National Study Shows How Marshes Are Changing with Rising Seas

A new study led by the National Estuarine Research Reserve System (NERRS) has found that as sea levels rise and other environmental conditions shift, marshes are undergoing significant changes. The study was funded by the NERRS Science Collaborative, which supports community-driven research to protect people and places on the coasts.

“For the first time, we can say with confidence backed by on-the-ground data that marshes around the nation are already impacted by sea level rise and not all in the same way,” says Christopher Peter, research coordinator at New Hampshire’s Great Bay Reserve and lead investigator for the study. “Understanding where, why and how these changes are happening is critical for coastal communities who rely on marshes for many benefits.”

Scientists from 20 Reserves and partnering Universities and state/federal agencies formed the National Marsh Synthesis Team in 2022. Together, they analyzed data from 85 marshes across 17 coastal states. They found that marshes exposed to faster rates of sea level rise and smaller tidal ranges are the most threatened, and those that are exposed to saltier waters closer to the coast are losing plant cover more quickly than those in more brackish waters.

With continued sea level rise and no room to move inland, marshes degrade and drown. As plants die and saltwater pushes farther inland, mudflats and ponding replace healthy marsh. This makes for weaker storm protection, less habitat for fish and wildlife, less carbon storage, and fewer natural filters for clean water, changes that make local communities more vulnerable to flooding, erosion, and economic loss.

While every estuary is different, the study revealed clear regional patterns. Marshes in the Northeast and Gulf of Mexico are degrading the fastest, while those on the West Coast appear least impacted for now. In the Southeast, responses are more variable, with mangroves moving into areas once dominated by marshes. Across all regions, one message is clear: as marshes change, so do the benefits they provide, from clean water and storm protection to wildlife habitat and shoreline stabilization.

At New Hampshire’s Great Bay Reserve, for example, Peter and his colleagues are seeing these dynamics play out in real time. 

“More frequent tidal flooding is turning low marshes into mudflats and high marshes into low marshes,” says Peter. “We’re losing plant diversity and critical habitat for species like the saltmarsh sparrow, along with the shoreline protection these marshes provide. That’s why we’re working with partners to restore them and help them persist as seas continue to rise.”

Alongside understanding how marshes are changing in the United States, the team has advanced the science and practice of marsh monitoring, creating resources and tools that can be used to study how tidal marshes are changing anywhere in the world. 

“Because Reserves around the country collect and analyze data in a uniform way, we can connect the dots and see trends that give all coastal communities knowledge they need to prepare for the future,” says Rebecca Roth, executive director of the National Estuarine Research Reserve Association (NERRA). “Collectively, Reserves are a unique living laboratory for advancing wetland science and a powerful incubator for new ideas and methods that can be shared nationally and internationally,”

To explore the results of this study, and to learn how marshes near you are responding to sea level rise, visit nerra.org.