Is Salt Marsh Expanding Northward?

A new study of 166 estuaries on all three U.S. continental coastlines, led by the U.S. Geological Survey, is the first to quantify and compare how rapid sea-level rise will drive landward migration of coastal wetlands on the Pacific, Atlantic, and Gulf coasts in the coming decades.

The findings illustrate how the movement of coastal wetlands – which are among the world’s most valued ecosystems – will drive ecological shifts under accelerated sea-level rise. The seaward edges of coastal saline or brackish marshes and mangrove forests are expected to transform to open water while their landward edges encroach upon freshwater wetlands and a variety of valuable landscapes farther upland. Understanding the transformative impacts of coastal wetland migration into adjacent ecosystems can help coastal managers sustain biodiversity and the ecological and societal benefits provided by coastal ecosystems in the face of rising sea levels.

“The potential landward migration of our saline coastal wetlands is expected to occur at the expense of valuable freshwater wetlands and upland ecosystems,” says Michael Osland, a USGS research ecologist with the Wetland and Aquatic Research Center. “Our findings provide awareness of the impacts and ecological threats we are up against and allow us to help land managers better anticipate and prepare for ecological losses and transformations due to rising seas and migrating wetlands.”

To adapt to rising sea levels, coastal wetlands can migrate landward at the expense of adjacent freshwater wetlands and upland ecosystems, but migration can be hindered by natural and anthropogenic barriers. This photo shows marsh migration into an upland forest in Maryland.

The study integrated data from 166 estuaries across the conterminous United States, including Washington D.C. and 22 coastal states along the Pacific, Atlantic and Gulf coasts. The study found that coastal wetlands will migrate landward, transforming coastlines but not compensating for the area expected to be lost on the seaward side. The study found that:

Two-thirds of potential migration is expected to occur at the expense of coastal freshwater wetlands.

One-third of the potential migration is expected to occur at the expense of valuable uplands, including croplands, forests, pastures, and grasslands.

The risk of landscape-scale wetland loss is high along the Gulf of Mexico and south Atlantic coasts, with hotspots on the Mississippi River Delta, Everglades, Albemarle-Pamlico and Chesapeake Bay estuaries.

Of the total potential wetland loss, 79% is expected to occur along the coasts of Louisiana, Florida, North Carolina, Texas and South Carolina.

By providing a look at the ecological shifts and transformative impacts of coastal wetland migration, the findings provide insights that decision-makers can use to help sustain and preserve coastal ecosystems.

The full study, “Migration and transformation of coastal wetlands in response to rising seas,” is available in the journal Science Advances.