The sun sets over the wetlands in Louisiana’s Barataria Bay. (Mario Tama/Getty Images)

Six years after the Deepwater Horizon oil spill devastated the shore of the Gulf of Mexico, scientists are still taking stock of the damage it caused. And increasingly, they’re reporting that widespread shoreline erosion and loss of wetlands — which can hurt important salt marsh ecosystems and leave coastal areas, and the city of New Orleans, more vulnerable to sea-level rise — was a major side-effect of the disaster.

A new study, published Thursday in the journal Geophysical Research Letters, reports extensive shoreline recession in the Mississippi River Delta as a result of the oil spill — and it finds that the spill’s impact was even more widespread than the erosion caused by Hurricane Isaac two years later.  

“Erosion is occurring [even] without the oil spill for lots of different reasons throughout the delta,” said Elijah Ramsey III, a research oceanographer with the U.S. Geological Survey who authored the paper along with Amina Rangoonwala of the USGS and Cathleen Jones of NASA’s Jet Propulsion Laboratory.

For decades, shoreline loss in the region has been linked to such factors as sea level rise, damming of the Mississippi River upstream (preventing it from delivering as much sediment to the delta as it once did), and oil and gas extraction. But the researchers wanted to see how these baseline erosion patterns may have changed following the Deepwater Horizon spill in 2010 and the impact of Hurricane Isaac in 2012.

The researchers focused on upper Barataria Bay, off the coast of Louisiana on the western edge of the Mississippi River Delta, which is characterized by marshland and numerous small islands. To document the recent changes it has undergone, the researchers relied on aerial radar data collected by NASA between 2009 and 2012.

Prior to the oil spill, the researchers observed that there was, indeed, some erosion occurring in isolated areas along the shoreline, which is made up of sensitive coastal wetlands composed of grasses, shallow waters and small islands. But these patterns dramatically changed in 2010. Following the spill, the researchers observed widespread erosion throughout the entire study area, in many cases leading to the fragmentation and even near-destruction of islands in the area. Most of the affected areas retreated 4 to 8 meters, although in the most severe cases, the researchers saw shorelines receding by 12 meters or more.

In areas that experienced the heaviest oiling, shoreline recession was most severe and often began to occur almost immediately, while some less severely impacted areas exhibited a slightly delayed response. Over the next two years, shoreline losses tended to remain highest in areas that had experienced the most severe oiling and the highest amounts of shoreline recession, even while the losses declined in places that had been hit less severely. This makes sense, given what scientists know about the way oil affects vegetation in coastal areas.

“What happens is the oiling weakens the roots of the vegetation [along the shore],” said Rangoonwala, a geophysicist with the U.S. Geological Survey and the paper’s lead author. Naturally, this effect will be more severe the more oil is present. These roots are what hold the soil in place to begin with, so after they’re lost, the marshland tends to fall apart more easily.

The study “shows that the shoreline erosion is connected to the severity of oiling,” Ramsey said. Other recent studies have come to similar conclusions. A recent paper, which focused on the shorelines of Louisiana, Alabama and Mississippi, found that marshes experiencing the most severe oiling after the Deepwater Horizon spill saw significantly accelerated erosion rates — up to 1.4 meters per year higher than normal.

Notably, the erosion experienced by the upper Barataria Bay was more widespread after the oil spill than after Hurricane Isaac, although the hurricane’s effects were actually more severe in the locations where they were observed. Overall, the spatial erosion patterns that occurred after the hurricane were more similar to the baseline patterns observed prior to the oil spill, the researchers note.  

This also makes sense: The impact of hurricanes and other storms is likely to be most severe in areas that are exposed to the most wave action, while parts of the bay that are more shielded will experience much less damage.

So the study suggests that, while disasters like hurricanes may cause more severe damage to the shoreline in localized areas, the impact of an oil spill may actually be more widespread along the coast. And this is a big concern for coastal communities that depend on an intact shoreline for protection against flooding.

Indeed, barrier islands and salt marshes provide a key defense against sea-level rise for coastal communities, helping to buffer the mainland against erosion. They’re also important in other ways, filtering pollutants and providing habitat for shellfish and other staples of human fisheries.

Unfortunately, marsh degradation is irreversible in many cases. This means the effects of disasters like the Deepwater Horizon spill can leave a permanent scar on the landscape. This is especially worrying in a time when concerns about rising sea levels worldwide — accelerated by the warming climate — are growing.

The coast of Louisiana is one of the U.S. regions expected to be hit hardest by sea-level rise. Earlier this year, residents of the state’s Isle de Jean Charles garnered national attention as the nation’s first “climate refugees” after receiving a $48 million grant from the federal government to be used for relocation. The island, which is populated mostly by members of the Biloxi-Chitimacha-Choctaw tribe, is steadily sinking.  

The new study suggests that disasters like oil spills may only weaken coastal communities’ resilience to the encroaching sea by wearing away their already vulnerable shorelines. But the findings may also help inform policymakers about what to expect if another spill occurs, suggesting that the effects of oil spills are both long-lasting and potentially even more extensive than those of other disasters common on the coast.