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Salt Marsh Ecology in an Era of Sea Level Rise

This project began in January 2004 and is projected to be completed in December 2018

Under the right circumstances, salt marshes have the ability to increase their elevation, and therefore may be able to “keep up” with sea level rise. We are working to understand the factors that result in vertical growth of salt marshes—specifically, the relationships between marsh surface elevation, rates of sediment accretion, and the growth of salt marsh vegetation. These data will be used to help predict long-term sustainability of coastal salt marshes under future sea level rise scenarios.

Why We Care
Salt marshes are an integral part of coastal ecosystems. They provide habitat for aquatic species, filter nutrients from upland sources, and help reduce the impact of coastal storms by absorbing wave energy. For salt marshes to persist despite rising waters, they must grow at a rate equal to or greater than the rate of sea level rise. This scenario requires that the marsh surface increases in height. The two processes by which this can occur are: 1) accumulation of root material in marsh soils, and 2) trapping of sediment that is carried into the marsh during flood tides. Our research focuses on the rates of both of these processes and the factors that control them.

As marshes plants grow, they remove carbon from the atmosphere through photosynthesis. The carbon that is used to produce root tissue often remains in the soil for long periods of time after the plants dies. This long term storage of carbon is referred to as “sequestration.” In addition to the previously mentioned ecosystem services provided by salt marshes, their ability to adjust to changing sea levels may be important to the global carbon cycle through sequestration.

What We Are Doing

  • Determining Optimal Growth Conditions – To predict long-term sustainability of salt marshes, we must first understand the conditions for optimal growth. To do so, we are using a combination of field surveys and experimental manipulations to evaluate the growth of above and below ground tissues of Spartina alterniflora (the dominant salt marsh plant) across multiple elevations and patterns of tidal inundation.
  • Modeling the Implications of Sea Level Rise – We have a series of surface elevation tables installed at marshes throughout coastal North Carolina. These devices are used to measure changes in marsh surface elevation over time. The data we collect concerning optimal growth conditions and marsh surface elevation will be incorporated in simulation models to predict the response of salt marshes to current and future sea level rise scenarios.
  • Carbon Sequestration in Salt Marsh Sediments – The term “blue carbon” is used to refer to the carbon photosynthesized by aquatic plants, like those that occur in salt marshes. We are evaluating the ability of North Carolina salt marshes to accumulate blue carbon by analyzing growth and decay rates of root material. These data will help us quantify the amount of blue carbon that can be sequestered through salt marsh growth, as well as the length of time that this carbon remains in the soil.

What We Are Finding

  • Long Term Salt Marsh Sustainability – Marshes within the study area are currently gaining elevation at rates ranging between 2.3 mm and 9 mm/yr. At current rates of sea level rise (~2.5 mm/yr), the marshes in our study are able to keep up. Our modeling results suggest that, at the current rate, marshes will persist into the next century. However, if the pace of sea level rise accelerates, salt marsh biomass is likely to decline precipitously over that same time period.
  • Carbon Sequestration – Our preliminary data indicate that more than half of the carbon that goes into creating below ground biomass resists decay for periods of > 1 year. The amount that is sequestered for longer time periods is a subject of ongoing research.

Next Steps
We are currently evaluating the relationship between below ground biomass and elevation to search for optimal growth conditions for roots and rhizomes. We are also working to develop a deeper understanding of the variability in rates of decay of below ground biomass across elevation gradients within a single marsh and among different marshes.

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