Tracking whales to assess Gulf spill impact
Officials say three-quarters of the 4.9 million barrels spilt in the Gulf of Mexico has been "dealt with". But what of the remaining oil - and how did the removal and dispersal affect existing ecosystems?
BBC World Service's Science in Action programme met two experts who will be monitoring the long-term impact of the spill to find out just how damaging the disaster has been.
Professor Christopher Clark is head of the bioacoustics research programme at Cornell Lab of Ornithology.
Along with his team, he is conducting surveys into the health and population of whales and dolphins in the Gulf of Mexico, to try to build a picture of the long-term damage caused by the spill.
"Our tool of choice is to listen, rather than to look," he explained.
"Light doesn't go very far in the ocean and shining any lights in the ocean doesn't help very much.
"So what we do is deploy instrumentation along the bottom of the ocean, or suspended in the water, that records sound for many, many months at a time.
"We deploy these strategically in areas we think the animals are most likely to be, and where the oil spill will most likely have an impact."
By listening to the various calls, songs, squeaks and creaks from whales and dolphins, Prof Clark and his colleagues can figure out how well the animals are hunting and interacting with each other - a key indicator of whether this important part of the ecosystem is struggling or not.
"I don't really know if we know the true extent of how much oil and gas was dispersed and flushed into the Gulf of Mexico, I think that number is still to be determined," he said.
"Yes, it's great that the surface situation seems to be improving, but now we have to contend with this invisible, highly-distributed massive amount of hydrocarbon. We can't forget that.
"We can't just think 'out of sight, out of mind'.
"We must understand that [it] will have very long term consequences on the health of the Gulf of Mexico."
Significant parts of the oil have been "biodegraded by naturally occurring bacteria", according to Jane Lubchenco, head of the National Oceanic and Atmospheric Administration (Noaa).
But John Kessler, professor of oceanography at Texas A&M University, has concerns how this now affects the levels of methane and other natural gases in the water.
"There's definitely a significant amount of oil and natural gas that's in these deep plumes. There's no doubt about that.
"The oil on the surface of the ocean seems to be disappearing at a rather rapid rate - it's a good thing - but unfortunately it seems like it's becoming 'out of sight, out of mind' in the general community."
His team has been studying methane levels in the Gulf since early June, just as the scale of the spill was becoming worringly apparent.
"Methane is a very potent greenhouse gas. We hear a lot about carbon dioxide due to fossil fuel combustion, but methane is actually, on a molecule-by-molecule basis, a more potent greenhouse gas than CO2 is.
"The oceans are by far the largest global resorvoir of methane, and there have been some implications that in the geologic past there have been significant emissions of methane from the sea floor that have contributed this greenhouse gas to the atmosphere, and potentially influenced climate as well."
For both Prof Clark and Prof Kessler, the work in the Gulf is very much in its infancy.
"Right now we're sort of in that immediate stage of doing a better job of describing what that ecosystem is like," Prof Clark said.
"Then overlay that with the information about the oil, what's happening to that hydrocarbon invasion - and are there differences in the way the whales are distributed now versus three, six months from now, versus what we thought they were doing a year ago.
"Our handicap is we don't have very much information about what they were doing a year ago."
For Prof Kessler, the analysis has already begun.
"What we're trying to do right now is get a better understanding of what our data is telling us from our first expedition," he said.
"Once we have all those results in hand, we can form a much more organised scientific plan, instead of just going out and randomly sampling.
"We are co-ordinating, not only with other people, but also with national funding agencies like Noaa and the National Science Foundation to get more expeditions out so that we can investigate what will change within this system in time."