The salt marshes of Cape Cod are as distinctive as they are important. These beautiful lowland wetlands are some of the most biologically productive ecosystems on Earth. They play a large role in the nitrogen cycle, act as carbon sinks, protect coastal development from storm surges, and provide critical habitats and nurseries for many fish, shellfish, and shorebirds.
Now, according to new research from the Marine Biology Laboratory (MBL), more than 90 percent of the world’s salt marshes are likely to be underwater by the end of the century.
The paper has been published in the journal macroenvironmental science.
The results came from a 50-year study at Great Sippewissett Marsh in Falmouth, Massachusetts. Since 1971, scientists from the MBL Center for Ecosystems have been mapping vegetation in experimental plots in this bog to examine whether increased nitrogen in the environment would affect bog grass species. Due to the length of the study, they were also able to detect the effects of climate change on the ecosystem, especially those resulting from accelerated sea level rise.
The researchers found that increased nitrogen favors higher levels of vegetation and bog surface build-up, but no matter what nitrogen concentration they apply to swamps, these ecosystems will not be able to outpace the inundation caused by global sea level rise.
“Places like the Great Sippewissett Marsh are likely to become shallow inlets by the end of the century,” says distinguished MBL scientist Evan Valleella, lead author of the study. “Even with conservative estimates of sea level…more than 90% of the world’s salt marshes are likely to be inundated and disappear or dwindle by the end of the century.”
“This isn’t a prediction from isolated scientists worried about a few details,” Valleella says. “There will be major changes on Earth’s surface that will alter the nature of coastal environments.”
Environmental system engineer
Salt marshes are gently sloping ecosystems and their plants have very narrow preferences for the heights at which they can grow. Different species grow at higher elevations (high bogs) versus lower elevations near the ocean (lower bogs) and have different responses to changes in nitrogen supply. When the change happens slowly enough, grasses can migrate to their preferred height.
In low-lying swamps, ropegrass (Spartina alterniflora) thrived as scientists increased the nitrogen supply. Among the high bog species, the abundance of bog bunting (Spartina patens) decreased in experimental plots with sea level rise. Saltgrass (Distichlis spicata) increased with the nitrogen supply and also acted as what the researchers called an “ecosystem engineer”—increasing the rate of bog upwelling. The accumulation of biomass left by the decomposing saltgrass offset the increased inundation caused by sea level rise in these areas.
“Saltgrass disappeared after a few decades, but it left a legacy behind,” says MBL research scientist Javier Lloret, adding that “it was very cool to see this interaction in the dataset.”
No matter how much nitrogen was added to the environment, the research showed that under projected current and future sea-level rise, low bog species would completely replace high bog species. As sea levels continue to rise, even these species will be submerged.
“At some point, if the sea level continues to rise at the rates we expect, there will be no more room for the low-lying bog plants. They will just be too waterlogged to survive,” Valleella says.
The only alternative is for the salt marshes to migrate inland.
Marshes around the world are facing what Loret calls “coastal pressure,” as sea level rise pushes in one direction and human development kicks in the other. A sea wall that may protect the house from flooding will prevent swamps from naturally draining to higher ground.
“These barriers, whether geographic barriers such as a hill or cliff, or people building along the edges of the ecosystem, constrain the potential for inland migration of swamps,” says MBL research assistant Kelsey Chenoweth. Moreover, sea level rise is accelerating and the marshes are having a hard time keeping up.
In a sea-level rise scenario like the one we’re facing, “the only solution for plants is to colonize new areas, to go higher,” says Laurette. But this migration may be impossible in some places ».
“Sea level rise is the most important threat to salt marshes. We really need to figure out what will happen to these ecosystems and figure out how to prevent some of the losses or try to adapt to them, so that the marshes can continue to play these important roles for nature as well as people,” says Lloret. .
Half a century of science
In 1971, scientists at the MBL Center for Ecosystems had no idea they would use their data to study global sea level rise.
“This was an experiment that started by looking at one environmental control (nitrogen), and then because of the longevity of the project, we were able to add new knowledge about this major accelerating agent of global change – global sea level rise,” says Valellala.
This is the benefit of long-term datasets like the one at the Great Sippewissett Marsh.
“You’re laying a foundation for problems that haven’t happened yet,” Chenoweth says.
When measuring environmental processes such as climate change and eutrophication, the data can ebb and flow over the years as the ecosystem responds to external stimuli. The changes operate on a much longer time scale than changes in other biological systems.
“To study a tree, you look at the changes through the seasons and you have to be able to see its full cycle. For a leaf, you look at the patterns between day and night. In single cells, you look at processes that occur on the time scale of minutes or seconds…but for For the entire ecosystem, we’re talking about many years or decades,” says Lloret. “You need to think on a scale of decades or even centuries to see fundamental changes.”
This study includes co-authors from the University of Massachusetts-Dartmouth and the Woods Hole Oceanographic Institution.
I. Valiela et al, Salt marsh vegetation change during half a century of experimental nutrient addition and climate-driven controls in the Great Sippewissett Marsh, macroenvironmental science (2023). DOI: 10.1016/j.scitotenv.2023.161546
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