Nearly two years after the Deepwater Horizon oil spill, the meticulous, long-term efforts of scientists finally yielded the official results: namely, that the brown, wilted, dying corals found at the Mississippi Canyon lease block 294 were indeed damaged by a plume of oil from the spill.

For many, it seemed a foregone conclusion. Back in December 2010, when news of the damaged corals first came out, their proximity to the leaking Macondo well seemed to be a "smoking gun" in its own right. What else could brown gunk (flocculent matter, if you are a scientist) covering damaged corals seven miles from the Deepwater Horizon site be, if not oil from the spill?

Yet, to this team of scientists, it was worth taking a closer look at the evidence with two-dimensional gas chromatography, sediment cores, coral samples, and mosaic imagery. Why? Because too much was at stake to base judgments on mere speculation.

In order to understand the damage in the deep, the scientists had to start by understanding what was down there to begin with.

To support that mission, enter USGS research benthic ecologist Dr. Amanda Demopoulos, who studies life on the sea floor to piece together what types of organisms typically live together in deep sea communities. Her work involves digging sediment cores from the bottom of the ocean and sorting through the many tiny forms of life found there.

In addition to deep sea coral ecosystems, Demopoulos studies communities in parts of the Gulf where oil naturally seeps up from the seafloor and is in fact a wellspring of life, not a source of damage. Chemosynthetic ecosystems -- the ones where food webs are based on chemicals rather than sunlight -- tend to have different forms of life, such as tubeworms.

Demopoulos was on the November 2010 research expedition which first discovered the damaged corals. Funded by the National Oceanographic and Atmospheric Administration and the agency now known as the Bureau of Ocean and Energy Management, the goal of that expedition was to gather the basic data needed to construct a scientific understanding of the various undersea ecosystems. It was part of a decades-long collaborative effort among federal and university scientists to explore deep sea ecosystems in an effort to provide sound baseline information for governance decisions about how to best balance natural resource use with protection. Demopoulos recalled watching the first images from the damaged site come in from remotely-operated vehicle.

"When we were watching the ROV video in the lab, I looked up at the video screen, and it looked starkly different from anything we'd ever seen before," Demopoulos said. "The corals were all dark grey and lumped over, and it was clear these animals were not healthy. We'd seen dead coral, but this was so different, we immediately knew it was worth investigating further. When we got closer, there didn't seem to be any secondary colonization, as we'd seen in the past on dead coral."

The fact that no new animals — such as barnacles or hydroids — had begun to attach to and grow on the dead corals suggested the coral deaths had been recent, noted Demopoulos. This process, known as secondary colonization, is commonly observed on dead corals, but takes time to occur.

The discovery of the damaged coral triggered a follow-up expedition to more carefully investigate the damage itself, supported by a special National Science Foundation RAPID grant. Demopoulos would later work with scientists from Haverford College, Penn State University, Woods Hole Oceanographic Institute, Temple University, and BOEM to assess the damage.

Demopoulos' part in the overall effort to understand life in the deep ocean has been to understand what lives in the sediments of different types of environments, such as deep-sea corals and chemosynthetic communities. Some species may be generalists found in a variety of environments, while others will be unique to one type of habitat. Demopoulos also pieces together new information about how these tiny organisms are connected through food webs.

Without a baseline for understanding what is typical, Demopoulos would not be able to assess how those sediment dwellers were affected by the oil spill. Based on her expertise with sediment samples, Demopoulos helped design the best approach for assessing the corals at the Mississippi Canyon lease block 294 for the presence of oil and the extent of damage.

"The challenge we faced in this study was piecing together what happened from multiple lines of evidence, because no one was sitting on the sea floor when the plume went by. The corals were the only witness,” said Demopoulos, “We had to consider the proximity to the Deepwater Horizon site and the fact a deep-water plume had recently passed over the site, then closely examine the corals for tissue damage and signs of stress, such as the presence of mucus, and of course, the chemical signature of the oil. It was truly an interdisciplinary effort.”

Demopoulos pointed out that the cumulative knowledge about deep-sea communities from previous expeditions provided the baseline for scientifically assessing what they saw at the site. “This is but one site in the Gulf of Mexico," she said, "but it has shown how important it was for us to have a frame of reference as to what a healthy deep-sea coral ecosystem looks like. We are still trying to understand the extent to which this is occurring elsewhere in the Gulf of Mexico.”

The results of the efforts were recently published in the Proceedings of the National Academy of Sciences of the United States.

Rachel Pawlitz writes for the US Geological Survey.



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