Salt Marsh 101
Learn about salt marshes and why they are important for our coasts.
What is a Salt Marsh?
Salt marshes exist where freshwater and saltwater meet. They store carbon, filter water, buffer against storm events, stabilize the coast, and are home to many species of plants, fish, and wildlife.
Zones of the Shoreline and Salt Marsh
Shoreline and salt marshes are characterized in zones. These zones are the product of how wet and salty the soils are along the shore, producing unique habitat for different sets of plants and animals.
The two zones of the shoreline that make up salt marshes are the Low Marsh and High Marsh.
Explore our infographics to see the differences between a natural, modified, and restored coast.
Click on the features or zones to learn more!
Below is an example of a healthy coastal ecosystem that is free of human-intervention. These are the plants, animals and natural coastal zones that we work to restore, protect and preserve through our work. Click on any animal, plant, or zone to learn more about the elements of natural coastlines.
This zone is important habitat for many organisms, such as fish, eel grass, and plankton. The subtidal zone is always underwater, even at low tide.
The mud flats are home to critters like crabs, mussels, and worms, all important blocks of the marine food web. This zone has fine sediments like clays and silts. The mudflat is submerged on every high tide.
Low Marsh Zone
This zone is dominated by smooth cordgrass. This plant can withstand daily flooding and high salt content in the soil, making it the only plant to grow in the low marsh. This zone is important habitat for crustaceans, birds, and fish. The low marsh is very good at storing carbon and is submerged during every high tide.
High Marsh Zone
The high marsh is a zone of competition amongst plants. The high marsh zone is more diverse than the low marsh because it is flooded less regularly. Its plants grow densely, taking carbon out of the air and sequestering it into the ground (this carbon is called blue carbon). It is important nesting habitat for birds. The high marsh is flooded only during the highest spring tides.
Upper Border Zone
The upper border sees saltwater only during storm events and in the form of salt spray. It is mostly a freshwater system with species that can tolerate occasional saltwater. The upper border is where species such as sweetgrass and brackish cattails can be found.
Like the upper border, the shrubland is only subjected to saltwater during storm events and in the form of salt spray. This zone is dominated by woody stalked vegetation rather than grasses. Shrubs like bayberry and rose are found in this zone.
The upland zone is not reached by the tides or saltwater. In natural landscapes this is often a forested stand (such as the Acadian Forest). Many species from the previous zones depend on the connectivity and health of this upper border for various aspects of their lifecycles.
Common Marsh Snail
Marshes can migrate both seaward and landward on the horizontal plane, though they tend to migrate landward over time as a product of long-term sea level rise. As the rates of sea level rise increase, so do the rates of marsh migration.
Marshes grow vertically through two major sources: sediment inputs and organic matter. These types of growth can result in minerogenic marshes (fed by sediments) or organogenic (fed by organic matter). A marsh will not survive if the rate of sea level rise is greater than the rate of its growth.
Below is an example of a coastal ecosystem that has been disrupted by human modification. Click on any development, invasive species, or zone to learn more about that element and how it impacts the health of a coastal ecosystem.
Sea Level & Temperature Rise
Our climate is changing as a result of the addition of greenhouse gases (GHG) into our atmosphere, causing an increase in the Earth’s average temperature. The ocean has a great capacity to absorb and distribute this heat. As the ocean warms it causes thermal expansion of water, the melting of sea and terrestrial ice and, as a by-product, sea level rise. If GHG emissions are not rapidly reduced, this process will continue.
Waste and Microplastics
Wastes and microplastics are a major source of pollution in terrestrial, ocean, and coastal ecosystems. Waste comes in all shapes, sizes, and materials. Typical waste found on the coast includes cigarette butts, plastic bottles, fishing gear, and microplastics. Microplastics are a significant problem as filter feeders (e.g. mussels and scallops) and fish consume these indigestible plastics. These animals are then consumed by predators, causing the plastics to accumulate in larger animals up the food chain.
The European green crab is an invasive species on the Atlantic coast of North America. This species can disrupt the balance of the coastal ecosystem by destroying eel grass beds and competing for food with the native species. The green crab can also increase salt marsh erosion as its burrows destabilize and damage the root mats of the marsh plants.
Excessive nutrient loading into our coastal systems can lead to the rapid growth of algal mats. These mats can form upon the salt marsh grasses and smother them, converting marsh into open water or mudflats. This nutrient pollution can come from multiple sources and zones, particularly the residential, road, and agricultural zones.
Marsh drowning describes the process of a marshland converting into open water. This process occurs when a marsh remains submerged for too long, stunting plant growth and degrading the ecosystem. Marsh drowning is caused by features like old mosquito ditches, agricultural sill, and dykes that retain pools of water on the marsh. Sea level rise also causes marsh drowning if the marsh is unable to grow at the same rate as the sea. It is important to note that, when naturally created, pool systems provide valuable habitat to a plethora of plants and animals. However, human-constructed pools (e.g. mosquito ditches) often do not provide these benefits and do not exist in equilibrium with the rest of the system.
Salt Panne Expansion
A Salt panne is a low lying, saline habitat found within the saltmarsh that provides habitat to many plants and animals. As sea levels and temperatures rise, coastal systems in warmer areas experience salt panne expansion. This is because pools and pannes that exist in the high marsh are being inundated by tidal waters from sea level rise and storm surge. When coupled with warmer summer temperatures, these pools evaporate, leaving their salt contents behind and causing salt panne expansion. This process can cause a feedback loop of increasing salinity in soils and decreasing plant cover on the landscape. Salt panne expansion can convert diverse marshes into desert landscapes.
Dyke Systems and Storm Surge
Storm surge is the process in which water levels rise in association with storms. The severity of storm surge is determined by multiple factors such as the volume of water carried by the storm into the coastline or the low atmospheric pressure associated with storms. As the climate continues to warm, the size and regularity of these storm events will increase.
As dyke systems interrupt the natural flow of water into and out of coastal ecosystems, they can exacerbate flooding events by blocking water drainage during high tides and significant storm events. The dyke system reduces the amount of water the landscape can accommodate from oncoming events as it reduces the former floodplain area. Imagine a splash pool filled to the brim – what happens if someone jumps in? The water is displaced and overflows. This same principle occurs in modified landscapes, but the size of the entire pool has been reduced. As sea levels rise and storm impacts worsen, the dykes must be topped (elevated with additional earth material) to keep the water out.
Agriculture and Nutrient Pollution
The agricultural zone occupies the former area of the high marsh. These fertile lands are no longer able to grow vertically via sediment or organic input and are therefore unable to keep up with the rate of sea level rise. Waters from the upland often flow down to the agricultural zone, carrying with them waste from the residential and roads zones. This waste then travels further downstream along with the soil and nutrient runoff from the farmland and enters the flow gate and larger water system. This results in the loss of soils and lowering of elevation as well as algal blooms in the water system. These blooms can decrease water quality and result in fish-kill by worsening the problems of eutrophication and algal mat cover.
Roads were built to connect landscapes as needed and can occupy any coastal zones. These structures can sometimes act as a barrier in the form of causeways and roads fitted with undersized culverts or bridges. The ditches in this zone and surrounding areas can house invasive species like phragmites, the common reed. When built directly on the coast, this is one structure that causes coastal squeeze. Particulate matter from vehicle emissions, debris, and gravel from this zone are often washed down into the watercourse below.
Common reeds, also called phragmites, exist in a native and an invasive form on the Atlantic coast of North America. This grass can grow in brackish waters and can occupy the transition zone between marsh and upland. This species outcompetes the native high marsh plants (including native phragmites), effectively reducing the biodiversity of the area into a monoculture. Phragmites can grow up to 5 meters tall, but these converted wetlands do not provide the same habitat value to native animals.
Straight pipes are pipes that dump raw sewage wastes into our waters. These pipes are typically found in older dwellings and cottages built before septic tanks and fields were common practice. Dumping raw sewage into the waters can contribute to numerous negative impacts including nutrient pollution and bacterial outbreaks, effectively reducing water quality.
Paved surfaces include roads, parking lots and driveways. These surfaces do not let water seep into the ground slowly, but rather force it to flow off the surface and downslope. As there is no vegetation on this surface to slow the water, runoff occurs more quickly. Fast-moving water can pick up and carry more pollutants, larger sediments, and can cause greater erosion. Lots of paved surfaces in a watershed can increase pollutants being washed into the waterway while worsening erosion from freshwater storm events.
Deforestation reduces the amount of rainwater that can saturate into the soil. Vegetation, such as trees and grasses, slows water that flows over their landscapes and can allow sediments to settle out. The roots of vegetation also act to bind the soils. The presence of vegetation can increase the amount of water it takes to saturate a soil while storing it for later.
The subtidal zone is always underwater, even at low tide. The subtidal zone gets larger as the low tide mark rises with the sea level. This is part of the coastal squeeze process. A rise in sea temperature also means warmer subtidal zones, which can potentially stress the native flora and fauna.
The mudflat is submerged on every high tide. As the subtidal zone increases due to sea level rise, this zone gets smaller (another element of the coastal squeeze process). The mudflat is also challenged by the introduction of the invasive European green crab and by deteriorating organic material from the low marsh that washes downslope.
The low marsh is submerged on every high tide and exposed at every low tide. This zone has gotten smaller with the addition of dykes, which sandwich the low and high marsh together into one zone. As sea levels rise, this zone will shrink even more due to coastal squeeze. The low marsh deteriorates from the process of marsh drowning and extensive algal mat cover.
The high marsh is a zone with high plant competition that is flooded only during the highest spring tides. This zone gets smaller with the addition of dykes, which sandwich the low and high marsh together into one zone. As sea levels rise, this zone will shrink even more due to coastal squeeze. The high marsh deteriorates due to marsh drowning, extensive algal mat cover, and salt panne expansion.
The agricultural zone is where we grow our food. This zone exists within the former high marsh zone. Flood plains are very fertile lands, which the Acadian settlers of Mi’kma’ki and Nova Scotia converted into agricultural land using the dyke system. Dykes (earthen walls) block the incoming tides and allow freshwater to drain at low tide through one-way flow gates (aboiteau), resulting in eventual desalination of the soil. These barriers ‘hold the line,’ blocking the marshes from migrating further upland and are one of many “hard structures” that contribute to coastal squeeze.
The road zone is where our transportation infrastructure exists. These corridors are extremely important for the transfer of goods, services, and for connecting our communities. This zone often replaces the upper border and shrubland zones. Roads built in coastal zones can act as barriers to the natural hydrology of the coast due to causeways and roads fitted with undersized culverts or bridges. The ditches in this zone and surrounding areas can house invasive species like phragmites, the common reed. When built directly on the coast, roads can contribute to coastal squeeze.
The residential zone is where we live. This zone often occupies the former shrubland and uplands of the coast. Residents have introduced invasive species, cleared the forests, and paved natural surfaces. In some cases, older homes are fitted with straight pipes.
Coastal squeeze is a process where the lowest zones of the coastline are pushed upland due to sea level rise, but are unable to migrate further as they are blocked by hard features in the landscape (e.g. sea walls, dykes, or undersized crossings). Reacting to rising sea levels, the low marsh zone cannot withstand constant submergence and will convert to an open water system. Similarly, the high marsh zone cannot withstand daily submergence and will seek to migrate upland. If blocked by hard features, the high marsh will be forced to convert into a low marsh system. As sea levels continue to rise, the converted low marsh will drown, resulting in the complete loss of the wetland in front of the hard feature. This process of zone migration is nature’s adaptation to rising sea levels; blocking the marsh migration corridor prevents this adaptation and destroys the wetland ecosystem.
Below is an example of a modified coastline that has been restored. Click on any restoration technique, adaptive management practice, or zone to learn more about how and why it is used for coastal restoration work.
This practice reintroduces natural processes and shoreline components to a modified coast. The natural processes include the movement of tides and sediments, while the natural components can include sediment, reefs, and plants. Living shorelines come in all forms and scales. These projects can integrate different combinations of hard engineering features (e.g. groins, sea walls, and jetties) and living features to secure and restore a more natural coast.
The restored subtidal zone hosts reef balls to create habitat for aquatic vegetation and critters while protecting the toe of the marsh from oncoming events. These structures help limit erosion while trapping sediment but do not limit it completely, maintaining the connectivity between the uplands and subtidal zone.
The upper edge of the restored mudflat zone has been fitted with oyster reefs and shoreline grass plantings. These reefs and grasses help further stabilize the shoreline, keeping the low marsh from falling apart into the mudflat. The filter feeders of the reef help improve water quality. These reefs and plantings will provide a buffer against wave and erosion but do not limit the area completely, maintaining the sediment connectivity between the uplands and the mud flat.
The restored low marsh zone has been fitted with runnels to assist the drowning marsh. Sediment nourishment and marsh plantings restore areas that have been converted to salt panne. The managed realignment has redistributed the footprint of the zones, allowing the low marsh to occupy an area that better matches its natural condition. The removal of the tidal barrier allows the low marsh to migrate upland as sea levels rise, maintaining the balance between the zones.
High Marsh / Agriculture
The managed realignment has separated the high marsh and the agricultural areas into two separate portions of the zone. This allows for the best of both worlds: the high marsh has a footprint that better matches its original area, while some of the fertile soils can still be farmed in the new dykeland. The removal of the tidal barrier will allow the high marsh to migrate upland as sea levels rise, maintaining the balance between the zones.
Upper Border Road
The removal of the tidal barriers (dykes and undersized culverts) allows for tidal waters to reach their highest extent and freshwater to drain freely. The managed realignment has restored a portion of the original footprint of the upper border zone along the slope of the roadway. The vegetation here is salt spray tolerant and it can withstand the occasional storm surge.
The managed realignment has restored a portion of the shrubland zone’s natural extent while designating it as the marsh migration corridor. The removal of the tidal barrier ensures that the lower zones will be able to freely move upland as sea levels rise, maintaining the overall balance between them. The removal of invasive species and planting of native species such as sweet grass on the lower border of this zone can restore a more natural habitat for the animals within.
The residential upland has become smaller in association with the managed realignment project. This change of area has provided the landowners with natural protection from sea level rise while establishing their connection to the coast. The landowners themselves have decided to help by moving their mow lines upland and replanting trees that once grew around them.
Reefballs are concrete domes with many holes that provide space for algae and animals to grow.
These structures also help to prevent shoreline erosion and slow down the water that passes over and through them. As the water is slowed, sediment can settle around and behind the reefballs, building the area up and making it suitable for marsh grasses.
Oyster Reefs/ Shoreline Plantings
Oyster reef restoration reintroduces oysters to marshes and mudflats they once occupied. This is accomplished by filling bags with oyster shells fitted with spat (baby oysters) and staking them along the coast. As the oysters grow and reproduce, they are able to reform the reefs and help stabilize the flat. Oysters are filter feeders and can help improve the water quality in an area. Reefs can also be completed with a variety of molluscs, such as mussels.
Shoreline plantings reintroduce native grasses to the coastline. These grasses help stabilize the marsh with their root mats while slowing oncoming waves with their stalks. As sediment settles on these plantings, they can grow up through it and build the marsh up vertically.
Runnels are narrow, shallow channels dug into the ground that allow impounded surface water to drain. This practice is a response to landscapes where human modifications have created pools on the marsh itself. The elimination of standing water on the site allows for the marsh to revegetate and avoid becoming an open water system.
Sediment Nourishment Marsh Plantings
Sediment nourishment is the process of adding material (often dredge) to an area that has eroded or submerged. This is done to restore the elevation of a drowning marsh or an area experiencing panne expansion. The nourishment provides material for plantings while ensuring they are at the right spot in the tidal frame.
Marsh plantings reintroduce native grasses to the coastline. These grasses help stabilize the marsh with their root mats while slowing oncoming waves with their stalks. As sediment settles on these plantings, they can grow up through it, building the marsh up vertically.
Managed realignment is the process of realigning zones of the coastline to better match their natural footprints, while maintaining critical infrastructure and land uses of our coasts. This practice is a response to heavily modified coasts where flooding has been worsened by dyke systems, flood gates, and road networks. These are typically large-scale projects.
Tidal Barrier Removal
Tidal Barrier removal is the process of replacing a feature that blocks tidal flow with a feature that does not. There are many forms of tidal barriers and a variety of methods to remove them. Barriers come in the forms of dykes, roads, causeways, undersized bridges, and culverts. When removing a barrier, it is best to remove it completely when possible (e.g. complete removal of an abandoned dyke). Water crossing barriers should be replaced with large bridges or open bottom culverts that fully accommodate water flow.
Invasive Removal and Sweet Grass Plantings
Invasive removal is the process of removing invasive species from our coastal system. The act of removing these species frees space where native plants can be replanted to restore lost habitat.
Mow Line Migration
Mow line migration is the practice where landowners change the areas that they mow. Low-lying and wet coastal areas are allocated as no mow zones. This simple practice is effective in creating habitat and enhancing coastal resiliency. This practice also helps maintain the marsh migration corridor.
Tree planting is the act of planting trees. Tree cover helps reduce the speed of runoff, limits erosion, and allows more water to saturate into the soil. This restoration technique is important since a naturalized upland will cause less stress on the zones downstream.