JUDITH S. WEIS / NJ ENVIRONMENT NEWS – Salt marshes, or coastal wetlands, the green areas at the edge of our estuaries, are among the most productive ecosystems on earth and provide ecosystem services that benefit humans everywhere. Salt marshes protect coastal communities from storm surges and waves, and attenuate flooding. Tidal wetlands in the northeast U.S. prevented an estimated $625 million in flood damages from Hurricane Sandy (much in New Jersey), according to a paper published in Scientific Reports. Salt marshes also remove excess nutrients from wastewater and runoff, trap pollutants, and sequester carbon, thus improving water quality and reducing the amount of climate change. In addition, they provide important habitat for a variety of invertebrates, fishes, and birds, and function as nursery habitats and feeding grounds for species of fish and crabs which are economically important.
Despite their importance, they have been underappreciated and filled in for centuries and are currently being damaged by many human activities. Coastal development, dredging, climate change, and pollution are some of the factors responsible for their degradation and loss. Climate change and pollution cause wetland loss by increasing drought, toxicity, and nutrient stress. Sea level rise (SLR) poses a threat of submergence (drowning) especially in locations where shoreline development prevents marsh plants from migrating inland. Similarly, subsidence (decreasing elevation) as a result of inadequate sediment supply or geologic factors is a serious threat, with similar results. While SLR is the major threat, excess nutrients (eutrophication) can also contribute to marsh loss by causing the marsh plants to become unstable and fall over, which causes creek-bank collapse followed by marsh converting to unvegetated mud. Overfishing of some fish has led to increased populations of herbivorous marsh crabs whose consumption of marsh grasses has caused marsh die-back in some areas.
SLR is the largest climate-related threat to salt marshes. It is estimated that one meter of SLR will eliminate 46% of the world’s coastal wetlands. The loss of marshes to rising sea levels is now a current and urgent issue, rather than one for the future. The rate of SLR is not identical at all locations, and some marshes are better able to keep up with SLR than others. Worldwide data indicate that many salt marshes are not acquiring enough new sediment to keep pace with SLR. Marshes in regions of higher local SLR are less likely to keep pace, and marshes with inadequate sediment supply are most vulnerable to SLR. When marsh area is reduced, the productivity of the estuary as a whole is diminished and there is reduced protection for human communities. With diminished marshes and more intense coastal storms, there will be less protection and greater storm damage. Reduced marshes, which are nursery grounds for fish, will also lead to reduced fish production.
A study of resilience of 16 marsh sites in the U.S found Pacific marshes were more resilient than Atlantic, with the least resilient in southern New England (none of the marshes studied were in NJ.) A study of Connecticut and New York marshes found that declining relative elevation led to increased tidal flooding, particularly in the high marsh. As flooding increased, organic matter continued to accumulate in the marshes, but mineral sediment deposition was seldom observed. A study of Jamaica Bay in nearby New York City found that those urban marshes had low inputs of mineral sediments, but organic matter increased enough to allow vertical accumulation to keep up with sea level — temporarily. This study concluded that marsh survival will require mineral sediment addition to the marsh surface.
Mitigating factors and possible remedies
If marshes cannot increase their elevation quickly enough, they may be able to move inland, provided there is open space behind them. This is rare in urban areas where coastal development may be located immediately behind the marsh. “Migration corridors” are particularly important to develop in estuaries where coastal development blocks marshes from moving inland in response to sea level rise. It is also difficult or not feasible for marshes to migrate inland in areas with a steep slope.
The invasive plant Phragmites australis, or common reed, causes biomass and carbon storage in marshes to increase greatly. This species, which is often removed in “marsh restoration” projects, also stores more nitrogen, thus reducing eutrophication, and its high amount of biomass also increases marsh elevation. Its litter (dead decaying plants) on the marsh surface traps more mineral and organic sediment than native marsh species due to greater litter production, which traps more sediment. This increasing of marsh elevation was once considered a negative effect, but now it is a positive and possibly essential effect.
One possible way of increasing marsh elevation is a process called “thin layer deposition” — spraying sediments from tidal creeks onto the marsh surface. It has been performed experimentally at sites in Stone Harbor, Avalon, and Fortescue in South Jersey. This is the salt marsh equivalent of beach renourishment. Its long-term effectiveness, the optimum amount of thickness to add, and how often the procedure will have to be done are as yet unknown and will undoubtedly be different at different locations.
When marshes are eroding at the edge, “living shorelines” in the form of oyster reefs or rocks can be placed at the marsh edge to prevent further erosion for a time. Living shorelines were found to enhance marsh resilience to hurricanes better than either hard edges (seawalls) or natural marshes. A few living shoreline projects have been created in NJ. Since these are relatively new approaches, their continued effectiveness in the face of SLR remains to be seen.
We need to learn about the status of our marshes throughout the state in terms of keeping up with SLR or not. And we need to evaluate the effectiveness of thin layer deposition and living shorelines as potential solutions and investigate the potential of leaving some Phragmites in place to allow the marshes to increase their elevation more rapidly. We need to explore and create pathways along which salt marshes can move inland in different locations in the state. Let us hope that some or all of these strategies in combination will be effective in saving our vital coastal wetlands for future generations.
Judith S. Weis is professor emerita of biological sciences at Rutgers University – Newark.