The world’s coastal marshes provide a critical —and embattled—meeting place between the sea and the shore. This amphibious terrain, part land and part water, cleanses pollutants, calms beach-eating waves, and stores climate-warming carbon.
Today, rising seas threaten to drown these marshlands. Dams and levees deprive them of critical land-building sediments. Coastal Louisiana is Exhibit A for the value of coastal marshes, and the perils that come with losing them. There, entire coastal towns are disappearing as marshland erodes into the Gulf of Mexico.
Louisiana agencies are preparing to spend more than $2 billion to reroute part of the Mississippi River and its sediment to revitalize just a portion of the state’s coastal marshes.
But further east, along the Atlantic coast, scientists have been uncovering the marsh-building power of a little creature with a far smaller price tag: the humble mussel. The small, blue-black shellfish turns out to be a marsh-engineering marvel. “This study shows that small and innocuous animals that live within an ecosystem can play a critical role in helping coastal systems persist in the face of climate change,” said Sinéad Crotty, associate director of science at the Carbon Containment Lab at the Yale University School of the Environment.
Anyone who has cooked a bucket of mussels can testify to their sediment-collecting abilities. As filter feeders, these shellfish siphon streams of seawater, capturing bits of detritus as well as grit and sand. Sediment that might collect in the bottom of a kitchen bowl will, in the wild, settle around the mussels.
How much does this add to salt marshes where mussels live? Computer models built to understand how water and the land interact on the coast don’t include them. Crotty, along with scientists from Florida and the Netherlands, tried to fix this oversight and understand how big a deal mussels really are.
Researchers set their sights on the South Atlantic Bight, a coastal area stretching from North Carolina to Florida that is home to 3,000 square kilometers of salt marsh—the largest concentration in the U.S. There, ribbed mussels (Geukensia demissa) congregate in small colonies, or mounds, at the headwaters of creeks formed as tides push saltwater into channels running through the grasses.
They gauged the mussel deposits in a variety of ways. They measured the shellfish’s “footprint” by mixing fluorescent chalk with sediment gathered from mussel mounds, then returning at night to see how far it spread. They also isolated sections of mussels for a month and caught all the sediment they excreted.
Finally, in 2017 they went big at Sapelo Island, a barrier island on the Georgia coast that’s home to a national research reserve, as well the tiny town of Hog Hammock. There, they plucked 200,000 mussels from one tidal creek and transplanted them to a nearby creek. They tracked the elevation of the marshland around these two creeks. Over three years, the mussel-deprived marsh sank an average of 1.7 centimeters each year compared to a neighboring creek left unchanged. By contrast the marsh with the extra mussels gained nearly half a centimeter every year, the scientists reported in February in Nature Communications.
The mussel’s effect on marsh height exceeded expectations, contributing to new land at roughly five times the rate predicting by custom-made computer models and stretching well beyond the immediate mussel mounds.
Researchers suspect the mussels could be pumping in marshes in several ways. They suck capture sediments floating in the water that might otherwise flow back out to sea, gluing them to mucus-infused clumps that sink. The mussels also deposit nutrient-rich feces that can spur plant growth.
The results indicate that the presence of these shellfish should influence decisions about which marsh lands to protect, or how to restore them, said Crotty. “Given that mussels are disproportionately important in driving accretion and other ecosystem functions, we should prioritize their protection, or outplanting.”
Crotty, et. al. “Faunal engineering stimulates landscape scale accretion in southeastern US salt marshes.” Nature Communications. Feb. 16, 2023.
Photo: ©Chesapeake Bay Program via Flickr