The 2010 BP oil spill accelerated the loss of Louisiana’s delicate marshlands, which were already rapidly disappearing before the largest oil spill in U.S. history, a new study reports.

As the oil washed into the marshlands, it coated and smothered thick grasses at their edge. When the grass died, deep roots that held the soil together also died, leaving the shore banks of the marshlands to crumble, said Brian Silliman, the University of Florida researcher who led the new study.

“We already knew that erosion leads to permanent marsh loss, and now we know that oil can exacerbate it,” Silliman said.

In Louisiana’s Barataria Bay, oiled marshes eroded at about twice the rate of non-oiled marshes, receding nearly 10 feet per year, Silliman’s team found.

“Doubling the rate of erosion is a huge number,” said Zoe Hughes, a marsh researcher at Boston University who was not involved in the research. “It’s very significant in areas where you have erosion anyway.”

Silliman’s team arrived in Louisiana in July 2010, three months after the destruction of the Deepwater Horizon drilling rig.

The oil “looked like a thick black belt on the shoreline — it went on and on,” Silliman said. “You could see the grasses underneath the oil were dying and decaying.”

While the oil killed the grasses at the edge of the marshes, these same thick grasses blocked oil from seeping more than about 30 to 45 feet inland, the team found, limiting damage to the shoreline of the marshes.

Silliman’s team measured erosion at three marsh sites that were moderately to heavily oiled and compared those sites with three areas that were not oiled. About 45 linear miles of Louisiana marshes were moderately to heavily oiled during the spill.

The researchers found that the accelerated erosion lasted about 18 months. By then, grasses had regrown on exposed mud flats in the formerly oiled marshes, securing the edges from further sped-up erosion.

But the marshlands lost during the spill are gone for good.

“When you get edge erosion, it’s gone,” Hughes said. “It’s not coming back.”

Before the oil spill, Louisiana was losing marshlands at a rate of at least a football field a day, Silliman said. The Mississippi River, forced into a narrow channel, fails to deliver land-building silt to the marshlands as it once did. And, as the Gulf of Mexico rises, the entire foot of Louisiana also is sinking, further accelerating loss of the marshlands.

The marshes were “drowning” before the oil spill, said the University of Houston’s Steve Pennings, who has documented how the spill killed insects and spiders in the marshes.

While “locally important,” marsh loss caused by the spill was not “extraordinary” compared with losses from the other, more permanent factors at work, Pennings said.

Situated at the foot of the Mississippi River, Louisiana’s salt marshes are a “lifeblood” for the state, Silliman said. Shrimp, oysters, clams and fish hatch their young in the marshes, making the areas vital to the state’s fishing industry.

The marshes also act as a two-way buffer; they protect the mainland from storms and attendant storm surges while simultaneously soaking up fertilizers and other runoff from farms that can cause damaging algae blooms in the Gulf of Mexico.

The study was published Monday in the Proceedings of the National Academy of Sciences. It was funded by a $200,000 grant from a $500 million research fund BP set up after the spill.