Was Deepwater oil spill a drop in the ocean?

A year on from the Deepwater Horizon oil spill in the Gulf of Mexico - the largest such incident in history - whom do you believe?

Do you side with Kenneth Feinberg, the independent administrator appointed by Barack Obama to deal with compensation claims, who has declared it is "reasonable to conclude" the area will have fully recovered by the end of next year?

Or do you believe the environmental campaigners at Greenpeace, who insist that "damage to the marine ecosystem in the Gulf of Mexico will run deep for decades"?

On the first anniversary of the explosion on the BP rig, which last April 20 killed 11 people and began releasing more than four million barrels (600 million litres) of oil into the ocean, the facts seem to be on Mr Feinberg's side.

Fish stocks are at record levels and beaches and wetlands are all open and looking good. While thousands of birds may have perished, prompt action appears to have saved about half of those affected.

And according to the US National Oceanic and Atmospheric Administration, the number of marine animals whose bodies show clear evidence of oil contamination amounts to just six dolphins and 18 sea turtles.

But environmentalists insist this apparent evidence against their claims of eco-apocalypse is misleading. They argue that counting visible carcasses radically underestimates the true numbers of animals killed, and that the real damage is far less obvious.

They point to evidence that organisms much further down the food-chain have been badly affected, and say it will be many years before the true extent of the ecological damage reveals itself.

Most scientists would agree it is far too early to give definitive answers. Yet such statements of the obvious are of little use to regulators and oil companies faced with making decisions right now about the lessons to be learnt. What they need is insight into the likely long-term effects.

The obvious place to turn is the outcome of other spillages. And the results are surprising - and salutary.

Until Deepwater Horizon, the worst accidental spillage was the Ixtoc 1 rig blowout of June 1979, in the southern Gulf of Mexico.

As with last year's event, oil continued to gush out of the undersea well for weeks. By the time the well was capped, 3.3 million barrels had been released.

There was huge concern about long-term ecological impact. And small wonder: beaches from Mexico to Texas were blighted by oil slicks and fish, octopus and crustacean catches all nosedived.

But then surprises started to emerge. First, the warmth of the water in the Gulf helped speed the evaporation of the slick, allowing recovery to set in much faster. The fact that the oil released was light crude, with a high content of volatile hydrocarbons, also helped.

Nature's ability to turn even apparent disasters into opportunities began to make itself felt. Oil spillages are not solely the product of humanity's greed for money and motive power: they have taken place in the oceans for hundreds of millions of years, where the earth's crust fractures releasing the hydrocarbons within.

This has led to the emergence of microbes capable of digesting oil and when presented with millions of litres of the stuff, they did not hesitate to gorge themselves.

As for the stocks of fish and other marine life, within two years these were back to record levels. The reason: the temporary ban on landing catch from the area had given them respite from relentless fishing.

To this day, there is no evidence that the huge ecological disaster predicted for the Gulf following the Ixtoc 1 spillage ever materialised. Some argue this may simply be because studies were discontinued too early.

But with environmentalists keen to seize on any evidence to back their claims, it is hard to believe that some huge eco-apocalypse has gone unnoticed.

Now, 30 years on, environmentalists are reprising their message, this time predicting massive long-term damage from the Deepwater spillage. Yet the parallels with the Ixtoc 1 event suggest that, once again, their fears will be misplaced.

The warm waters of the Gulf again helped accelerate the break-up of the slick, while the oil-eating microbes did their bit - even at the far greater depth of the Deepwater well.

Studies by US scientists found the bugs made short work of a vast plume of oil rising up, multiplying so rapidly that after two weeks it no longer reached the surface.

And, in another repeat of 1979, the fish stocks seem to have taken advantage of the temporary ban on landing catches and replenished themselves.

In short, while it may lack the potency of mathematical proof, past experience suggests that nature is well able to take the Deepwater spillage in its stride.

So was the former BP chief executive Tony Hayward on to something with his vilified claim last year that the spillage was "relatively tiny"?

It is certainly true that while more than half a billion litres is a colossal amount in human terms, it is piffling compared to the millions of billions of litres of seawater into which it spewed.

Ironically, the biggest cause of ecological damage could prove to be the man-made chemicals used to disperse the oil.

The rush to give nature a "helping hand" to avert ecological disasters has a sorry history. Detergents used to help wildlife cope with the notorious Torrey Canyon supertanker spillage off south-west England in 1967 proved toxic, while the hot water blasting methods used to clean up the slick left by the Exxon Valdez in Alaska left the coastline looking pristine - but sent oil down into areas previously untouched, increasing the area affected.

In the case of Deepwater Horizon, almost 3 million litres of dispersants were used, and studies suggest they lingered for months in the cold, dark depths of the ocean. The intent may have been right, but the consequences are, as yet, unknown.

Even if the long-term effects do prove to be minor, no one could argue that oil spillages no longer pose an ecological threat.

It is clear that when it comes to recovery, the Gulf of Mexico has unusual characteristics that could not be counted on elsewhere. But it is also clear that environmentalists need to tone down their apocalyptic rhetoric, lest they prompt a rush to cures worse than the ailment.

Robert Matthews is visiting reader in science at Aston University, Birmingham, England.