Visualize coastal wetland assessment methods, processes, and concepts used and studied as a part of the Mid Atlantic Coastal Wetlands Assessment Program.
Surface Elevation Tables &
Surface Elevation Tables (SET) are installed in marshes to study precise changes in surface elevation. Each SSIM station has three deep rod SETs and nine feldspar marker horizons (MH).1-5 An in depth review of methodologies and theories of SET-MHs can be found on the USGS Patuxent Wildlife Research Center’s website.
Surface Elevation Tables track precise cumulative elevation changes. Marker horizons are used in conjunction to monitor short term accretion on the marsh platform. MH measure surface elevation changes while SETs capture shallow subsidence. Shallow subsidence (SS) is the reduction in elevation, or sinking, of the wetland surface between the bottom of the SET benchmark and the bottom of the marker horizon. Subsidence dynamics are controlled by subsurface processes and hydrology. SS is equivalent to the difference between cumulative elevation change and the rate of accretion.5 A marsh platform is considered to be “keeping pace,” or vertically stable, when the cumulative elevation change is greater than or equal to the local rate of sea level rise.
Understanding minute changes in wetland accretion and erosion over time is valuable to scientists studying marsh response sediment delivery and ultimately to sea level rise. The SET is a portable and nondestructive, making it ideal for scientists to use in marshes. Scientists will measures SETs multiple times a year.
Graphic created by LeeAnn Haaf
The resilience of coastal tidal marshes to sea level rise is governed by dynamic processes that consist of several interconnected positive and negative feedbacks.6-17 In the simplest sense, marsh elevation is controlled by sediment subsidies18 and plant productivity. These processes work together and are co-dependent, maintaining the processes that allow the marsh to avoid drowning.6-17 Sedimentation, in addition to plant productivity, comprises net surface accretion. This is a critical component of each marsh’s natural ability to keep pace with rising sea levels.12 Changes to the rate of surface accretion might influence the degree of impact that subsurface processes have on elevation maintenance. If shallow subsidence out-paces surface accretion, elevation is lost regardless of sea level. This process may take years to have discernible signs of degradation, but is expedited as sea level rises.19
The amount of sediment needed to sustain natural elevation maintenance and keep plants in their optimal growth ranges is site specific and complex.7, 11, 12, 13, 14 Any alteration to the marsh’s sediment subsidy and/or impacts to plant productivity would have consequences for resilience. Reduced resilience or poor condition (degradation) are not exclusive of the amount of sediment a marsh receives as there are many other factors that contribute to plant productivity.
Graphic created by LeeAnn Haaf
Given their position at the nexus of land and sea, coastal wetlands fall vulnerable to human disturbance from the landward edge but are also at the mercy of sea level rise and destructive oceanic storms. Most coastal wetlands in the Mid Atlantic have been anthropogenically disturbed to some degree, thereby reducing overall condition and enhancing vulnerability to drowning as sea levels rise. Sustaining coastal wetlands through these disturbances and vulnerabilities has been a key focus of coastal management in recent decades.
Although coastal wetland responses to sea level rise are well studied, landward processes, specifically the inland movement of coastal wetland grasses into forests, are less understood. Landward migration of coastal wetlands, or transgression, will likely be one of the major strategies to sustaining coastal wetland acreage into the future. Spatial and temporal variability exists in the degree of opportunity to manage areas for transgression as the potential for coastal wetlands to migrate can be dictated by a suite of factors 20. Better understandings of landward transgression processes and management tactics are needed to ensure that guidance tools for practitioners are robust for all predicted situations which may evolve with rising sea levels.
Wetland Productivity Graphic created by the Coastwide Reference Monitoring System in Louisiana.
This Wetland Productivity graphic was created by the Coastwide Reference Monitoring System21 in Louisiana. It represents information from the Louisiana coast, but it summarizes the various relationships and processes within coastal wetlands very well. We encourage you to follow this link to discover the interactive component of this graphic
1. Cahoon, D. R. 2015. Estimating Relative Sea-Level Rise and Submergence Potential at a Coastal Wetland. Estuaries and Coasts, 38(3), 1077–1084. http://doi.org/10.1007/s12237-014-9872-8
2. Lynch, J. C.; Hansel, P.; Cahoon, D. R. 2015. The Surface Elevation Table and Marker Horizon Technique A Protocol for Monitoring Wetland Elevation Dynamics.
3. Cahoon, D. R., Lynch, J. C., Perez, B. C., Segura, B., Holland, R. D., Stelly, C., … Hensel, P. (2002). High-Precision Measurements of Wetland Sediment Elevation: II. The Rod Surface Elevation Table. Journal of Sedimentary Research, 72, 734–739. http://doi.org/10.1306/020702720734
4. Cahoon, D. R., Reed, D. J., & Day, J. W. 1995. Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited. Marine Geology, 128(1-2), 1–9. http://doi.org/10.1016/0025-3227(95)00087-F
5. Cahoon, D. R. 2015. Estimating Relative Sea-Level Rise and Submergence Potential at a Coastal Wetland. Estuaries and Coasts, 38(3), 1077–1084. http://doi.org/10.1007/s12237-014-9872-8
6. Boumans, R., M. Ceroni, D. M. Burdick, D. R. Cahoon, and C. Swarth. 2002. Sediment Elevation Dynamics in Tidal Marshes: Functional Assessment of Accretionary Biofilters 2002: 1–44.
7. D’Alpaos, Andrea, S. M. Mudd, and L. Carniello. 2011. Dynamic response of marshes to perturbations in suspended sediment concentrations and rates of relative sea level rise. Journal of Geophysical Research: Earth Surface 116: 1–13. doi:10.1029/2011JF002093.
8. Fagherazzi, S., M. L. Kirwan, S. M. Mudd, G. R. Guntenspergen, S. Temmerman, J. M. Rybczyk, E. Reyes, C. Craft, and J.Clough. 2012. Numerical models of salt marsh evolution: Ecological, geormorphic, and climatic factors. Review of Geophysics 50: 1–28. doi:10.1029/2011RG000359.1.
9. Ganju, N. K., M. L. Kirwan, P. J. Dickhudt, G.R. Guntenspergen, D.R. Cahoon, and K. D. Kroeger. 2015. Sediment transport-based metrics of wetland stability. Geophysical Research Letters 42: 7992–8000. doi:10.1002/2015GL065980.
10. Kirwan, M. L., and G. R. Guntenspergen. 2012. Feedbacks between inundation, root production, and shoot growth in a rapidly submerging brackish marsh. Journal of Ecology 100: 764–770. doi:10.1111/j.1365-2745.2012.01957.x.
11. Kirwan, M. L., and J. P. Megonigal. 2013. Tidal wetland stability in the face of human impacts and sea-level rise. Nature 504: 53–60. doi:10.1038/nature12856.
12. Kirwan, M. L., and S. Temmerman. 2009. Coastal marsh response to historical and future sea-level acceleration. Quaternary Science Reviews 28. Elsevier Ltd: 1801–1808. doi:10.1016/j.quascirev.2009.02.022.
13. Mariotti, G., and S. Fagherazzi. 2010. A numerical model for the coupled long-term evolution of salt marshes and tidal flats. Journal of Geophysical Research: Earth Surface 115: 1–15. doi:10.1029/2009JF001326.
14. Morris, J. T., P. V. Sundareshwar, C. T. Nietch, B. Kjerfve, and D. R. Cahoon. 2002. Responses of coastal wetlands to rising sea level. Ecology 83: 2869–2877. doi:10.1890/0012-9658(2002)083[2869:ROCWTR]2.0.CO;2.
15. Mudd, S. M., S. Fagherazzi, J. T. Morris, and D. J. Furbish. 2004. Flow, sedimentation, and biomass production on a vegetated salt marsh in South Carolina: toward a predictive model of marsh morphologic and ecologic evolution. American Geophysical Union: 165–187. doi:10.1029/CE059p0165.
16. Mudd, S. M., S. M. Howell, and J. T. Morris. 2009. Impact of dynamic feedbacks between sedimentation, sea-level rise, and biomass production on near-surface marsh stratigraphy and carbon accumulation. Estuarine,Coastal and Shelf Science 82. Elsevier Ltd: 377–389. doi:10.1016/j.ecss.2009.01.028.
17. Mudd, S. M., A. D’Alpaos, and J. T. Morris. 2010. How does vegetation affect sedimentation on tidal marshes? Investigating particle capture and hydrodynamic controls on biologically mediated sedimentation. Journal of Geophysical Research: Earth Surface 115: 1–14. doi:10.1029/2009JF001566.
18. Mendelssohn, I. A., and N. L. Kuhn. 2003. Sediment subsidy: Effects on soil-plant responses in a rapidly submerging coastal salt marsh. Ecological Engineering 21: 115–128. doi:10.1016/j.ecoleng.2003.09.006.
19. Cahoon, D., and G. R. Gutenspergen. 2010. Climate change, sea level rise, and coastal wetlands. National Wetlands Newsletter, 32(1):8-12.
20. Torio, D. D., & Chmura, G. L. (2013). Assessing coastal squeeze of tidal wetlands. Journal of Coastal Research, 29(5), 1049–1061. https://doi.org/10.2112/JCOASTRES-D-12-00162.1