Conceptual Synthesis

Synopsis:

This project is the first of its kind to assimilate historical aerial photography in grey scale (1940's-1960's), modern-era color IR photography (1970's-present day), and satellite remote sensing data (MODIS: 2000 - present) and combine these data with advanced mapping and habitat classification algorithms, vegetation biophysical index algorithms, and landscape fragmentation analyses to begin to better synthesize the rates and potential impacts of future coastal marsh change through simulation and modeling. The study compared two immediately adjacent sites that span the range of coastal marsh sustainability, from a sediment starved, high salinity, retreating delta (Grand Bay), to a heavily sedimentary and freshwater-dominated, large riverine delta (Pascagoula River). The key issue we are concerned about in this research can be captured by this image of coastal marsh erosion and retreat from Louisiana (Figure 1), which represents an increasingly common observation being reported from many places around the country and the globe.

Marsh sustainability over longer time scales (decades to centuries) is influenced by the combination of: (1) the ability of the marsh platform to build up and maintain elevation relative to the mean tidal height, and (2) the ability of the marsh plants to retreat landward as mean water levels increase with rising sea level (Figure 2). Coastal vegetation retreat is a natural process that has occurred in the historical past when marsh platforms were inundated, however, in today's built-up coastline there is often no longer green space available for upland marsh migration. As a consequence, coastal marshes are starting to become increasingly fragmented habitats with the end result that they can no longer provide many important and valuable ecosystem services, such as coastal protection, sediment and nutrient removal, estuarine nursery habitat, and carbon burial.

Future coastal marsh sustainability is therefore largely going to be affected by the ability of marsh to (1) build elevation and (2) retreat inland. A simple matrix showing the interaction between these two processes was developed to help explore the different potential sustainability scenarios (Figure 3). The current/historical conditions are shown on the top, with marshes expanding and retreating (horizontal green arrow) in response to relative elevation change (vertical brown arrow). This is a dynamic process influenced by relative sea-level, external sediment inputs, and marsh-produced organic matter from roots and leaf litter. In areas with large amounts of sediment input, such as river deltas, marshes can accrete and expand seaward over time (e.g. Mississippi River delta in Louisiana).

Over time, especially with rising sea levels and potentially changing land-use patterns that alter sediment supply, coastal marshes face increasing challenges to maintain spatial extent which affects future sustainability. In scenario A (top left), the marshes can retreat upland into a forested buffer area, while still having sufficient sediment supply to maintain elevation - this scenario corresponds with undeveloped portions of the Pascagoula River delta in our research. In scenario B (top right), the marshes can still retreat upland, but have lost sediment inputs - this scenario corresponds most closely with our study area in Grand Bay. In scenario C (bottom left), the marshes can no longer retreat upland as there are houses, roads and other built environs in a developed coastline, however, external sediment supply can help to maintain elevation for some period of time into the future - this scenario corresponds with urbanized portions of the Pascagoula River delta in our research. Finally, in scenario D (bottom right), the marshes are facing a highly unsustainable future from lack of upland retreat due to the built environment, and lack of sediment supply, resulting in marsh platform elevation not maintaining pace with sea level rise - many small coastal marshes in Mississippi and Alabama currently face this situation.

Our research in this project has added evidence for another source of marsh loss, happening in interior portions of extant marshes along the northern Gulf of Mexico. Subsidence of the marsh platform, especially in Louisiana, but also at Grand Bay and in selected areas of the Pascagoula River, allows interior areas with existing marsh habitat to convert to open water. Once this happens, additional marsh edge loss can occur along this new marsh-water margin, resulting in more rapid fragmentation of marsh, and a faster rate of conversion of former marsh to open water. The Mississippi delta in Louisiana is a good example of this problem, but we have found that even in the absence of human factors like oil/gas extraction, this subsidence process is evident in marshes present in Mississippi and Alabama.

Figure 3: Current marsh condition and four different scenarios for future marsh condition. Sustainability of future marsh health is strongly related to space available for upland retreat (horizontal green arrow) and sediment supply (vertical brown arrow).

Societal consequences:

The implications of this and other research suggest a future where coastal marshes are less extensive and provide fewer ecosystem services than they have in the past. An important ecosystem service provided by marshes that many coastal residents rely on is protection from high water and storm surge. Coastal marshes help to reduce wave energy and coastal erosion, providing protection for built infrastructure like houses and roads (Figure 4). However, as our research has demonstrated, marshes are already eroding and subsiding, and this process will continue to speed up as sea level rise accelerates into the future. Ongoing, but gradual, coastal erosion brings the water closer to built structures increasing the risk of catastrophic damage during storm events. Therefore, loss of coastal marshes with sea level rise increases the cost of insuring, rebuilding, or relocating property along the coastline.

Another important set of ecosystem services provide by coastal marshes are related to their role as habitat and as filters. Coastal marshes remove sediments, nutrients, and pollutants from the water, improving coastal water quality. These nutrients and sediments help the plants to grow, providing both structure and food for many commercially and recreationally valuable fisheries species during the juvenile phase. This is why marshes are often called the "nursery" for the coastal estuary. Loss of marshes, therefore, also affects viability of coastal economies through loss of fisheries production, loss of clean water, and loss of tourism and recreational opportunities (Figure 5). The loss of these economically valuable resources is an often under-appreciated consequence of coastal marsh loss, as it may be less visible than loss from flooding and storm damage. Finally, a recently recognized and important ecosystem service provided by coastal marshes is that of carbon burial or sequestration, more popularly termed "blue carbon". As atmospheric carbon dioxide continues to increase there is increasing recognition of the importance and societal benefits associated with carbon removal from the atmosphere, as this greenhouse gas is one of the primary drivers of climate change. Therefore, loss of coastal marshes with sea level rise will affect not only the built environment, but also has additional costs related to economic benefits, environmental benefits, and long-term climate mitigation benefits.

Individual choice:

Maybe this is what you are thinking right about now: "This sounds like a scary future. What can I do to help?". As with many of the pressing societal problems today, you have a choice on how to move forward into a more sustainable future for yourself, your family, and your children's children (Figure 6). Climate change and sea level rise are not going away, and will unfortunately continue for a long time (hundreds of years). Taking action that can help to reduce the problems that cause climate change is an important step and will help to reduce the magnitude of future coastal marsh loss. Further, supporting actions that will help coastal marshes to remain as resilient as possible in the face of future sea level rise can also help to offset the consequences outlined above. This can include actions such as:

• Commit to reducing your carbon footprint.
• Support the protection and conservation of wetlands, and other natural resources, in your local area.
• Support coastal restoration and the use of living shorelines to help improve coastal resilience.
• Support appropriate land use and planning to minimize flooding and allow room for coastal retreat of marshes.