Brian Barton
Salt marshes are coastal wetlands that are regularly flooded by seawater and are found mainly in temperate regions around the world. They are highly productive ecosystems that provide a range of benefits to humans, such as flood control, water quality maintenance, carbon sequestration, and support for fisheries and tourism. However, natural conditions and human activities are causing salt marsh decline, with pollution being a significant contributor. Salt marshes are under pressure from various pollution sources, including stormwater runoff carrying pollutants like excess fertilizers, sediment, pet waste, litter, and chemicals, as well as direct sources like factory pipes and smoke stacks. The introduction of invasive species and the impacts of climate change, such as sea level rise, also pose threats to the health and survival of salt marshes. Understanding and addressing these pollution and environmental challenges are crucial for preserving the valuable ecosystem services that salt marshes provide.
| Characteristics | Values |
|---|---|
| Salt marsh loss | 561 square miles (1,453 square kilometers) over the past 20 years |
| Carbon emissions from salt marsh loss | 16.3 million metric tons per year |
| Contributors to salt marsh decline | Climate change, conversion to aquaculture, coastal erosion, eutrophication, drainage, mangrove encroachment, invasive species, urban sprawl, and severe storms |
| Salt marsh benefits | Buffer during storms and rising sea levels, flood control, water quality filtration, carbon sequestration, support for fisheries and tourism, highly productive ecosystems |
| Pollution sources | Stormwater runoff, fertilizers, pesticides, pet waste, litter, excess freshwater, petroleum products, chemicals, nutrients, fecal bacteria, sediments |
| Invasive species | Tiger shrimp, macroalgae Gracilaria, reed Phragmites |
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What You'll Learn
Stormwater runoff
The high volume and velocity of stormwater runoff can cause erosion in salt marshes, and the pollutants it carries can degrade water quality and harm the health of the marsh. This form of non-point source pollution is a significant threat to the biodiversity of salt marshes, which are considered one of the most biologically productive ecosystems on Earth. Salt marshes provide vital habitat and nurseries for many coastal plant, animal, and bird species, as well as recreation areas for humans.
The loss of salt marshes due to stormwater runoff and other factors has led to the release of significant amounts of carbon dioxide and other greenhouse gases into the atmosphere. A NASA-led study found that the world lost 561 square miles (1,453 square kilometers) of salt marshes over a 20-year period, resulting in estimated net global emissions of 16.3 Terragrams of carbon dioxide across the study period. This is equivalent to the annual emissions of approximately 3.5 million motor vehicles.
To address the issue of stormwater runoff, best management practices such as rain gardens, rain barrels, vegetative buffers, and native plant landscaping can be implemented to slow down water flow and allow for on-site infiltration. These practices can help reduce the amount of runoff that enters salt marshes and mitigate the negative impacts on these valuable ecosystems.
Additionally, public education programs have been initiated to raise awareness about the issue and promote simple steps that communities can take to reduce the amount of impervious surface and pollution entering salt marsh and tidal creek systems.
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Nonpoint source pollution
In the context of salt marshes, stormwater runoff is a primary contributor to NPS pollution. As more people move to coastal areas, the development of roads, bridges, rooftops, and other impervious surfaces increases. This urban sprawl prevents rainfall from slowly filtering into the soil. Instead, it quickly funnels stormwater, laden with pollutants, into salt marshes.
Stormwater runoff carries a range of pollutants, including excess fertilizers, sediments, pet waste, litter, nutrients, chemicals like gasoline and oil, and pathogens like fecal bacteria. These pollutants degrade water quality and harm the health of salt marshes. The sheer volume of freshwater entering salt marshes through runoff can also disrupt the natural balance of these ecosystems.
The impacts of NPS pollution on salt marshes are concerning. Salt marshes are highly productive ecosystems that provide essential services, such as flood control, water filtration, carbon sequestration, and support for fisheries and tourism. NPS pollution can disrupt the delicate balance of these ecosystems, leading to the decline of salt marshes and the loss of these vital services.
Addressing NPS pollution requires collective efforts. Residents in areas surrounding salt marshes can play a crucial role in mitigating NPS pollution by adopting responsible practices, such as reducing the use of fertilizers and pesticides, properly disposing of litter and pet waste, and supporting initiatives that promote low-impact development and runoff management. Additionally, public education programs can empower communities to make informed choices that reduce the introduction of chemicals, nutrients, and pathogens into salt marsh ecosystems.
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Climate change
Salt marshes are coastal wetlands that are flooded and drained by saltwater brought in by the tides. They are highly effective carbon sinks, absorbing carbon dioxide from the atmosphere and locking it into the ground. Salt marshes also provide coastal protection from flooding and storms, and they serve as habitats for many plant, bird, and marine species.
However, salt marshes are vulnerable to the impacts of climate change, particularly sea-level rise. When excess carbon dioxide is released into the atmosphere from burning fossil fuels, it causes the atmosphere to warm, leading to the melting of polar ice caps and the expansion of ocean water, resulting in rising sea levels. An increase of even a few inches in sea level can significantly impact the distribution of plants and animals in salt marshes. For the salt marsh to survive, sediment accumulation and Spartina growth must occur at the same pace as or faster than the rising sea level. If the marsh surface cannot keep up, Spartina and other marsh plants will drown, and the salt marsh will convert to tidal flats.
Human activities have also impacted the resilience of salt marshes to climate change. Nutrient and pollution loading from agriculture and sewage, invasive species, and tidal restrictions can reduce the ability of salt marshes to cope with rising sea levels and temperature changes. Furthermore, coastal development and urban sprawl have led to the destruction of salt marshes for infrastructure and agricultural purposes, reducing the overall area of salt marshes and their capacity to sequester carbon.
The inclusion of blue carbon ecosystems, such as salt marshes, in international policies and agreements is critical for addressing climate change and achieving emission reduction targets. The protection and conservation of salt marshes are essential not only for their carbon sequestration potential but also for the ecosystem services they provide, such as coastal protection and habitat provision for numerous species.
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Coastal development
Salt marshes are coastal vegetated wetlands generally covered by two species of cordgrasses and are made up of two distinct habitats: high and low marsh areas. Salt marshes are highly productive ecosystems, and they can help reduce wave erosion on sea walls designed to protect low-lying areas from wave erosion.
Salt marshes have historically been perceived as wastelands, leading to their loss and change through land reclamation for agriculture, urban development, salt production, and recreation. In the past century, the conversion of marshland to upland for agriculture has been overshadowed by conversion for urban development. Coastal cities worldwide have encroached onto former salt marshes, and in the US, the growth of cities has looked to salt marshes for waste disposal sites. Estuarine pollution from organic, inorganic, and toxic substances from urban development or industrialization is a worldwide problem, and the sediment in salt marshes may fix this pollution, causing toxic effects on floral and faunal species.
The natural beauty of the salt marsh-tidal creek ecosystem is a major factor attracting people to the coasts. As the population continues to grow, it becomes increasingly important to protect these ecosystems. However, coastal development in the Southeast is consuming forests at a rate much faster than the human population is growing. This pattern of development, called urban sprawl, creates communities with large amounts of paved areas, such as roads and parking lots. These impervious surfaces prevent rainfall from filtering slowly into the soil and instead flush it quickly into marshes, causing stormwater runoff, or non-point source pollution. This runoff contains nutrients, chemicals like gasoline and oil, pathogens like fecal bacteria, and sediments.
Salt marshes in Rhode Island are already showing evidence of being overwhelmed by rising sea levels. In many places along the Massachusetts coast, salt marsh migration may be restricted due to a lack of adjacent inland or upland habitat due to coastal development. In some areas, marshes are still rising or "rebounding" after the retreat of ice sheets from the last ice age, a process that further combats sea level rise. While salt marshes are capable of adapting to rising sea levels, it is unclear if they will be able to keep up with future rates of change.
In the 1970s, Federal and State governments began to develop and implement legislation to protect the remaining coastal wetlands, regulating activities that could occur in the salt marsh-tidal creek ecosystem. Communities are starting to encourage low-impact development best management practices like pervious concrete and upland vegetative buffers that filter larger amounts of runoff through soils and vegetation to sequester pollutants.
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Invasive species
Salt marshes are particularly vulnerable to invasive species due to their role as a habitat and nursery for many organisms. Several invasive species have been identified in salt marshes, including the tiger shrimp, the macroalgae Gracilaria, and the reed Phragmites australis.
Gracilaria has invaded mud flats in the Southeast that have not historically supported significant amounts of algae. It has changed the mud flat habitat by providing vertical structure, altering species compositions, and affecting oyster growth. It is not yet known how Gracilaria will further alter the salt marsh and tidal creek ecosystem in the future.
Phragmites australis is a tall reed that can reach heights of 10 to 12 feet (3-3.5m) and is native to Europe and Asia, where it is valued as an important wetland species. In the US, there are three separate subspecies of Phragmites australis, two of which are native and one is an invasive subspecies from Europe, likely introduced through ballast water. The invasive subspecies is more common on disturbed habitats and forms denser stands than the native species, reducing plant diversity. It can shade out shorter native plants and decrease the quality of the salt marsh habitat, altering the natural function of the ecosystem.
Smooth cordgrass (Spartina alterniflora), a plant originally introduced to stabilize the shoreline, has become invasive in San Francisco Bay Marsh ecosystems. It cross-breeds with native California cordgrass, producing "super hybrids" that grow taller and denser and can survive at lower and higher points in the intertidal zone. These hybrids could fill mudflats used by shorebirds, reducing their foraging areas and altering the food web for small creatures in the broader salt marsh ecosystem.
Despite the negative impacts of invasive species, it is important to recognize that their effects can be complex and context-dependent. In some cases, invasive species may provide benefits that are overlooked. For example, Phragmites provides support for many birds and other land animals, and it has been suggested that removing this species, especially along vulnerable coastlines, may be expensive and unnecessary.
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Frequently asked questions
Salt marshes are under threat from human activities and climate change. A recent NASA-led study found that the world lost 561 square miles of salt marsh between 2000 and 2019, with the US and Russia accounting for 64% of the total loss. Salt marshes are vital ecosystems that provide a range of benefits to humans, including flood control, water quality maintenance, carbon sequestration, and support for fisheries. However, they are vulnerable to pollution from sources such as stormwater runoff, which carries pollutants like excess fertilizers, sediment, pet waste, and litter into the marsh.
One of the main sources of pollution in salt marshes is stormwater runoff, which occurs when increased urbanization leads to more impervious surfaces, such as roads, parking lots, and roofs, that prevent rainwater from filtering slowly into the soil. Instead, the water flushes quickly into the marshes, carrying pollutants such as nutrients, chemicals, pathogens, and sediments. Pollution can also come from direct sources, such as factory pipes or smoke stacks, or from indirect sources like farms and lawns, which contribute pesticides and fertilizers that degrade water quality.
Pollution has several negative impacts on salt marshes. It can disrupt the marsh food web, killing off some species and causing population explosions of others. It can also lead to the accumulation of toxins in the water, soil, and sediments, which can poison animals and contaminate seafood. In addition, pollution can contribute to the decline and die-off of salt marshes, leaving dead or dying patches. These impacts can have economic consequences for industries such as commercial and recreational fisheries, tourism, and culinary arts, which depend on healthy salt marsh ecosystems.
