At Depth, the Hidden Cost of Oil?

Marine scientist Tracy Villareal on the search for deep water oil plumes in the Gulf of Mexico.

Summer 2010 could go down in history as the summer of the great BP Deepwater Horizon oil spill.

As oil poured forth from the damaged well on the floor of the Gulf of Mexico, the nation held its breath. The numbers were frightening: 4.1 billion barrels of oil released, 1.8 million gallons of dispersants applied, 11 people killed, oil slicks the size of Kansas. The spill provided an acute awareness of the environmental costs of a petroleum-based fuel economy.

And then, something strange happened. The surface damage wasn’t as bad as predicted. Birds and sea turtles were affected, but not in the numbers expected given the immensity of oil spilled, and marshes weren’t coated in as much goop as many predicted.

As a nation, we’ve since turned our attention elsewhere, but the questions remained at the end of the summer: “Where was the rest of the oil, and what was it doing?”

So in August, marine scientist Tracy Villareal embarked on a month-long cruise with more than 30 other scientists in search of the deepwater oil plume, hoping to find answers to those questions.

Villareal was chief scientist on the R.V. Cape Hatteras, one of a two-vessel team that danced around the northern Gulf’s so-called “steel archipelago” (its 4,000 or so oil platforms) equipped with advanced lab equipment for measuring water from the ocean’s surface to its great depths.

On their journey, he and his fellow scientists found signatures of the sprawling oil plume as deep as two-thirds of a mile beneath the surface. They found—in places—increases in the acidity of the water, oxygen drawdown and increases in particles in the water. They also discovered changes in nutrient levels where naturally occurring oil-eating bacteria had metabolized hydrocarbons. The team found that oil had—in places— permeated inches deep on the ocean floor.

“What we now know,” says Villareal, a professor at the Marine Science Institute, “is that the high methane signature seen around the wellhead very early in the spill was rapidly metabolized by bacteria. There was no persistent signature of methane in the plume. We also know that the subsurface oil plume was well defined, but had a high degree of variability, with eddies, swirls and holes in it.”

In the area of the plume, he said the oxygen drop was pronounced, but not catastrophic.

“The levels of decline [of oxygen in the plume] did not reach thresholds where the biology would have had a problem,” says Villareal. “Personally, I think that the Gulf has a tremendous ability to absorb this sort of insult, and that the spill, despite its size, will have only a moderate long-term impact. That being said, the jury is still out on certain aspects of the impact, such as what happened to pelagic fish eggs.”

Villareal says that may only be known over time if larval fish populations decrease next year and beyond.

“There was clearly oil on the bottom around the spill site that led to high benthic mortality,” he says. “The question that’s still unanswered is how extensive that mortality was.”