How much damage has the BP oil spill done?

Fish and birds affected by oil There have been many devastating images from the Gulf

In the months since the start of the BP oil spill in the Gulf of Mexico there have been harrowing images of birds coated in oil and dead dolphins, but just what do we know about the scale of the environmental damage done?

WHAT WILL HAPPEN TO THE OIL?

"The good news is that oil is a natural product and is relatively easily degraded," says Prof Ed Overton, an environmental scientist at Louisiana State University.

Oil skimmers Some of the oil will be picked up by skimmers

Oil which has not been dispersed or washed up on shore will be targeted by microbes.

"They use the oil as a food," says Prof Overton.

There is an advantage in that the spill happened in the warm Gulf of Mexico, where conditions are good for decomposition. In colder climes, things can be harder.

"Contrast that with Exxon Valdez where you still have beaches where you can kick over cobblestones and still have pools of oil beneath them," says Stan Senner, director of conservation science for Ocean Conservancy.

"In the Gulf of Mexico it is a different environment. There is some greater capacity for that environment to handle hydrocarbons."

But that is not to say the oil will totally vanish.

There may be oil which becomes buried on shore, and oil may end up at the bottom of the sea in anaerobic areas - places where there is no oxygen to allow the microbes to do their work.

"We have never seen these clouds or plumes of oil dispersed in tiny droplets in the water," says Mr Senner. "We don't know how much is ending up on the bottom. Onshore, we don't know how much is being buried."

WHAT WILL THE EFFECT BE ON THE WETLANDS?

The oil can be deadly to both plants and animals, as can be seen in this audio slideshow.

Blue heron flies over marshland The Gulf Coast's wetlands are home to a diverse range of wildlife

"The wetlands that are already impacted, if only the stems and the grass you see on the surface are affected then the recovery will be within one or two years," says Dr Larry McKinney, director of the Harte Research Institute for Gulf of Mexico Studies.

"If the oil has penetrated down to the roots, then you are going to lose those areas altogether. [Wetlands] will go to open water and will never recover."

It's hard at this juncture to tell what the eventual effect will be on the northern Gulf Coast's habitat.

"A coating on the leaves will kill the leaves but it doesn't necessarily kill the plant," says Prof Overton.

WILL ANIMAL LIFE RECOVER?

"With events like this their impacts occur in different phases," says Mr Senner. First there is the initial wave of deaths, the animals that get covered in oil and die.

Turtle covered in oil Longer-lived animal populations will take years to recover

The death toll Mr Senner is aware of already includes a "thousand bird carcasses - half of them are oiled, others are just carcasses, a few hundred turtles, 50 or so dolphins".

But those small numbers reflect the fact that only a small percentage of carcasses are recovered.

"The assumption is that the actual mortality rate is many times what has been recovered," says Mr Senner. "The rule of thumb for the bird carcasses is that they find one in 10, but that could be low."

The question everybody will be asking is how quickly can animal numbers return to normal. It all depends on the life span of the animal.

In the Ixtoc spill of 1979, there was a 60-70% reduction in shrimp in the year of the spill, but they were back to normal within one or two years, says Dr McKinney.

Then there are longer-lived animals like dolphins, whale sharks and sea turtles. If a single generation has been largely wiped out, numbers might not fully recover for 10-20 years.

And then there are the truly long-lived organisms.

"For deepwater solitary coral communities, their lifespan is in hundreds of years," says Dr McKinney.

UNANSWERED QUESTIONS

  • What will the effect of widespread use of the dispersant Corexit be?
  • Will migrating birds be severely affected?
  • How much oil has actually been spilled?
  • How much is buried in fragile wetland habitats?
  • What is the effect of oil on deep-sea ecosystems?

They could potentially be vulnerable to the oil or to patches of oxygen-depleted water created by the oil-eating microbes. When the microbes degrade the oil, oxygen is used up and carbon dioxide is produced.

Dr McKinney is worried about "huge clouds of low-oxygen oil dispersant mix, [which is] methane-heavy".

The plus side for fish stocks on the other hand is that, severe as the damage might be, it will be mitigated by the break in fishing.

"They have closed such large areas of the Gulf that the pressure has been reduced," says Dr McKinney. "Reducing the fishing pressure will allow more fish to remain alive and reproduce."

So stocks of fish like red snapper could be back to normal within two to four years.

But, the effect of the spill on species that are already under immense pressure - like Atlantic bluefin tuna - could be severe.

HOW WILL ECOSYSTEMS BE CHANGED?

"In ecosystems, when you wipe out large segments of them, the ecosystem responds to the absence of those things and other things come in to take their place and you don't return to the way things were," says Mr Senner. "Ecosystems are always dynamic. What we see in the Gulf of Mexico is an ecosystem that already had a number of stresses on it."

Some animals and plants may be badly affected by the disruption of the spill and not regain their previous place in the ecosystem once conditions return to normal. But they will be replaced by other organisms.

THE UNKNOWN

A lot of the thinking about the effects of the BP oil spill is informed by the work done following the Ixtoc spill of 1979.

But there are some aspects of the latest spill about which scientists find it hard even to speculate.

Marine biologists will admit that not a great deal is known about the effects of oil on organisms in deep water.

"We know almost nothing about the ecology in the deep ocean," says Prof Overton.

It may offer only a crumb of comfort, but the 2010 spill will one day provide that knowledge.

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