Shifting Patterns of Ribbed Mussel Distribution and Ecosystem Services in Response to Sea Level Rise

Throughout the salt marshes of the US Atlantic Coast, ribbed mussels (Geukensia demissa, Dillwyn, 1817) and smooth cordgrass (Spartina alterniflora Loisel) form an important mutualistic relationship. Spartina provides habitat and promotes settling of ribbed mussels, which, in turn, stabilize and fertilize the Spartina and sediment. This relationship, however, is at risk of interruption due to sea level rise, erosion, and coastal development. Among the most at-risk segments of the marsh, the front (waterward) edge of the marsh is also where ribbed mussels and their ecosystem services are concentrated. Despite their importance of ribbed mussels to the salt marsh ecosystem, very little is known about the spatial distribution. in order to address these questions, we had the following objectives: 1) to identify spatial factors influencing mussel distribution across the landscape, 2) to quantify the contribution of ribbed mussels to nitrogen removal in the presence of Spartina, and 3) to assess how the distribution of the population and its ecosystem services are likely to change by the year 2050. We conducted field work in the summers of 2015 and 2016 to survey ribbed mussel populations in 30 marshes around the Chesapeake Bay. Ribbed mussel population density and distribution was positively related to the number of Spartina stems, the exposure of the site, and to a minor degree, the amount of agriculture within 300 m. The amount of forested land cover within 60 m was negatively related to ribbed mussel density. With these factors, we built a model to estimate ribbed mussel populations in the first two meters (edge) of the marsh, and estimated the presence of 805 million mussels along the edges of Virginia\u27s marshes. Sediment core incubations revealed that when ribbed mussels are integrated with Spartina, the ammonium and particulate removal is enhanced, relative to when mussels occur separately, but that the overall rates vary dramatically by the location of the marsh whence the cores were collected. Spatial application of a 0.62 m sea level rise scenario and local erosion rates altered the distribution of both marshes and ribbed mussels. Overall, ribbed mussel abundance declined by 3.6% between 2018 and 2050; however, most locations saw moderate to large declines, while a very few locations saw very large increases (\u3e 100%). Declines in abundance were greatest in urban areas dominated by fringing marsh and extensive shoreline armoring, while gains were greatest in agricultural areas with extensive marshes. The projected redistribution of mussels by 2050 will have important implications for water quality improvement goals that will need to be addressed by local and state authorities. This dissertation has focused on the seascape ecology and management of ribbed mussels in the Chesapeake Bay. The work has demonstrated the importance of applying spatial techniques to study and understand organisms and ecosystems at the interface between land and water. Only through further study and proactive planning will we be able to plan for and address the coming impacts of anthropogenic climate change and sea level rise