When marine toxicologist Susan Shaw set out to investigate the Gulf of Mexico oil spill, she didn't do it from behind a desk.

Late in May, a few miles offshore of Louisiana's Pass a Loutre marshlands, Shaw donned a wetsuit, coated her exposed skin with a protective coat of petroleum jelly, and dove into the oil slick. "What I witnessed was a surreal, sickening scene beyond anything I could have imagined," Shaw wrote a few days later in The New York Times:

There were patches of oil on the gulf’s surface. In some places, the oil has mixed with an orange-brown pudding-like material, some of the 700,000 gallons of a chemical dispersant called Corexit 9500 that BP has sprayed on the spreading oil...

[O]nly a few meters down, the nutrient-rich water became murky, but it was possible to make out tiny wisps of phytoplankton, zooplankton and shrimp enveloped in dark oily droplets. These are essential food sources for fish like the herring I could see feeding with gaping mouths on the oil and dispersant.

BP refused an Environmental Protection Agency order in late May to significantly cut down its use of dispersants, as well as another to find and use a less toxic substance than Corexit, saying that it "continues to believe that Corexit EC9500A is the best alternative" available in the necessary amounts,

Dr. Shaw, director of the Marine Environmental Research Institute, is calling for the Obama administration to halt BP's use of Corexit in response to the BP blowout -- or any other dispersant. Last week she told OnEarth why.

Q: How is use of Corexit increasing the scope of the oil disaster?

A: Dispersed oil is actually more toxic than regular oil because it's broken up. There's more surface area for exposure, so there's higher exposure of anything that it comes in contact with. The hydrocarbons are much more bio-available, let's put it that way.

And then it's moving in a different way: Up at the surface, you have the big slugs of oil, which can be vacuumed off. But when they disperse the oil, it starts breaking up and breaking up -- until at the mid-level of the water column, it's now formed these clouds, which are expanding the more dispersants they put on. So you've got this moving, dispersed cloud.

Q: Is that what people are calling the plumes?

A: Yep. This dispersed oil is highly toxic to plankton and all the organisms at the the base of the food web. So what we're looking at, for me, is the trophic cascade story.

Q: A "trophic cascade" means a bottom-up collapse in the food web, right?

A: Yes, but it can be much more complex. In the Exxon Valdez spill, they thought that the what would happen would be that kelp would wipe out, and then the little fish. But actually what worked out was very, very different. A certain kind of kelp on the rocks, an algae rather, was broken up. A certain kind of barnacle that was holding everything onto the substrate was replaced by an invasive barnacle, and then the algae was replaced by another invasive species.

What happened was you had an invasion of these species there were less stable on the substrate, so that currents would come along and easily rip [them] off.

Also, what replaced this algae was not the favorite food for the mussels, or for the upper periwinkles and snails. So then this whole lineup at the trophic snail level wiped out. Eventually this got to the sea ducks. We lost over two hundred thousand sea ducks [after the Exxon Valdez spill], and the've never come back.

Q: How could a trophic cascade play out in the Gulf of Mexico?

A: We're trying to figure that out. But if these plumes get over to the reef [off the southern coast of Florida], they'll decimate the coral, and that will wipe out the tiny fish that are in the coral and all the [other] symbiotic organisms.

I'm worried about that trophic effect. Plus, the toxicity of the Corexit itself. In some cases the synergism is more toxic than the original substance.

Q: Do you have any idea how much Corexit BP has used?

A: I found an estimate that was 1.8 million but you'd have to track that.

Q: When it is good to use a dispersant in an oil spill?

A: It's hard to say, but I don't think this toxic dispersant should be used in any oil spill. There are water-based ones, one is called Dispersit, and one is called Sea Brat 4. But of course no one has invested in developing those.  Sea Brat 4 is the same tox profile as Corexit 9500, however; Dispersit seems to be better.

Those two are water-based, whereas Corexit has petroleum solvents in it. So with Corexit, we're putting petroleum on a petroleum spill. We're increasing the hydrocarbons in the water.

Q: So what's the point of using a dispersant?

A: To keep oil off the shore. It does -- it keeps it down in the water column. That's where the death is occuring now.

Q: In a sense, then, if the oil is off the shore, it's out of people's minds.

A: Right. Some people have said it was a PR thing for BP.

It's hard to keep oil off the shore, but it can be done, and there's such a weak effort going on now.

Q: The Coast Guard and BP keep showing the public pictures of skimmers, and boats burning oil.

A: Right, and all that stuff is shallow-water drilling methods from the 1970s that are so old, they don't work. A shallow-water spill is a whole different thing, because the oil can actually weather rather quickly. This is deep water, it's a deepwater system. In deep water the oil can't weather as quickly, because it's colder water.

It's just like a house of cards folding. When you wipe out the base and the middle of the food web like this, then the top is going to crash, and we'll see dead bodies, with dolphins, sea turtles -- which are endangered already -- and manatees.

I don't think we've even begun to see the impacts of this spill. It's become more and more horrifying over time.

Q: You were in Louisiana in late May. How did people you met then feel about the crisis?

A: There was anger, but there was this belief -- people wanted to believe that things were going to get better. The local people were still saying, 'Well, we think this is pretty bad, but we really trust BP and they're going to make this right. They'll clean this up."

No one ever thought it would go this far, and be this extent of damage, and we'd still be fighting with whose boat was going to be out there.

Q: So these reports that the response efforts have been disorganized, in your estimation those are true?

A: Right. When we were out there, we were out 40 miles, and we saw only two skimmer boats. And one was broken down and being towed by the other one.

Q: So neither was acutally skimming oil, in other words?

A: No!

Q: Did the federal government grasp what was really going on?

A: I don't think so. Everyone's just praying that Obama will get in there. I think he's going to have to.

Q: What would getting in there mean?

A: Go down there and be there, camp out down there. One thing he could do would be to set up a branch of the Cabinet, make it a "Gulf restoration agency," and welcome volunteers that want to come from around the world. Make this a bigger picture thing.

Q: Will you continue to monitor and assess the ecological conditions in the Gulf?

A: My feeling is no one knows what they're quite doing with the big picture. All the agencies are just in the trenches, doing what they're supposed to do. They don't have the funding, they're doing about 13 times more work than they ever have done. They're up to their necks.

So I'm working with networks and agencies, advising them, developing monitoring plans and defining what should be monitored, long-term and short-term. I'm going back down there. I hope to have the partnerships in place in the next few weeks.

Image courtesy of Marine Environmental Research Institute