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Issue 26
, 2010
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The Dead Zone in the Gulf of Mexico

Source: ENN, Date: , 2010

While the Deepwater Horizon oil gusher continues to mire the Gulf of Mexico, another threat could be growing below the oil slicked surface. This is the "Dead Zone". Currently the most well known dead zone is about 8,500 square miles in size and lies in the Gulf of Mexico, where the Mississippi River dumps high nutrient runoff from its vast drainage basin, which includes the heart of the U.S. agriculture business from the Midwest. This is equivalent to a dead zone the size of New Jersey. Dead zones are hypoxic (low oxygen) areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s.

Dead zones can be man made or naturally caused. Natural causes include coastal upwelling and changes in wind and water circulation patterns. Runoff from sewage, urban land use, and fertilizers can also contribute to eutrophication that can lead to these zones.

Eutrophication can be caused by an increase in chemical nutrients (particularly nitrogen and phosphorus) in the water. These chemicals are the fundamental building blocks of single celled, plant like organisms that live in the water column, and whose growth is limited in part by the availability of these materials. Eutrophication can lead to rapid increases in the density of certain types of these phytoplankton, a phenomenon known as an algal bloom. Although these algae produce oxygen in the daytime via photosynthesis, during the night hours they continue to undergo cellular respiration and can therefore deplete the water column of available oxygen. In addition, when algal blooms die off, oxygen is used up further during bacterial decomposition of the dead algal cells. Both of these processes can result in a significant depletion of dissolved oxygen in the water, creating hypoxic conditions. In other words a dead zone is created due to a depletion of oxygen in the water. Many life forms are effectively suffocated or forced to migrate.

Notable dead zones in the United States include the northern Gulf of Mexico region, surrounding the outfall of the Mississippi River, and the coastal regions of the Pacific Northwest, and the Elizabeth River in Virginia Beach. These zones fluctuate in size and intensity.

For the Gulf a study of the remains of organisms found within sediment layers near the mouth of the Mississippi River indicate four hypoxic events before the advent of artificial fertilizer. In these sediment layers, anoxia-tolerant species are the most prevalent remains found. The periods indicated by the sediment record correspond to historic records of high river flow recorded by instruments at Vicksburg, Mississippi.

The nation's current worst oil spill could worsen and expand the oxygen starved region (dead zone) of the Gulf labeled suggests Michigan State University professor Nathaniel Ostrom. It could already be feeding microbes that thrive around natural undersea oil seeps, he says.

"At the moment, we are seeing some indication that the oil spill is enhancing hypoxia," or oxygen depletion, Ostrom said. "It's a good hint that we're on the right track, and it's just another insult to the ecosystem; people have been worried about the size of the hypoxic zone for many years."

The original Gulf dead zone is believed to stem from urban runoff and nitrogen based fertilizers from farmland swept into the Gulf by the Mississippi River. Higher springtime flows carry a heavier surge each year, nourishing algae blooms that soon die and sink (consuming water dissolved oxygen). Those decay and are eaten by bacteria that consume more oxygen, driving out other marine life and killing that which can't move, such as coral.

Oil hungry microbes can be expected to consume more oxygen from the water as they feast on hydrocarbons, Ostrom says. But the oil slick and chemical dispersants also could reduce the flow of oxygen from the atmosphere to the ocean, and possibly reduce the sunlight available to nourish oxygen producing marine plant life.

In essence the oil release is providing a strong food source for some microbes who will consume the oil but use up all of the oxygen resulting is a dead zone that may grow even larger.

Dead zones are reversible. The Black Sea dead zone, previously the largest dead zone in the world, largely disappeared between 1991 and 2001 after fertilizers became too costly to use following the collapse of the Soviet Union and the demise of centrally planned economies in Eastern and Central Europe. Once the dead zone went away the local fishing improved.