Study: Oil from Deepwater Horizon disaster killed marsh plants, accelerated coastal erosion

March ecosystems also helped prevent more extensive damage by blocking oil

The oil slick from the 2010 Deepwater Horizon disaster rotates slowly in the Gulf of Mexico.

By Summit Voice

SUMMIT COUNTY — Oil from the 2010 Deepwater Horizon oil disaster killed off salt marsh plants up to 30 feet away from the shoreline along the Gulf Coast, resulting in a doubling erosion rates along some beaches in the area, to more than 10 feet per year of lost shoreline.

“Louisiana is already losing about a football field worth of wetlands every hour, and that was before the spill,” said Brian Silliman, a University of Florida biologist and lead author of a new study that examined the oil spill impacts.

“When grasses die from heavy oiling, their roots, that hold the marsh sediment together, also often die. By killing grasses on the marsh shoreline, the spill pushed erosion rates on the marsh edge to more than double what they were before. Because Louisiana was already experiencing significant erosive marsh loss due to the channelization of the Mississippi, this is a big example of how multiple human stressors can have additive effects,” Silliman said.

Marshes are the life’s blood of coastal Louisiana because they act as critical nurseries for the shrimp, oysters and fish produced in these waters while helping to sequester significant amounts of carbon. They also protect coastlines from flooding and guard estuarine waters from nutrient pollution.

But the marshes have been suffering for decades as a result of the channelization of the Mississippi River, which has starved them from needed sediments to deter erosion.

Then came the oil spill.

Researchers observed minimal oil on the surfaces of grasses located more than 45 feet from the shoreline, indicating that significant amounts of oil did not move into interior marshes.

Instead, the researchers found that the tall grasses along the marsh edge acted as wall-like trap to incoming oil slicks, concentrating oil on the marsh edge. This concentration of oil on the shoreline protected interior marshes from oiling but worsened already extreme erosion on the shoreline. As oiled plants died, their roots that hold tight to the sediment perished as well. Already eroding sediment was now exposed to wave action without the effect of the gripping plant roots.

The encouraging results, Silliman said, included significant declines in the oil concentration on the marsh surface over 1.5 years and that unaffected, healthy marsh plants in the marsh interior quickly grew back into marsh die-off areas that had not yet been lost due to heightened erosion.

When the new marsh plant growth grew into the erosive edge of the marsh, Silliman said, the recolonization of the area by the gripping plant roots shut down the oil-elevated erosion rates and returned them to those seen at marsh sites where oil coverage did not occur.

The researchers also found that polyaromatic hydrocarbons, or PAHs, a carcinogenic byproduct of oil, was 100 percent greater at the Barateria Bay testing site than in reference marshes. This finding provides chemical evidence to support their visual observations that marshes in the affected areas were laden with oil while those in reference areas did not receive significant oiling.

By adding Biochar, a charcoal-based substance, to marshlands, Silliman’s team is also using new bioremediation tactics to try to break down PAHs into organic material. If this method is successful, he said, it could be used to supplement naturally occurring microbes in the marsh mud that already oxidize the oil carcinogen. The team is soon to publish those findings.

“This is a new idea applied toward cleaning up PAHs,” said UF chemistry professor Andrew R. Zimmerman, a co-author on the paper. “It’s possible there’s a bunch lurking at the bottom of the bay.”

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