Leslie Kaufman / New York Times News Service

In the year since the wellhead beneath the Deepwater Horizon rig began spewing rust-colored crude into the northern Gulf of Mexico, scientists have been working frantically to figure out what environmental harm really came of the largest oil spill in U.S. history.

What has emerged in studies so far is not a final tally of damage, but a new window on the complexities of the gulf, and the vulnerabilities and capacities of biological systems in the face of environmental insults.

There is no doubt that gulf water, wildlife and wetlands sustained injury when, beginning on April 20 last year, some 4.9 million barrels of oil and 1.84 million gallons of dispersants poured into the waters off Louisiana. But the ecosystem was not passive in the face of this assault. The gulf, which experiences a natural seepage of millions of gallons of oil a year, had the innate capacity to digest some of crude and the methane gas mixed with it. Almost as soon as the well was capped, the deep became cleaner to the eye. By the same token, dozens of miles of marsh still remain blackened by heavy oil, government crews are still grooming away tar balls that wash up ceaselessly on beaches and traces of the dispersants are still found floating in the currents.

Biologists are nervously monitoring as yet unexplained dolphin strandings this year, trying to come up with a realistic count of birds and mammals killed during the spill and working to understand what happens when the gulf floor is covered with the remains of oil-eating bacteria.

“It is really kind of hard to get a grasp of the big picture, and it is not for a lack of trying,” said Christopher Reddy, a senior scientist at the Woods Hole Oceanographic Institution who studies long-term consequences of oil spills. “Hundreds of scientists are working day and night trying to carve out a piece of that giant puzzle, but it is an entire region and it is complicated.”

How the regional ecosystem has responded, its strengths and weaknesses, will keep scientists busy analyzing data for years and help them in understanding the effects of environmental disasters.

An army in hot pursuit

After an oil spill, the government is responsible for toting up the ecological damages in something called a Natural Resource Damage Assessment. The document, which requires battalions of researchers, makes the case for damages that the companies responsible for the spill should pay to restore the ecosystem to its pre-spill health. The companies hire their own teams of assessors, who might paint a very different picture. The two sides settle or go to court.

At of the end of January, the government said its scientists alone had taken 35,000 images, walked more than 4,000 miles of shoreline and culled more than 40,000 samples of water, sediment and tissue.

The scientists are also testing how to estimate what they can’t count precisely, like animal deaths. One group of evaluators is scattering bird carcasses offshore and measuring how many sink and how many wash ashore. Those numbers will be used to calculate how many birds may have died in addition to the ones that were found and counted.

For all this effort, it will take time for some of the consequences to manifest themselves. It was three years after the Exxon Valdez spill in Prince William Sound in Alaska, for example, that the herring fishery suddenly collapsed.

During the Deepwater Horizon disaster, as the slick was spreading, the federal Fish and Wildlife Service moved about 28,000 eggs from turtles’ nests on at-risk beaches in Alabama to the coast of Florida. While 51 percent of the eggs hatched — roughly consistent with normal survival rates — it will be another two decades or so before the hatchlings that survive come back to Florida as adults to lay eggs. Only then will anyone know how successful the rescue effort really was.

Many of the results that have been gained so far, by government or private industry, are not yet public; they are awaiting rigorous review before eventual release. Moreover, in some key cases, scientists must keep their findings confidential because of continuing legal actions.

“We have a real responsibility to make sure that we come out of this process with as much compensation as is appropriate for the damages,” said Bob Haddad, chief of the assessment and restoration division of the National Oceanic and Atmospheric Administration, which is taking the lead in coordinating the damage assessment. “I don’t want to get tripped on issues like inadmissibility of evidence.”

Oil and water do mix

Still, there has been some independent scientific work done in the gulf, and it has produced some good news. Because the spill occurred at very high pressure a mile beneath the ocean’s surface, some of the oil was reduced to tiny droplets that remained suspended thousands of feet deep in a fine mist.

Terry Hazen, who leads the ecology department at Lawrence Berkeley National Laboratory, took 170 samples from around the Deepwater Horizon between July 27 and Aug. 26 last year, just weeks after the wellhead was capped.

Hazen was looking to track the fate of the underwater oil as it spread and instead found it to be entirely gone. “We can detect down to two parts per billion,” he said, “but nothing was there.”

His work was financed by a grant his lab won from BP, the owner of the well, long before the spill, and it was not in any way reviewed or influenced by the company, he said.

The results showed that the oil had not just been diluted with water but that it had largely been eaten by naturally occurring bacteria. Researchers worried early on that such bacteria might not exist thousands of feet down or that the process of digestion might be particularly slow because of colder temperatures at these depths. But Hazen’s group found bacteria that specialized in oil eating in frigid temperatures.

Another byproduct of the spill was roughly 200,000 metric tons of methane gas. In June 2010 there was as much as 100,000 times as much methane dissolved gas in the gulf as normal. Scientists worried that it could remain dissolved in the water column, depleting oxygen levels, for years.

But by fall, researchers from the University of California, Santa Barbara, and Texas A&M took water samples from 207 sites near the spill and found that methane proportions were back to normal.

John Kessler, an oceanographer at Texas A&M, said: “It appeared that the methane would be present in the gulf for years to come. Instead, methane respiration rates increased to levels higher than have ever been recorded.”