Each spring and summer fertilizer from the fields of the U.S. Midwest runs off into the Mississippi River. Old Muddy carries the nutrients down the length of the continent before dumping them into the Gulf of Mexico. Once introduced, the nitrogen and phosphorus prompts a bloom in algae, phytoplankton and other microscopic plants. After the plants die they drift to the bottom and their decomposition sucks the oxygen out of the seawater. The result is a vast dead zone, lethal to sea life that cannot swim out of the way, in inhabitable waters near the Gulf Coast that is sometimes as large as New Jersey—and the as much as 3.8 million liters of oil now spilling into the Gulf per day may make it worse.

"The oil is in the area of the annual low-oxygen zone that develops off the Mississippi River," says biological oceanographer Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium (LUMCON), who has measured an early start to the annual dead zone this year in March. "There will be localized low-oxygen areas under the surface of the slick."

The oil spill may exacerbate the shallow-water dead zone through a variety of physical and biological processes. But it could also help minimize the dead zone through similar means. Overall, the response of the Gulf dead zone to the oil spill is quite uncertain, with oxygen levels being tugged up and down by numerous factors, leaving the future of this habitat in question.

At the same time, further from shore, the oil is having a host of potential oxygen-depleting effects from the surface waters all the way to the seafloor. So the questions remain: Will the oil spill create more dead zones in those deeper habitats? Or could it simply help to minimize the one we already have near the Louisiana coast?

Let's get physical
Oil creates a slick that rides on the water's surface. First and foremost, this physical coating prevents seawater from absorbing oxygen from the atmosphere. As the oil washes into ever-shallower waters, that barrier will particularly choke off oxygen in the estuaries and wetlands that serve as habitat and nurseries for much sea life, in essence asphyxiating larvae and other inhabitants.

Of course, the smaller Exxon Valdez oil spill never ended up creating such dead zones near shore. The 41.5 million liters of oil spilled off southern Alaska dropped oxygen levels in the water by as much as 50 percent but currents there minimized the damage to sea life, and the large-scale movements of seawater may work similarly in the Gulf. "We never saw any fish kill or other event from hypoxia there, and I'm not anticipating it here," says microbiologist Ronald Atlas of the University of Louisville, who evaluated responses to the Alaskan oil spill. "I'm not expecting with currents that we're going to go to zero or a low enough number to have a fish kill."

At the same time, however, shallow-water sediments coated in oil can help boost algal growth. "Frequently after an oil spill sediment that has become oiled will turn green—algae will proliferate," says ecologist John Fleeger at Louisiana State University. "The grazing invertebrates will die off [from the oil], and the algae has a boom. It's an unexpected situation where you think everything will be harmed, but some organisms can do better." That could mean less oxygen as the algae grows, dies and decomposes.

But the physical oil coating on the water's surface might also reduce the number of algae by blocking sunlight—potentially minimizing the near-shore dead zone. "The trick [for this sunlight-blocking] is getting the slick to stand still long enough," says marine biologist Robert Diaz of The College of William and Mary. "If the slick is just passing by, the depression in light and photosynthesis would likely be short-lived and minor," and therefore do nothing to prevent the seasonal dead zone underway.

The oil itself is directly toxic to some algae as well, and can turn even more toxic when interacting with sunlight, an effect known as phototoxicity, which could also diminish the dead zone. In a smaller spill off Panama's coast in the 1980s such phototoxicity played a key role in killing coral and other shallow-water sea life. "In retrospect, a good part of the damage was from photo-enhanced toxicity," says environmental chemist Jeffrey Short of environmental group Oceana, who has studied the after effects of the Exxon Valdez spill and others. "It's not very well understood in the field. This oil spill may give us an opportunity to learn a lot more about how much of a factor that is."

In addition, the dispersants that have been used to break up the oil, such as Correxit 9500, are toxic to phytoplankton. The National Academy of Sciences noted in a 2005 report that 20 parts per million of it depressed growth in 50 percent of Skeletonema costatum, a Gulf of Mexico diatom, within 72 hours. "If BP was over there in the area of the dead zone applying the dispersant at the same rate as they are at the wellhead, they might be on to something in terms of controlling the dead zone," Diaz says.

In deep water
Regardless of what may happen near shore, the deeper waters of the Gulf—where the oil is directly spilling—may form their own dead zones, thanks to the busy work of microorganisms breaking down the oil and consuming oxygen in the process. An example of this busy work can be found in the famous Mobile Bay jubilee—a mass migration of deeper dwelling sea life, such as crabs and shrimp, into shallower waters as a result of certain weather conditions. Jubilees happen when a low-oxygen zone rapidly forms in deeper waters as organic sediment is broken down by microorganisms, prompting deep-sea crabs, shrimp, flounder and other fish to migrate quickly to shallower, more oxygenated waters. Stunned by the low-oxygen conditions, they are easy to harvest in copious quantities—hence the name jubilee. Now, low- or no-oxygen (anoxic) conditions may follow in the wake of the oil spill and the increased microbial activity that it will spur.

Similarly, the far offshore waters of the Gulf of Mexico may see more or expanded areas of deeper waters with low or no oxygen. Low oxygen concentrations in the middle depths of the Gulf persist naturally, from roughly 100 meters to 1,000 meters, dropping as low as three milligrams per liter compared with an average concentration of as much as six milligrams per liter in other waters. (By comparison, the shallow-water dead zone has oxygen levels less than two milligrams per liter.) That means life in these depths is used to little oxygen but bacteria, fungi and other microbes chewing on the oil could expand these zones into surface or bottom waters or make low-oxygen zones anoxic—replicating the effects of a jubilee.

Less or no oxygen kills sea life that cannot move fast enough (or at all) to escape it, such as the immobile Lophelia coral anchored on the sea bottom or relatively immobile animals, such as worms and crabs. "Increased microbial respiration as the oil is broken down may expand this zone," biological oceanographer Lisa Levin of the Scripps Institution of Oceanography wrote in an e-mail. "Low oxygen zones in the water will lead to animal migrations and habitat compression, but also mortality in less motile organisms."

Already, thanks to climate change warming the waters, these zones are expanding. "You could get a similar response from oil as an organic substrate," Diaz says.

Shrimp, to take just one example, may be forced to migrate into denser clusters in different depths as a result of oxygen levels that are too low in their preferred habitat. "It's only going to make them more stressed, and the prediction would be negative in terms of the population of mobile animals," Diaz says. "Multiple stressors lead to higher levels of mortality."

Is dilution the solution?
Fortunately, the sheer volume of water in the Gulf of Mexico will help to tamp down the impacts from even the worst oil spill in U.S. history, which may have already dumped more than 150 million liters of petroleum into the ocean. "Dilution is not the solution to pollution entirely but a pretty large volume of water already has low oxygen levels as part of that deep water," LUMCON's Rabalais notes. "The volume [of water] is just too great."

Biogeochemist David Valentine of the University of California, Santa Barbara, calculates that 20,000 barrels per day added more than 30 days to an area of roughly 40,000 square kilometers of ocean with a depth of 1.5 kilometers coupled with microbial degradation rates would produce a "1 percent oxygen drawdown. On a regional scale, it's probably not going to cause mass death. Localized, it could be more of an issue."

For example, no one knows for sure what will happen on the Gulf's seafloor, a region that is not well understood by scientists. The drilling mud used in an attempt to muscle the oil back into the well at the spill site might exacerbate low-oxygen conditions in the immediate vicinity of the Macondo well. After all, much of that mud leaked back out, laced with hydrocarbons, and settled to the sediment around the wellhead. "There should be some localized drawdown of oxygen where that drilling fluid is being put out," Rabalais says. "It will have some localized effects on the benthic [deep-sea] fauna."

And dispersants are being applied directly to the oil in the subsurface, resulting in yet more rich-feeding for oil-eating microbes as well as subsurface plumes, which BP's chief operating officer Doug Suttles noted at a press conference on May 29 that the oil company would not be able to clean. Company CEO Tony Hayward continues to deny such subsurface plumes might exist.

"We're relying on dispersion to reduce concentrations below oil toxicity threshold limits," explains microbial ecologist Kenneth Lee, director of the Center for Offshore Oil, Gas and Energy Research with Fisheries and Oceans Canada, who has been assisting with the disaster in the Gulf. "Of course, the dispersants can also stimulate microbial growth."

Ultimately, an enormous science experiment is underway in the Gulf of Mexico, as oil slicks interact with the seasonal shallow-water dead zone and dispersed oil droplet plumes provide rich feeding to microbial life. A host of science expeditions hope to assess and better understand how the oil spill will impact everywhere from tidal estuaries to 1,500 meters beneath the sea's surface. "No one knows yet to what extent any of these potential problems will alter our ecosystems and their functions," Scripps's Levin says, "but significant effects are likely."