Marsh restoration is the process of assisting the recovery of a wetland ecosystem that has been degraded or destroyed. Marshes are a specific type of wetland dominated by herbaceous, or soft-stemmed, plants rather than woody vegetation. The goal is to rebuild a lost wetland or repair the ecological processes of a declining one, returning its natural functions and attributes.
Ecological Importance of Marshes
Healthy marsh ecosystems provide functions that benefit the environment and human communities. They serve as habitats for thousands of species of plants and animals. For many commercially and recreationally important fish and shellfish, marshes act as sheltered nurseries for their young. These areas also offer feeding and resting grounds for migratory birds, with some species using them as primary hunting grounds.
Coastal marshes also act as natural barriers that protect shorelines. The dense plant life absorbs wave energy, reducing storm impacts and buffering communities from flooding. This vegetation helps stabilize shorelines by trapping sediments, which counteracts erosion. The slow water flow allows sediments and pollutants from runoff to settle, improving overall water quality.
Marshes play a part in the global carbon cycle as effective “blue carbon” sinks, capturing and storing large amounts of atmospheric carbon dioxide. Hectare for hectare, coastal wetlands can sequester carbon at a rate up to ten times greater than mature tropical forests. If left undisturbed, the carbon stored in the deep peat soils of a marsh can remain locked away for thousands of years.
Common Threats to Marsh Ecosystems
Several factors contribute to marsh degradation.
- Coastal development and historical land use: For many years, marshes were misunderstood and drained or filled for agriculture, industry, and housing. The construction of roads and other infrastructure can also alter water flow, starving marshes of necessary water and sediment.
- Sea-level rise: As sea levels increase, coastal salt marshes can become permanently inundated, drowning vegetation adapted to specific water depths. This process is often compounded by subsidence, the natural sinking of coastal land, which further reduces the marsh’s elevation.
- Pollution: Runoff from agricultural and urban areas often carries high loads of nutrients, like nitrogen and phosphorus, leading to eutrophication. Other pollutants, including pesticides and industrial chemicals, can accumulate in marsh sediments, harming local wildlife.
- Invasive species: Non-native plants can outcompete native marsh grasses that form the foundation of the habitat. This displacement can alter the marsh’s physical structure, reduce its value for wildlife, and lead to increased erosion.
The Restoration Process
Marsh restoration begins with a site assessment to understand the causes of degradation and establish clear goals. This phase involves studying the site’s hydrology, soil composition, and existing plant and animal communities. Project goals might range from restoring habitat for a specific species to enhancing coastal protection for a nearby community, which guides the selection of restoration techniques.
Hydrological restoration focuses on re-establishing natural water flow. In many degraded marshes, structures like dikes or dams have restricted tidal flushing. Projects often involve removing or modifying these barriers to allow tides to transport water and sediment into the marsh. In permanently flooded areas, creating new channels can drain excess water and restore conditions for marsh vegetation.
Sediment augmentation is used in marshes that have lost elevation. This technique involves adding sediment, often dredged from navigation channels, to raise the marsh platform to an ideal height for native plants to colonize. Sometimes, this new sediment is shaped into terraces or mounds to create sheltered areas that reduce wave energy and encourage plant establishment.
Revegetation is often a final step, involving the active planting of native marsh grasses like Spartina. These plantings help stabilize new sediment, accelerate plant community recovery, and provide immediate habitat structure. These restoration methods are frequently used in combination to address a site’s specific problems.
Evaluating Restoration Outcomes
Determining the success of a restoration project requires a monitoring phase to track progress toward its goals. This process relies on scientific metrics to measure the recovery of the ecosystem’s structure and function. Monitoring often begins before work starts to establish a baseline and can continue for years to assess the long-term trajectory.
An indicator of success is the health of the plant community. Scientists measure the survival, growth, and spread of native vegetation. They look for increasing plant density and biomass over time, which demonstrates that conditions are suitable for a self-sustaining ecosystem. The presence of diverse plant species signals the return of a healthy community.
The return of wildlife is another measure of success. Monitoring efforts track the use of the site by target species, such as specific birds, fish, or invertebrates. For example, biologists may conduct fish surveys to see if the marsh is functioning as a nursery or use bird counts to determine if migratory species are using the area. The reappearance of these species indicates the habitat is providing food and shelter.
Evaluations also assess the recovery of ecosystem functions. This can involve measuring improvements in water quality by testing for reductions in pollutants and excess nutrients. Scientists also analyze sediment accretion rates to determine if the marsh is building elevation at a pace that can keep up with sea-level rise.