Washington’s Blog 
Sept 26, 2010
I previously pointed out :
Some experts have also said that the use of Corexit has prolonged by decades  the presence of toxic crude oil, because the dispersant sinks the oil  beneath the ocean surface, where it cannot be quickly broken down by sun, waves and microbes.
And the head of Lawrence Berkeley National Laboratory’s Ecology Department – Terry Hazen – argues  that the use of dispersants can delay recovery of ocean ecosystems by decades:
Hazen has more than 30 years experience studying the effects of oil spills. He says the oil will be damaging enough; toxic dispersants will just make it worse. He points to the 1978 Amoco Cadiz Spill off the coast of Normandy as an example. He says areas where dispersants were used still have not fully recovered, while areas where there was no human intervention are now fine.
As Hazen has noted :
“The untreated coastal areas were fully recovered within five years of the Amoco Cadiz spill,” says Hazen. “As for the treated areas, ecological studies show that 30 years later, those areas still have not recovered.”
Admittedly, chemicals other than Corexit were used in the Amoco Cadiz spill. But the precautionary tale still holds: chemicals should not be applied to oil spills unless scientists are positive that they will provide a net long-term benefit.
I have just learned that Corexit 9500 was actually the dispersant used in the Amoco Cadiz spill. 9500 is the dispersant used in the Gulf (another – even more toxic version – 9527 – has also been applied ).
Even in the turbulent, highly oxygenated waters of France’s Breton coast, it took at least seven years after the 1978 Amoco Cadiz spill for local marine species and Brittany’s famed oyster farms to fully recover, according to French biologist Philippe Bodin. An expert on marine copepods, Bodin studied the long-term effects of the spill from the grounded tanker. He believes the impact will be far worse in the generally calmer, lower-oxygen waters of the Gulf, particularly because of the heavy use of the dispersant Corexit 9500. BP has said the chemical is no more toxic than dish-washing liquid, but it was used extensively on the Amoco Cadiz spill, and Bodin found it to be more toxic to marine life than the oil itself. “The massive use of Corexit 9500 in the Gulf is catastrophic for the phytoplankton, zooplankton, and larvae,” he says. “Moreover, currents will drive the dispersant and the oil plumes everywhere in the Gulf.”
For the sake of thoroughness, I should point out that detergents as well as Corexit were apparently applied at the Amoco Cadiz spill site. The detergents might not have helped either.
Still, I find it stunning that Corexit was the dispersant applied in France which scientists say delayed recovery by decades.
Why does Corexit delay recovery of natural ecosystems?
There are a number of reasons (above and beyond the direct toxicity which they may pose to sealife).
Initially, dispersants break oil into tiny droplets, which can be eaten by small critters who think it is food . Bigger critters eat the smaller guys, and then the whole food chain is effected.
In addition, dispersants affect  fish so that they absorb more toxic chemicals from the oil, and then are less able to get rid of them from their bodies.
And National Geographic hints at another reason: Dispersants sink the oil, shielding it from the natural processes like sun, waves and microbes which break it down:
[Texas A&M University coral reef expert Wes Tunnell] stood in the clear, waist-deep water of the protected reef lagoon holding what appeared to be a three-inch-thick slab of sandy gray clay. When he broke it in two, it was jet black on the inside, with the texture and smell of an asphalt brownie. Here on the lagoon side, where the reef looked gray and dead, the Ixtoc tar mat was still partially buried in the sediments. But on the ocean side of the reef, where winds and waves and currents were stronger, no oil remained. The lesson for Louisiana and the other Gulf states is clear, Tunnell thinks. Where there is wave energy and oxygen, sunlight and the Gulf’s abundant oil-eating bacteria break it down fairly quickly. When oil falls to the bottom and gets entrained in low-oxygen sediments like those in a lagoon—or in a marsh—it can hang around for decades, degrading the environment.
People might not have understood in 1978 how harmful Corexit was to the ecosystem, but they surely should have known by 2010.
Have we finally learned our lesson … or will Corexit or similar dispersants again be dumped into the ocean the next time there is a major oil spill?