Gulf Coast oil spill damage could be lasting
By The Washington Post | Tuesday, Sept. 27, 2016, 11:10 p.m.
The 2010 Deepwater Horizon oil spill has been called one of the worst environmental disasters in American history — and more than six years later, scientists are still investigating how much damage it actually caused. Now, a new study suggests the spill may have permanently marred one of the Gulf of Mexico shore's most important ecosystems.
The study, published Tuesday in the journal Scientific Reports, finds the oil spill caused widespread erosion in the salt marshes along the coasts of Louisiana, Alabama and Mississippi. And the researchers say there's a chance these marshes might never completely grow back.
Marshes “provide a variety of important services,” said lead study author Brian Silliman , a marine conservation biologist at Duke University. “They benefit humans, including acting as pollution filters, absorbing nutrients as they run off from the land before they get into the estuary, helping to suppress harmful algal blooms. They also act as breakwaters and buffer the shoreline from erosion.”
They're also important carbon sinks — in fact, research suggests that coastal wetlands may absorb several times more carbon per unit of area than tropical forests do. And they also provide habitat to a wide variety of animals that are staples of human fisheries.
So scientists have been concerned about how the marshes fared after the oil spill, believed to have affected at least 1,300 miles of shoreline from Texas to Florida.
For the new study, the researchers investigated the relationship between the amount of oil that ended up on marsh grasses following the spill and how much these marshes eroded in the years since. Specifically, they looked at the height of the grasses and how high on the stems oil was found at each location. They analyzed data taken from more than 100 sites in Louisiana, Alabama and Mississippi.
The researchers found that marshes with low or moderate levels of oiling did not erode any faster than usual. But in places where oil covered at least 90 percent of the plant stems, erosion rates accelerated significantly. The study suggested that erosion in these places was more than three times higher than in marshes with no oil at all., and on average was about 1.4 meters per year higher than normal. These elevated erosion rates persisted for up to two years following the spill.
The results suggest that marshes have a kind of threshold beyond which the effects of the oil begin to manifest themselves. And in the places that are hit the hardest, the effects can be significant.
“Identifying thresholds is something that ecologists and conservation biologists and policymakers are focused on in all ecosystems right now,” Silliman said. “Understanding that breaking point can help them in restoration, conservation. We know some of these systems will be able to take some disturbance, but that threshold — that's where it can break down and the losses can be permanent.”
And at some of the study sites, these changes may very well be irreversible, Silliman noted. Past a certain point of erosion, the edge of the marsh moves beyond the low-tide point, and it becomes difficult for the grasses to grow back.
Active restoration efforts — that is, physically restoring the sediments that were eroded away — could give the marshes a shot at returning. But otherwise, Silliman said, “the chance of natural recovery is very slim at best.”
The study “documents one of the largest declines in an ecosystem following the Deepwater Horizon oil spill,” Silliman said. But other research suggests that marsh grasses certainly aren't the only organisms that have suffered.
A report released by conservation organization Oceana earlier this year synthesized research pointing to a wide range of effects. Endangered sea turtles, bottlenose dolphins, coral reefs, shorebirds and all kinds of fish suffered increased mortality, developmental defects or reproductive declines as a result of exposure to the oil. And our understanding of these consequences may continue to grow, as there are still scientists conducting ongoing research on the changes along the Gulf shore.
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Deepwater Horizon Oil Spill Caused Widespread Marsh Erosion, Study Shows
September 27, 2016
Oil from the 2010 Deepwater Horizon spill in the Gulf of Mexico killed marsh grasses where it washed ashore in Louisiana’s Barataria Bay. (Credit: Duke Univ.)
DURHAM, N.C. -- The Deepwater Horizon oil spill six years ago 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 percent 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 heavily oiled sites were between 1.4 and 1.6 meters per year higher than scientists had expected, based on findings from similar areas that weren’t hit with oil.
The April 20, 2010 explosion on the Deepwater Horizon offshore oil rig killed 11 workers and pumped more than 100 million gallons of crude oil into the northern Gulf of Mexico, making it the worst environmental disaster of its kind in U.S. history.
“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,” said Brian R. Silliman, Rachel Carson Associate Professor of Marine Conservation Biology at Duke’s Nicholas School of the Environment, who led the new large-scale analysis.
The outlook is more optimistic for marshes where oil covered less than 90 percent of plant stems.
“In these marshes, erosion rates did not accelerate, likely reflecting less oil impact,” Silliman said. “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, published Sept. 27 in the peer-reviewed journal Nature Scientific Reports, 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 percent threshold because of the Deepwater Horizon spill.
“Our analysis reveals that accelerated erosion, and likely land loss, occurred in these areas,” Silliman said.
Most of the erosion occurred within one to two years after the spill, he noted. 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.
The new findings corroborate an earlier study led by Silliman that showed elevated erosion at a limited number of heavily oiled sites. They also support studies led by Mark Hester and Jonathan Willis at the University of Louisiana’s Institute for Coastal and Water Research, which found that there was widespread die-back of marsh plants at these sites.
“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 said. “By identifying the 90 percent threshold above which spill-induced erosion occurs, our study provides key knowledge to more accurately predict loss of marsh ecosystems following future spills.”
The study was conducted by scientists at Duke; Iowa State University; the University of California-Santa Cruz; the University of Louisiana at Lafayette; Abt Associates of Boulder, Colo.; and the Instituto de Investigaciones Marinas y Costeras of Mar del Plata, Argentina with funding from the State of Louisiana, the National Oceanic and Atmospheric Administration, British Petroleum, the National Science Foundation, the Stolarz Foundation, and the Gulf of Mexico Research Initiative.
CITATION: “Thresholds in Marsh Resilience to the Deepwater Horizon Oil Spill,” Brian R. Silliman, Philip M. Dixon, Cameron Wobus, Qiang He, Pedro Daleo, Brent B. Hughes, Jonathan M. Willis, Mark W. Hester. Nature Scientific Reports, Sept. 27, 2016. DOI: 10.1038/srep32520
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