The 2010 BP oil spill hastened the loss of Louisiana’s already fragile salt marshlands, a new study says. In a comparison of erosion rates at three healthy marsh sites and three areas affected by the oil spill, University of Florida scientists found that oil from the spill coated thick grasses on the outer edge of some wetlands, killing off salt marsh plants 15 to 30 feet from the shoreline. When those grasses died, the deep roots that held the soil sediment died as well, causing the rate of erosion on shore banks to more than double. In Louisiana’s Barataria Bay, for instance, oiled marshes have receded nearly 10 feet per year after the spill — about twice the normal rate of erosion in a region already losing huge areas of marshland as a result of channelization of the Mississippi River and rising sea levels. “We already knew that erosion leads to permanent marsh loss, and now we know that oil can exacerbate it,” said Brian Silliman, a University of Florida biologist and lead author of the study, published in the Proceedings of the National Academy of Sciences.