The importance of coastal wetlands
Salt marshes and mangrove stands are plant-dominated areas adjacent to the coast that contain plants (salt marsh) and trees (mangroves) that have developed the ability to tolerate being periodically flooded with salt water. Collectively, both of these habitats are referred to as coastal wetlands. Coastal wetlands are important habitats for both terrestrial and aquatic animals and provide water filtration by taking up pollutants and excess nutrients in coastal environments. They also act as a buffer against storm erosion by absorbing wave energy and holding the soil together with complex root structures.
One of the consequences of global climate change is that the level of the ocean is rising due to the melting of arctic glaciers and the expansion of seawater as it warms. With a large portion of the human populations living close to the coast, sea level rise represents a serious threat to infrastructure and homes. In the United States, approximately 39% of the population (123.3 million people) live in counties that are directly adjacent to the coast.
Salt marsh in the Narragansett Bay Estuarine Research Reserve. Photo courtesy of NOAA. Available at: https://oceanservice.noaa.gov/facts/saltmarsh.html
Coastal wetlands and sea level rise
In recent years, extensive research has been conducted to determine the vulnerability of coastal wetlands to sea level rise. Both salt marsh and mangroves can increase the elevation of the coastline in several ways. First, the aboveground plant material can trap sediment from the water column, which will end up at the base of the plants and build up over time. Second, the root structure of the plants can increase the soil elevation by increasing the amount of plant material present. Below ground roots also hold the soil structure in place and decrease soil compaction.
The vulnerability of coastal wetlands to sea level rise is determined by comparing how quickly plants can add elevation to the marsh surface to how quickly the sea is rising. Mangroves, which are generally restricted to tropical areas, have been migrating to areas that were previously dominated by cold-tolerant salt marsh plants as global temperatures increase. Previous research has reported that there may be differences in the pace of elevation gain between mangrove and salt marsh plants that can have important consequences for these areas as sea level continues to rise.
Coastal Wetlands in the Mississippi River Delta
The Mississippi River Delta is located in Louisiana where the Mississippi River meets the Gulf of Mexico and was formed by sediment that was transported by the river to the coast. This area contains almost 40% of the coastal wetlands in the United States. The Mississippi River Delta has been undergoing high rates of land loss and decreased by 1,194,260 acres between 1932 and 2016 due to a combination of land sinking and human activities.
In the Port Fourchon area of the Mississippi River Delta, mangroves have increased in area from 101 acres in 1993 to 1,656 acres in 2011. Due to the replacement of salt marsh plants by mangroves in this region, Karen McKee and William Vervaeke set out to determine if there were differences in the vulnerability of mangrove, salt marsh, and mixed mangrove-salt marsh patches to sea level rise. They set up small plots of only black mangrove, only salt marsh grass, and mixed mangrove-salt marsh grass and tracked changes in elevation plots over 6 years. They also applied disturbances to half of the plots that decreased the amount of above ground plant material to see if disturbance had an impact on elevation changes. Mangroves naturally die back due to freezing temperatures, so the researchers froze the mangrove trees in assigned disturbance plots. Salt marsh plants can die from being smothered by floating plant debris (i.e. plant wrack) that is deposited on the marsh during high tide. In assigned disturbance plots with salt marsh plants, the researchers added plant wrack to simulate this process.
Are coastal wetlands in the Mississippi River Delta vulnerable to sea level rise?
McKee and Vervaeke showed no significant elevation differences between the patches over the course of the experiment, meaning that mangrove, salt marsh grass, and mixed mangrove-salt marsh grass patches all experienced similar amounts of elevation gain, ranging from 4.1 to 5.4 mm per year. Elevation gain was a combined measurement that took into account the change in the amount of soil that was being trapped on the surface by the plants (10.0 to 12.4 mm per year) and changes in the depth of the root zone and the area below the roots (-8.0 to -4.6 mm per year). Disturbance had no effect on elevation changes in any of the plots.
Once McKee and Vervaeke measured elevation gains in their experimental plots, they needed to compare these gains to how fast sea level was rising to assess the vulnerability of these coastal wetlands. In the Port Fourchon region of the Mississippi River Delta, sea level has been rising an average of 5.0 mm per year based on long-term data, but during the study interval, the sea-level rose at a pace of 14 mm per year. When they calculated the difference in the elevation change of their plots taking into account sea level rise, they concluded that the plots had an average elevation deficit of 4 mm per year, meaning that sea level was rising faster than the elevation of the plots. While replacement of salt marsh grass with mangroves did not have an impact on elevation change, these areas were still vulnerable to sea level rise.
The results of McKee and Vervaeke contrast with previous studies in different areas, which highlights the fact that the vulnerability of coastal wetlands to sea level rise will vary regionally. The vulnerability of wetlands will also depend on the species composition and how far from the edge of the coast the plants are located, since elevation in wetlands varies with distance from the water’s edge. These differences make large-scale predictions extremely difficult. Even so, many cities and states are exploring conservation efforts to maintain coastal wetlands and to assess the threat of sea level rise on infrastructure. In my home state of Rhode Island, the Executive Climate Change Coordinating Council has been conducting analysis on the impacts of sea level rise since 2014. These local and regional efforts aim to establish future policies to mitigate the impact of sea level rise in coastal communities
McKee, K.L. and W.C. Vervaeke. 2018. Will fluctuations in salt marsh-mangrove dominance alter vulnerability of a subtropical wetland to sea-level rise? Global Change Biology 24: 1224-1238. https://doi.org/10.1111/gcb.13945