Though little visible evidence remains of the oil that gushed into sea when the Deepwater Horizon rig exploded, it could still cause environmental problems. Some clean-up efforts were likely not helpful, scientists say.
When the BP rig exploded in the Gulf of Mexico in April 2010, 11 workers died and millions of barrels of oil from the Macondo well leaked into the waters. It was a disaster for local seabirds, marine mammals and turtles. Many died after being covered in the sticky oil. Even during the clean-up process, some nesting places were damaged.
The results can still be seen today. For instance, the local birth rate among dolphins is lower than it was before the rig went down.
But much of life has returned to normal in the Gulf of Mexico. Fishing fleets are back in action, and they're catching tasty fish. Tourists have even started to return, and they're not afraid to swim in the sea.
But just because it's hard to see the oil, it does not mean it is gone. When a tanker splits its sides, you tend to see large clots of oil on the surface of the water, but with the Deepwater Horizon disaster, much of the oil stayed in the depths of the ocean.
"The oil reservoir had such an incredible negative pressure - or vacuum - and so it was all sprayed through a very thin pipe, in the finest droplets," Antje Boetius, a biologist at the Alfred Wegener Institute for Polar and Marine Research, told DW. "The drops - just a thousandth of a millimeter in size - had such a large surface area compared to their buoyancy that they were unable to rise. All they could do was drift with the mass of water."
A fluffy carpet of oil
These droplets combined with suspended matter such as plankton and minerals at a depth of up to 2,000 meters. They clumped, then sank to the ocean bed, where they formed a fluffy carpet of oil.
"It looks like dust bunnies at your house," Boetius said, adding that the detritus found on the ocean bed and on deep sea corals is red and brown.
Oil is almost constantly discharged naturally into the Gulf of Mexico. That means there are bacteria in the Gulf that can digest oil with special enzymes. This cluster of bacteria is now slowly working its way through the carpet of oil that was created by the Deepwater Horizon leak.
The oil is slowly disappearing - but it can lead to toxicity, said Detlef Schulz-Bull, a chemist at the Leibnitz Institute.
"One part of the oil consists of a highly poisonous compound, which decomposes very slowly," he told DW. "When this interacts with the bacteria, it's even harder to evaluate its toxicity. But both the toxicity - and the compound's complexity - probably increases."
Fatty acids in the food chain
The main problem is that the bacterial decomposition products, including the fatty acids, are far more water-soluble and organisms ingest them faster so they quickly end up in the food chain.
Another problem is that the bacteria need a lot of oxygen, which can lead to an oxygen deficiency in the ocean. This, in turn, gives rise to other bacteria that do not need oxygen.
"The second group of bacteria produce sulfide, resulting in hydrogen sulfide, but this sulfide is bad for the sea worms, shell fish, and other fish," said microbiologist Boetius.
That is why biologists were concerned that the marine ecosystem at the Gulf of Mexico would collapse - which has not happened because the Gulf's strong currents ensure there is a constant supply of oxygen. The current also dilutes the oil and the decomposition products produced by the bacteria.
Worst US offshore spill
However, it has also become clear that the recovery workers may have made a mistake in using the solvent Corexit to stop the oil reaching the coast on the water's surface. The oil and the Corexit both dissolved into the water and poisoned bacteria, fish larvae and micro-organisms.
Schulz-Bull said it makes more sense not to use solvents when dealing with oil catastrophes.
"You can't see the oil, but it's still there," Schulz-Bull said. "All you're doing is pouring chemicals into the ecosystem that are highly toxic. You're not helping anything."
The one thing that does help is an oil barrier to create a containment zone, the chemical expert said. "Then, pump off the oil." If that doesn't work, Schulz-Bull said, "We should be using special wood materials and try to get the oil to stick at the water's surface where it can later be skimmed off."