Coastal marshes are wetland environments found along coastlines, characterized by their regular flooding and draining by tidal saltwater or brackish water. These areas feature soils composed of deep mud and peat, which is formed from decomposing plant matter. The frequent submergence by tides and high organic content can lead to low oxygen levels in the peat, sometimes resulting in a distinct “rotten-egg” odor.
Characteristics and Global Distribution
Coastal marshes are shaped by tidal and saline influences. The vegetation in these areas consists primarily of salt-tolerant plants, known as halophytes, which include various grasses, herbs, and low shrubs. Mangrove forests, found in tropical regions, are also considered a type of coastal wetland, differing from salt marshes by being dominated by trees rather than grasses.
Regular tidal flooding delivers sediments, nutrients, and water to these ecosystems. Salinity levels can vary across the marsh, with lower salinity found at higher elevations less frequently inundated by tides. They are globally distributed, found along coastlines on every continent except Antarctica. Extensive marshlands exist along the U.S. East Coast and Gulf of Mexico, where about half of the nation’s salt marshes are found in the Gulf Coast region. In 2020, the estimated global extent of tidal marshes was around 52,880 square kilometers, across 120 countries and territories.
Ecological Contributions
Coastal marshes are productive habitats, supporting diverse wildlife. They provide shelter, food, and nursery grounds for a portion of coastal fishery species, including shrimp, crab, and various finfish; over 75% of commercial fish and shellfish species rely on these areas for part of their life cycle. Many migratory waterfowl and shorebirds also use these wetlands as stopover points, finding resting, feeding, and breeding grounds. The decomposition of plant matter in marshes forms rich organic material, which fuels a broad food chain from microorganisms to larger animals.
Beyond supporting wildlife, coastal marshes filter sediments and pollutants from land runoff, improving water quality before it reaches the ocean. They effectively absorb nitrogen, reducing by 50% or more the amount entering open waters. This purification helps prevent eutrophication, a condition caused by excessive nutrients that can lead to harmful algal blooms and low oxygen “dead zones” in coastal waters.
Coastal marshes also sequester carbon, often called “blue carbon” ecosystems. They capture and store large quantities of carbon dioxide from the atmosphere, at a rate estimated to be 10 times greater than tropical forests, storing three to five times more carbon per acre. Most carbon is stored in anaerobic marsh soils, decomposing slowly and persisting for hundreds or even thousands of years.
These ecosystems buffer against storms and erosion, providing coastal protection. They absorb wave energy during storms, reducing erosion and mitigating coastal flooding by decreasing wave heights. Studies in the United States estimate that coastal marshes can provide over $20 billion in annual damage reduction from hurricanes and other coastal storms. For example, during Superstorm Sandy in 2012, coastal wetlands reduced property damages by more than $625 million across the impacted area, with an average of about 10% damage reduction in each state.
Major Threats
Coastal marshes face threats, primarily from human impacts. Coastal development, driven by increasing populations in coastal counties, leads to marshland destruction. Between 2004 and 2009, coastal wetlands in the contiguous United States declined by over 360,000 acres, an average loss of over 80,000 acres per year.
Pollution also impacts these ecosystems. Agricultural runoff with fertilizers and pesticides can overload the marsh’s filtration capacity, leading to toxin and excess nutrient accumulation that harms marsh plants and wildlife. Industrial waste and stormwater runoff also carry heavy metals and other chemicals into marsh environments, where they can be stored for long periods. Alterations of water flow, often from roads or railroads severing the marsh’s connection to the sea, disrupt tidal flooding and can change soil and water chemistry, stressing or drowning marsh plants.
Environmental threats, like sea-level rise, endanger coastal marshes. Rising sea levels can inundate marshes, converting high marsh areas to low marsh or even open water if the marsh cannot accrete sediment fast enough to keep pace. Invasive species, like the common reed (Phragmites australis), exacerbate these vulnerabilities. These non-native plants can outcompete native marsh vegetation, forming dense stands that reduce habitat quality and alter ecosystem function.
Preservation and Recovery Initiatives
Efforts are underway to protect and restore coastal marsh ecosystems, recognizing their ecological and economic contributions. Government-led protection programs, such as Georgia’s Coastal Marshlands Protection Act, aim to prevent development endangering marshes and secure existing marshlands. These legislative measures involve strict zoning and permitting to ensure marsh preservation and allow for natural migration.
Community-based conservation efforts involve local groups, agencies, and individuals in planning, monitoring, and implementing restoration projects. Partnerships like the South Atlantic Salt Marsh Initiative, with over 300 members, work to conserve approximately 1 million acres of salt marsh across the southeastern United States. These initiatives focus on restoring and protecting existing marshland and conserving adjacent areas for future marsh migration.
Restoration techniques revitalize degraded marshes.
- Sediment replenishment (augmentation or placement) involves adding dredged material to eroded areas to rebuild marsh elevation and support new vegetation.
- Restoring hydrological flows by removing barriers like dikes, dams, tide gates, or undersized culverts that restrict tidal exchange.
- Replanting native vegetation, such as salt-tolerant grasses, to stabilize sediments, improve water quality, and provide wildlife habitat.
- Controlling invasive species through manual removal or chemical control to ensure the health and diversity of native plant communities.