"Eutrophication" from the oil spill may present a deadly challenge to sea life.
(This is a series of comments I made in some open threads, and some people suggested I put it in a diary to make it more widely available. So here it is.)
My first job when I moved to Florida was with the Public Interest Research Group, fighting against offshore oil drilling in Florida. So while I make no claims whatever to be an expert, I do know a few things about it. And I am very concerned about the potential effects of the underwater oil plumes that have been detected in the Gulf, and I haven't heard any talking heads talking about it . . .
Basically what the dispersants have done is break the oil slicks into small droplets that have formed large underwater clouds. This is done for a particular reason--it's to create a larger surface area on the oil so that bacteria can get to it more easily. For bacteria, oil is nothing but old rotted plants and animals---in other words, oil is food, and they will happily eat it.
But here's the problem . . .
The bacteria need oxygen to metabolize the hydrocarbon, and they get that by taking it out of the surrounding water. The process is similar to what happens when algae blooms form in ponds or lakes---that happens a lot here in Florida when lawn fertilizer gets washed off by rain and runs into a nearby body of water--it causes a huge growth of algae to appear, and then suddenly die once the fertilizer is all used up. The dead algae are then eaten by bacteria, who use oxygen from the water in their digestion process. The result is known as "eutrophication"---essentially all the oxygen in the water gets used up by the bacteria, oxygen-breathing creatures like fish can no longer extract oxygen from the water, and they suffocate, resulting in a fish kill. Even before the spill, there were already underwater dead zones in the Gulf, caused by agricultural fertilizers being washed by rain into the Mississippi River and out into the Gulf.
I am very concerned that the same thing could happen inside the underwater oil plumes---as the bacteria eat the oil, they too will extract oxygen from the surrounding seawater, and this could potentially create very large dead zones of oxygen-less water, killing virtually all life within it. It could be a fish kill on a truly huge scale that would dwarf the current loss of life on the shorelines.
As an aside, the fact that bacteria eat oil and sunlight breaks it down chemically are the primary reason why oil slicks don't float around forever. On the surface of the ocean, they are exposed to sunlight and bacteria, and they usually break down fairly quickly. That is why oil spills in Alaska don't float all the way down to California--they get broken down through natural processes long before that. It's why the Deepwater Horizon spill probably won't make it all the way to the Florida peninsula--it will get broken down by sunlight and bacteria before it can float that far.
So it is likely that the Gulf oil will be gone from the water within just a few weeks of the leak stopping---it will be broken down by natural processes, including being eaten by bacteria and being chemically broken down by sunlight. Even in places where the oil comes ashore, it usually breaks down relatively quickly from the soil surface--it is food for bacteria, and they take care of it. That is a lucky thing for us.
But in many cases, oil that reaches shore also sinks down below the ground surface into the anaerobic layer--the area a few inches below the surface where oxygen from the atmosphere doesn't diffuse. The bacteria that break down oil are aerobic--they require oxygen to live, and they can't survive in the anaerobic layers. While there are anaerobic bacteria that will eat oil, they are far less efficient at it than the oxygen-using aerobic microbes--so any oil that gets into the anaerobic layers will stay there for quite a long time. That is why, at the site of the Exxon Valdez spill, there is no longer any surface oil, but there is still oil remaining a few inches below the ground surface--the aerobic bacteria can't get down there to eat it. While that subsurface oil doesn't come into contact with animals (except the burrowers), it does affect plant roots. This is particularly dangerous in swamps and marshlands, since these areas typically have very thick layers of muck where oxygen can't penetrate (that's why the mud in a swamp stinks so bad when you step in it)--and any oil that gets into that layer of muck will remain a long time. That's why it's so important to keep the oil from getting into the marshlands in the first place--once it gets into the anaerobic muck, it's pretty much impossible to get it out.
So although the oil will disappear quickly from the open water once the leaking stops, it will remain for quite a while beneath the soil surface in areas where it comes ashore.
But to me, the big unknown danger is what could happen to the underwater plumes. We know very little about how these plumes will act. They may rise relatively quickly to the surface and be quickly degraded by sunlight and bacteria. They may float around underwater for long periods like clouds and persist for a long time. And to me, the biggest danger is that they form large underwater clouds that produce eutrophication. We just don't know what will happen. But it is an enormous potential danger.