Some coastal salt marshes are keeping pace with sea level rise

The world’s coastal salt marshes serve as crucial carbon storage hotspots, building up sediments and plant material to maintain their elevation above sea level. However, as sea levels rise at an increasing rate, the scientific community is divided on whether these wetlands can continue to win the race against rising waters. 

A new study has now shed light on how salt marshes along the U.S. East Coast have responded to accelerating sea level rise by building elevation more quickly over the last century.

Published in the AGU journal Earth’s Future, the study examined soil accretion rates across nine salt marshes from Maine to Georgia using soil cores that provide over 100 years of marsh history. The researchers dated the soil to calculate how quickly the wetlands grew over time and how those rates changed. 

According to Nathaniel Weston, an ecosystems ecologist at Villanova University who led the study, this research is the first to document what’s happening on this scale.

Two primary factors determine how quickly a salt marsh accumulates soil: the amount of sediment deposited during tidal floods and the amount of organic matter from the marsh’s plants that escape decomposition.

Human activities, such as building dams or clearing land for agriculture, can disrupt sediment supply to marshes. Additionally, cooler temperatures facilitate the accumulation of more organic matter.

A coastal wetland can keep pace with rising waters if sea level rise is slow, sediment is abundant, and vegetation thrives. However, if the sea level rises too rapidly, this delicate balance is lost. “These feedback cycles allow a marsh to keep up and increase its rate of soil accretion up until a point where it just can’t,” said Weston. “After that, it falls off the other end and can no longer exist as a marsh. And it’s very likely that will happen in many places.”

While soil accretion rates increased in all nine marshes studied, only six managed to build soil at roughly the same rate as sea level rise over the past century. Three marshes in North Carolina, South Carolina, and Virginia fell behind due to rapid sea level rise, low sediment supplies from damming, and warmer temperatures. Weston suggests that many coastal salt marshes worldwide might be accelerating their growth for now, but the future remains uncertain.

Molly Keogh, a coastal geologist at the University of Oregon not involved in the study, expressed concern about the future of these wetlands. 

“There’s got to be a tipping point where wetlands simply can’t keep up with these increasingly fast rates of sea level rise,” said Keogh. “There are places, like the Mississippi Delta, that have already reached these tipping points. At this point, the drowning of [coastal] wetlands is essentially inevitable.” 

Slowing sea level rise is essential but poses an immense and slow-moving challenge that might not come quickly enough to save salt marshes.

To mitigate inundation, communities could implement “living shorelines” of vegetation to retain sediment, said Weston. A more expensive and temporary solution involves spraying a slurry of mud and water over the marsh, which adds a few millimeters of sediment and can offset sea level rise for a few years. However, the long-term survival of these critical ecosystems ultimately depends on finding ways to slow down the rising sea levels.

Coastal salt marshes are important for several reasons, as they provide a variety of ecological, economic, and protective functions:

1. Carbon storage: Salt marshes are highly effective in capturing and storing atmospheric carbon dioxide, a greenhouse gas that contributes to climate change. They sequester carbon in the form of plant biomass and sediment, making them essential for mitigating the impacts of climate change.
2. Biodiversity: Salt marshes are rich, productive ecosystems that support a wide range of plant and animal species, including birds, fish, crustaceans, and other marine life. They serve as critical habitats, nesting grounds, and nurseries for many species, contributing to the overall health and diversity of coastal environments.
3. Water quality: Salt marshes act as natural filters for pollutants, absorbing and breaking down contaminants that enter the ecosystem from land runoff and industrial discharges. By improving water quality, they protect the health of marine organisms and maintain the ecological balance of coastal waters.
4. Shoreline protection: Salt marshes serve as natural barriers against coastal erosion and storm surges, as their dense vegetation helps to dissipate wave energy and trap sediments. This function is particularly important in mitigating the impacts of sea-level rise and protecting coastal communities from flooding and storm damage.
5. Economic value: Coastal salt marshes contribute to local economies by supporting commercial and recreational fisheries, as well as tourism-related activities such as birdwatching, boating, and nature photography. The ecosystem services they provide, such as water filtration and shoreline protection, also have significant economic value.
6. Education and research: Salt marshes offer unique opportunities for scientific research, environmental monitoring, and educational programs, helping to advance our understanding of coastal ecosystems and their response to environmental changes.

Coastal salt marshes are vital ecosystems that play a crucial role in maintaining the ecological balance, protecting coastal communities, and supporting local economies. Their conservation is essential for the long-term health and resilience of our coastal environments.

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