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The University of Texas at San Antonio Online Magazine

What Lies Beneath

More than two years after the worst oil spill in the Gulf, signs of a larger, hidden impact are found beneath the ocean floor

Researchers looking at tiny worms, algae and other microscopic organisms among grains of sand at the water's edge have developed a remarkable, if incomplete, picture of the sub-surface impact on sea life caused by the BP oil spill, the most devastating ever to take place in the Gulf of Mexico.

Now-clean beaches off the Louisiana and Alabama Gulf Coast that were heavily impacted by the April 2010 Deepwater Horizon oil platform blowout stand in sharp contrast to the biological changes below the surface, a federal study concludes.

The study of DNA from organisms like coastal intertidal nematode worms, some as small as one millimeter, was led by researchers from the University of New Hampshire, along with scientists from Auburn University and UTSA. It was funded by a $200,000 grant from the National Science Foundation.

The yearlong project was part of the government's time-sensitive, post-disaster study of the damage caused by the massive oil leak occurring more than a mile below the surface, which took three months to cap.

"Recovery is taking place, there is no doubt, but at what pace, and what is the process of that recovery, we just do not know," said Jyotsna Sharma-Srinivasan, a research assistant professor in UTSA's Department of Biology.

Official government estimates indicate that over an 89-day period, about 2.6 million gallons of oil a day poured out of the sea floor as a result of a methane gas explosion far below the seabed. Some estimates calculated that significantly more than 231 million gallons were released into the ecosystem in a huge, dark plume that eventually made landfall along the upper Gulf Coast.

Samples from beaches around Alabama's Dauphin Island and Mobile Bay were collected shortly after the blowout but before the plume made landfall. Five months later, a second set of samples was collected after the coastal beaches were cleaned. Samples were also collected from Grand Isle, off the Louisiana coast, during a period in September 2010 when those beaches were heavily oiled.

Holly Bik, the study's lead author, took a spoonful of pre-spill sand from each beach and extracted millions of DNA genetic barcodes from every organism in that spoonful, Sharma-Srinivasan said.

231 million gallons of oil were released into the ecosystem.

A diverse mix of organisms has been wiped out, and only a few species of fungi and nematodes have replaced them.

"We then sequenced a specific 'barcode' region of DNA from our samples and compared these pieces of DNA to online databases to determine what species were living on each beach," she added.

The research team concluded that the oil had caused "massive harm to the microscopic creatures in coastal sands" and that a diverse mix of those organisms that existed before the spill has been obliterated, replaced by only a few species of fungi and nematodes.

"Shrimp and oysters were deeply impacted—their spawning grounds were destroyed—and the juveniles were wiped out," she said.

But it was not until the DNA sequencing samples were taken from the worms that a clearer picture of the impact on the ecosystem emerged.

In her portion of the study, Sharma-Srinivasan found that while there had been more than 70 different species of nematodes in the sampled Gulf waters before the accident, only about six species were found afterward.

"Some worm [species] turned out to be very resistant to the oil, maybe able to eat fungi that were able to break down the hydrocarbons," while other species were not, she said.

The scientists noted that they still don't have a clear picture of how long the spill's impact will be felt, whether the microscopic organisms' diversity will return to pre-spill numbers, or whether the shoreline will be repopulated by new organisms.

What is known is that microorganisms like nematodes are critical to maintaining a healthy marine habitat. They are "the machinery that keeps the ecosystem working," Bik said.

"In this instance, the worms act as a sort of miner's canary," Sharma-Srinivasan said, adding that the microscopic organisms are a crucial element in the complex food chain and play a major role in key ecosystem functions like nutrient recycling.

She noted that a cursory view of the surface water fails to reveal what apparently has been occurring far below.

"If people look at the Gulf waters and see no oil on the surface, they often conclude the area has recovered to its original state, but that often is not the case," she said. "The oil sinks below the sediment and may affect the entire ecosystem."

Nematodes, like other microscopic organisms, play a critical role in the ecosystem by serving as food for fish and shrimp, which are a valuable source of revenue for the area's fishermen. The worms also serve to keep the system healthy by eating bacteria and decaying matter, contributing to the decomposition process that is vital to the ecosystem's overall health, she said. They also introduce carbon and other important minerals back into the food chain.

"What struck me was that you wouldn't have known there was an oil spill—most of our sample sites looked like normal beaches," said Bik, who at the time of the study was a researcher at the University of New Hampshire, but who now works at the University of California–Davis. "But when we analyzed the genomic data, there seemed to be all these biological repercussions going on."

The research was published in the journal Public Library of Science ONE.

In their report, the scientists concluded that the "marine habitats experienced visible, heavy impacts following the [spill], yet our scant knowledge of [what was there before the spill] has precluded a thorough assessment of this disturbance."

Yet, the study reported, based on the researchers' analysis of organisms before and after the massive spill, "our data suggest considerable initial impacts across Gulf beaches may be ongoing, despite the disappearance of visible surface oil in the region."


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