Researchers find ‘tipping points’ in salt marsh survival

Marshes are as particular as goldilocks, Duke University researcher Anna E. Braswell says. If they never get flooded, they’ll turn into uplands. But if they get flooded too much, they’ll drown.

But climate change is bringing an expected rise in sea levels, putting these delicate ecosystems in peril.

“It’s really important to understand how they will survive with sea level rise,” said Braswell, a PhD graduate of Duke’s Nicholas School of the Environment and lead author of a study of coastal wetlands published in Ecosystems.

Braswell and co-author James B. Heffernan, assistant professor of ecosystem ecology and ecohydrology at the Nicholas School, analyzed Atlantic and Gulf Coast estuaries from Maine to Mexico to determine how much of each estuary was occupied by wetlands and what factors controlled their spread and their resilience to change. Then, they assessed what conditions support marsh survival.

The factors differed based on different scales. At the local level, the ability of coastal marshes to persist depended on the balance between erosion eating away at the edge of the marshes and vegetation stabilizing them.

But at a broader level, other factors played a role. The depth, size, shape and even latitude of an estuary became important predictors for their persistence. The shape and orientation of nearby coastlines and the depth of near-shore waters mattered, too. The amount of replenishing sediment being carried into the estuary by rivers or incoming tides was also a key indicator of marshes’ resilience to change.

“There are some places where we might see tipping points,” Braswell said, “where we might see marshes that don’t have conditions anymore that support their persistence. Going forward, those local changes might mean we can see local losses moving into bigger losses into the next century.”

The researchers analyzed each site at site at five different geographic scales, from individual tributaries to the adjacent coastline and watersheds, to search for patterns they might otherwise miss

“I think the thing that really surprised us was how clearly differentiated those factors were at the bigger scale,” Braswell said.

Coastal salt marshes provide a long list of ecosystem services that benefit humans, including shoreline protection, pollution filtration, flood prevention and carbon sequestration. They also provide nesting habitat and forage for a wide range of wildlife, from sharks to sea turtles to shorebirds.

“This is really fertile ground,” Braswell said, “when you think of the coastline for habitat and food resources and nurseries.”

Header Image: The costal marsh at Chassahowitzka National Wildlife Refuge, about 65 miles north of St. Petersburg, Florida, is home to a multitude of species. ©Joyce Kleen/USFWS