Laura Erickson's For the Birds

When I was visiting the Gulf of Mexico, I sat down for a conversation with Dr. Paul McKay, a physical oceanographer based in New Orleans.

I had arrived in New Orleans after spending several days in Grand Isle, Louisiana, where I visited a badly oiled beach at Grand Isle State Park on July 30th. That beach had been oiled a month earlier during a storm surge in the aftermath of Hurricane Alex. There were still huge deposits of oil on the surface, and when a park naturalist dug her hands into the sand, she pulled up huge globs of thickly oiled sand, extending more than two feet below the surface.

I asked Dr. McKay how oil could get that far below the surface, and how long it might last there.

PAUL MCKAY: When you look at sand, especially wet sand at the beach, you sort of look at it as being nearly solid. It’s kind of firm, you don’t sink terribly far into it, but really it’s a very porous media. There’s a lot of open space in between the grains of sand, which is filled with water.

When you have water washing up in a wave, especially in water with oil in it, it washes up, and when the wave comes in, you’ve got a few inches of water on top of the sand, and that water is pushing downwards, exerting pressure, and that drives some of the water with oil coming along with it for a ride down into the sand, and then the rest of it flows back out on the wave. The next wave comes up, it pushes a little bit more water, and it pushes the original water and oil further into the sand, and this can penetrate down several inches or several feet.

Once the oil is into the sand, there’s not much that’s going to happen to it. Oil degrades due to the action of weathering, sunlight on it, degrading it, and it degrades due to biology, the microorganism attacking it and eating it. Once you get into the sand, there’s no more sunlight shining on it so that it’s not going to weather anymore, and once you get into the sand, the amount of microorganisms available to attack the oil is much, much less, so oil, which gets into the sand, the pores between the sand, can potentially stay there for years or decades, and it will pretty much stay there inert until there’s a big wave or there’s a storm, which will put more pressure on it. This can pull it back out of the sand and drive it back out into the ocean and re-oil the ocean.

It’s also an issue for creatures that live in the sand—burrowing crabs and worms, and for things like shorebirds that eat these burrowing crabs and worms.

Next, I asked Dr. McKay what impact the oil that currently remains in the water can be expected to have on microorganisms in the Gulf.

PAUL MCKAY: Well, actually, the main impact is not so much the oil on the microorganisms, it’s what the microorganisms do with the oil. A large amount of oil was dispersed at the wellhead and subsurface, and it has to still exist out there. The fact that we don’t know exactly where it is doesn’t mean that it has disappeared. It’s been dispersed into very, very small droplets that probably you wouldn’t even notice if you pulled a sample of water out. However, microorganisms can notice this.

Oil is a very rich source of hydrocarbons, a very rich source of nutrients for certain specialized bacteria that have evolved to consume them. When they encounter these plumes of dispersed droplets of oil, it’s a feast. As creatures do, when there’s a feast, they tend to eat it. When they tend to see there’s a lot of oil, they tend to reproduce and make a lot more creatures that want to eat all this oil.

In some ways, this is good, because this is how we’re going to process and get rid of this oil. But when you have a very large population explosion of bacteria consuming oil, eventually they’re going to run out. They’re going to eat it all, which again, this is what we want to happen. But once they run out of oil, they run out of food, and then they’re going to die.

Most animals tend to some settle out, but most of them, especially in shallow, warm waters, are going to decay. Decay is a redox reaction. It consumes oxygen. All of these animals, all these microorganisms bacteria are going to reproduce out of control and are going to consume as much oil as they possibly can until they run out, then they’re going to die, and they’re going to decay, and that’s going to suck oxygen out of the water.

This is the same sort of thing that happens every year when nutrients are washed down from the Mississippi River, causing algae blooms, which reproduce out of control until they consume all the nutrients, and then they die and decay and suck the oxygen out. But this is a whole new source.

So on top of the regular anoxic conditions, the nutrification from the nutrient load in Mississippi, we’re going to have this whole other anoxic event caused by oil. So we can expect to see dead zones in the Gulf of Mexico very much larger than we normally see every summer.

Paul McKay and I also talked about the potential impact of the oil on crabs and how that, in turn, will affect other important organisms in the food web. That part of our conversation will air later this week. I’m Laura Erickson, speaking for The Birds.