On the rocky beaches of Alaska, scientists plunged shovels and picks into the ground and dug 6,775 holes, repeatedly striking oil — still pungent and dangerous a dozen years after the Exxon Valdez infamously spilled its cargo.

More than an ocean away, on the Breton coast of France, scientists surveying the damage after another huge oil spill found that disturbances in the food chain persisted for more than a decade.

And on the southern gulf coast in Mexico, an American researcher peering into a mangrove swamp spotted lingering damage 30 years after that shore was struck by an enormous spill.

These far-flung shorelines hit by oil in the past offer clues to what people living along the Gulf Coast can expect now that the great oil calamity of 2010 may be nearing an end.

Every oil spill is different, but the thread that unites these disparate scenes is a growing scientific awareness of the persistent damage that spills can do — and of just how long oil can linger in the environment, hidden in out-of-the-way spots.

At the same time, scientists who have worked to survey and counteract the damage from spills say the picture in the gulf is far from hopeless.

“Thoughts that this is going to kill the Gulf of Mexico are just wild overreactions,” said Jeffrey W. Short, a scientist who led some of the most important research after the Exxon Valdez spill and now works for an environmental advocacy group called Oceana. “It’s going to go away, the oil is. It’s not going to last forever.”

But how long will it last?

Only 20 years ago, the conventional wisdom was that oil spills did almost all their damage in the first weeks, as fresh oil loaded with toxic substances hit wildlife and marsh grasses, washed onto beaches and killed fish and turtles in the deep sea.

But disasters like the Valdez in 1989, the Ixtoc 1 in Mexico in 1979, the Amoco Cadiz in France in 1978 and two Cape Cod spills, including the Bouchard 65 barge in 1974 — all studied over decades with the improved techniques of modern chemistry and biology — have allowed scientists to paint a more complex portrait of what happens after a spill.

It is still clear that the bulk of the damage happens quickly, and that nature then begins to recuperate. After a few years, a casual observer visiting a hard-hit location might see nothing amiss. Birds and fish are likely to have rebounded, and the oil will seem to be gone.

But often, as Dr. Short and his team found in Alaska, some of it has merely gone underground, hiding in pockets where it can still do low-level damage to wildlife over many years. And the human response to a spill can mitigate — or intensify — its long-term effects. Oddly enough, some of the worst damage to occur from spills in recent decades has come from people trying too hard to clean them up.

It is hard for scientists to offer predictions about the present spill, for two reasons.

The ecology of the Gulf of Mexico is specially adapted to break down oil, more so than any other body of water in the world — though how rapidly and completely it can break down an amount this size is essentially unknown.

And because this spill is emerging a mile under the surface and many of the toxic components of the oil are dissolving into deep water and spreading far and wide, scientists simply do not know what the effects in the deep ocean are likely to be.

Still, many aspects of the spill resemble spills past, especially at the shoreline, and that gives researchers some confidence in predicting how events will unfold.

Remarkable Persistence

In 1969, a barge hit the rocks off the coast of West Falmouth, Mass., spilling 189,000 gallons of fuel oil into Buzzards Bay. Today, the fiddler crabs at nearby Wild Harbor still act drunk, moving erratically and reacting slowly to predators.

The odd behavior is consistent with a growing body of research showing how oil spills of many types have remarkably persistent effects, often at levels low enough to escape routine notice.

Jennifer Culbertson was a graduate student at Boston University in 2005 when she made plaster casts of crab burrows. She discovered that instead of drilling straight down, like normal crabs, the ones at Wild Harbor were going only a few inches deep and then turning sideways, repelled by an oily layer still lingering below the surface.

Other researchers established that the crabs were suffering from a kind of narcosis induced by hydrocarbon poisoning. Their troubles had serious implications for the marsh.

“Fiddler crabs normally play a crucial role in tilling the salt marsh, which helps provide oxygen to the roots of salt marsh grasses,” Dr. Culbertson said about her study.

http://www.nytimes.com/2010/07/18/science/earth/18enviro.html?_r=1&hp