Erosion rate tapered off over time
Heavily oiled areas in Barataria Bay showed twice the normal land erosion rates in the year and a half after the Deepwater Horizon/BP oil leak, according to a new scientific study released Monday.
The study, published by the Proceedings of the National Academy of Sciences, found that oil coverage killed marsh plants, which led to the higher rates of erosion, but also said the rate of erosion has since returned to more normal levels.
Brian Silliman, associate professor of biology at the University of Florida and lead author of the study, said the normal rate of 5 feet per year of erosion in the bay shoreline doubled to 10 feet per year. However, after a year and a half, healthy marsh started infilling into the shoreline again, and as soon as the roots took hold, erosion rates went back to pre-leak rates.
“It was a temporary event,” he said.
The increased erosion rates are caused when significant oiling covered the marsh grass and the rhizomes, or underground stems that shoot out roots, died. It took between 70 to 80 percent coverage of a stem of marsh grass to actually kill it, Silliman said.
When oil got into the soil, it’s likely the oil’s toxicity also killed the rhizomes underground, he said.
“Once those rhizomes die, those gripping roots are not holding together the sediment anymore,” Silliman said, and that accelerated land loss for a period of time.
The marshes were already eroding before the spill because they are starved of sediment, but the oil spill brought additional stress that amplified the impact, he said.
“Relative to the land loss over 50 years it’s (the oil-related erosion) relatively small,” Silliman said.
In addition, the study found that most of the oiling was kept to the front of the shoreline within the first 15 to 30 feet, he said. He said the theory is that the tall marsh grass acted as almost a wall to keep the concentration of oil on the perimeter.
In another part of the study, Silliman said researchers transplanted marsh grass into areas that had been oiled and also in areas that had been oiled and eroded. In areas that the land had eroded, the marsh grass died as it basically drowned from being at too low of an elevation to survive, he said.
However, Silliman said, some of the best growth occurred in areas that were oiled and went through marsh die-off but didn’t erode. That makes sense, since any competition would have been eliminated along with the marsh grass making it easier for the transplanted material to grow, he said.
“We can infer that it was no longer lethal,” Silliman said about the soil where the marsh grass grew back.
“Our results suggest that there are reasons for both optimism and concern about the impact of this oil spill on Mississippi deltaic marshes of Louisiana,” according to the report. The study results show that marsh plants have an ability to be resilient to oil but that the cost of an oil leak on the shoreline can be high in terms of accelerated erosion, according to the report.
The report is available online at http://www.pnas.org.