zones of bodies of water very quickly. It likely entered Back Bay during the Ash Wednesday Storm via a break in the islands (Pomposini 2017, p. 3). Because of its nonnative status and ability to grow so densely, watermilfoil likely contributed to most of the wave attenuation by SAVs in Back Bay in the late twentieth century.

Role of Submerged Aquatic Vegetation and Marshes in Improving Ecosystem Quality

In a 2007 study, researchers examined the effect of marsh terrace size on the abundance of nekton in the area. They measured biomass and fauna density among three sizes of marsh terrace cells and found that while there was no significant difference in biomass and density of nekton, there was a drastic difference in nekton when comparing the areas with terraces to the areas without any vegetation. 

Marsh grasses play an important role in regulating nutrient loading and pollution. They can reduce the degree of impact of various pollutants (MIT News, 1996). SAV and marsh grasses also improve water quality by decreasing wave energy. Environments with higher wave energies often involve more suspended solids, which prohibit photosynthetic processes and negatively impact filter feeders. Furthermore, higher suspended solid counts can make the waterway less aesthetic, degrade fisheries, and increase water treatment costs (Bilotta, 2008). 

Back Bay National Wildlife Refuge: About Us. (n.d.). U.S. Fish and Wildlife Service. Retrieved March 18, 2024, from https://www.fws.gov/refuge/back-bay/about-us

Bilotta, G. S. (2008). Understanding the influence of suspended solids on water quality and aquatic biota. Water Research, 42(12). https://doi.org/10.1016/j.watres.2008.03.018

Chandler, D. L. (2021, October 18). How marsh grass protects shorelines. MIT Office of Sustainability. Retrieved April 9, 2024, from https://sustainability.mit.edu/article/how-marsh-grass-protects-shorelines

Eismann, E., Thomas, C., Balazik, M., Acevedo-Mackey, D., & Altman, S. (2021). Environmental Factors Affecting Coastal and Estuarine Submerged Aquatic Vegetation [PDF]. Ecosystem Management and Restoration and Research Program, 41-49. https://erdc-library.erdc.dren.mil/jspui/bitstream/11681/42185/1/ERDC-EL%20SR-21-6.pdf

Henderson, S. M. (2019). Motion of Buoyant, Flexible Aquatic Vegetation Under Waves: Simple Theoretical Models and Parameterization of Wave Dissipation [pdf]. Coastal Engineering, 152, 1-3. https://www.sciencedirect.com/science/article/abs/pii/S0378383918305349?via%3Dihub

Mitchell, M., Herman, J., & Hershner, C. (2020). Evolution of tidal marsh distribution under accelerating sea level rise. Wetlands, 40(6), 1789-1800.

Modification of Water Quality. (n.d.). Maryland Department of the Environment. Retrieved March 21, 2024, from https://mde.maryland.gov/programs/water/wetlandsandwaterways/aboutwetlands/pages/quality.aspx#:~:text=Wetlands%20help%20maintain%20good%20water,the%20sediment%20load%20of%20water.

The Natural Communities of Virginia Classification of Ecological Groups and Community Types. (2021, March). Virginia Department of Conservation and Recreation. Retrieved March 21, 2024, from https://www.dcr.virginia.gov/natural-heritage/natural-communities/ncea3

Osorio, R. J., Linhoss, A., Skarke, A., Brasher, M. J., French, J., & Baghbami, R. (2022). Modeling wave climates and wave energy attenuation in marsh terrace environments in the northern Gulf of Mexico [PDF]. Ecological Engineering, 176, 1. https://doi.org/10.1016/j.ecoleng.2021.106529

Paul, M., Bouma, T. J., & Amos, C. L. (2012). Wave attenuation by submerged vegetation: combining the effect of organism traits and tidal current [PDF]. Marine Ecology Progress Series, 444(31-41), 1-3. https://doi.org10.3354/meps09489

Pomposini, Amanda N. "Biology and Impacts of Leech Infestation in Largemouth Bass (Micropterus Salmoides) in Back Bay, Virginia" (2017). Master of Science (MS), Thesis, Biological Sciences, Old Dominion University, https://doi.org/10.25777/1tcx-bx69

Priest, W. L., Silberhorn, G. M., & Dewing, S. (1989) City of Virginia Beach Marsh Inventory: Volume 3 Back Bay and Tributaries. Special Reports in Applied Marine Science and Ocean Engineering No. 300. Virginia Institute of Marine Science, College of William and Mary. https://doi.org/10.21220/V5QB0B

Priest, Walter I. and Dewing, Sharon, "The Marshes of Back Bay, Virginia" (1991). III. Flora. 3. https://digitalcommons.odu.edu/backbay1990_flora/3

Rozas, L.P., Minello, T.J. Restoring coastal habitat using marsh terracing: The effect of cell size on nekton use. Wetlands 27, 595–609 (2007). https://doi.org/10.1672/0277-5212(2007)27[595:RCHUMT]2.0.CO;2

Searles, K. (2024). Marsh Restoration in Back Bay. City of Virginia Beach Public Works. Retrieved March 21, 2024, from https://pw.virginiabeach.gov/stormwater/flood-protection-program/stormwater-green-infrastructure/marsh-restoration-in-back-bay

Shepard CC, Crain CM, Beck MW (2011) The Protective Role of Coastal Marshes: A Systematic Review and Meta-analysis. PLoS ONE 6(11): e27374. https://doi.org/10.1371/journal.pone.0027374