The 2010 Deepwater Horizon oil spill in the Gulf of Mexico caused widespread marsh erosion that may be permanent in some places, according to a new Duke University-led analysis of 270 miles of the Louisiana, Mississippi and Alabama coasts.

At the hardest hit of 103 Natural Resource Damage Assessment (NRDA) sites, where oil covered more than 90% of plants’ stems, widespread die-off of grasses at the marsh edge occurred, followed by up to two years of accelerated erosion as dying plant roots lost their grip on marsh soil. Erosion rates at these sites were 1.4-1.6 meters higher per year than scientists had expected, based on findings from similar areas that were not hit with oil.

Where oil covered more than 90% of plants’ stems, widespread die-off of grasses at the marsh edge occurred. Image credit: NOAA. “Marshes that experienced elevated erosion due to high levels of oiling didn’t recover; they’re now gone, having been converted to mudflats in the shallow underwater environment of the Gulf,” says Brian Silliman, associate professor of marine conservation biology.

The outlook is more optimistic for marshes where oil covered less than 90% of plant stems.

“In these marshes, erosion rates did not accelerate, likely reflecting less oil impact,” Silliman says. “So long as the marsh platform elevation is sufficient and the rooting system of the vegetation is healthy and intact, these marshes could potentially recover over time.”

The team’s research is the first empirical study to identify the threshold at which spill-induced marsh erosion occurred across a large geographic area. They used data collected as part of the NRDA, which was conducted by state and federal Natural Resource Trustees in the aftermath of the Deepwater Horizon spill.

Various sources have estimated that between 60 and 100 linear kilometers of salt marsh experienced plant stem oiling above the 90% threshold because of the Deepwater Horizon spill.

Most of the erosion occurred within one to two years after the spill, he notes. Once the erosion front— created by the black belt of oil that layered over and killed grass on the marsh edge—ran into healthy vegetation farther back, the land loss due to erosion slowed.

“Given the vital roles coastal salt marshes play in protecting shorelines from erosion and flooding, providing habitat for wildlife and helping clean our water, scientists need to understand the thresholds of salt marsh resilience to human disturbances like oil,” Silliman says. “By identifying the 90% threshold above which spill-induced erosion occurs, our study provides key knowledge to more accurately predict loss of marsh ecosystems following future spills.”