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Into the Dead Zone

EPA researchers are creating a state-of-the-art three-dimensional model of the Gulf of Mexico's dead zone.


Underwater image of dying aquatic life

Every summer a massive area of oxygen-starved water (up to 20,000 square kilometers or 7,772 sq. mi., roughly the size of New Jersey) forms across the bottom of the Gulf of Mexico, adjacent to the mouth of the Mississippi River, killing bottom-dwelling marine life below and chasing some creatures farther out to sea.

This dead zone of hypoxic water is the result of an overload of nutrients—from fertilizers and storm runoff to untreated sewage and atmospheric nitrogen—being flushed down the Mississippi and into the Gulf. The nutrients fuel an explosion of plankton in the warm surface water, which then die and are consumed by bacteria below, robbing the bottom waters of oxygen.

The biggest casualties of the dead zone are sedentary animals such as shellfish and worms that live on the bottom. Other fish, such as croaker and menhaden, have been victims of massive fish kills as a result of hypoxia.

This phenomenon has been well-documented by scientists for decades, but now EPA researchers are putting together a state-of-the-art three-dimensional computer model of hypoxia to better predict where it will occur each year, and perhaps suggest ways to reduce its negative effects.

Riparian Buffers Keep Nitrogen
Pollution at Bay
Photo of trees by a river

Letting trees and other vegetation grow along stream and river banks is a relatively inexpensive and environmentally effective way to reduce the amount of excess nitrogen runoff reaching the water.

But how wide do these riparian buffer areas need to be to achieve the desired effects?

EPA scientists Paul Mayer, Tim Canfield, and their colleagues are conducting research to provide answers. Their studies reveal that 50-meter wide buffers reduce nitrogen pollution by about 85 percent; while 26 to 50 meter buffers reduce nitrogen by about 70 percent. Buffers 5 to 10 meters wide are much less effective, only occasionally reducing excess nitrogen runoff that reaches these flowing waterways.

Since Mayer's and Canfield's work was published, it has been used to develop new management practices by several states, including North Carolina, Pennsylvania and Maryland, as well as local water resource management entities in Washington, North Dakota and Colorado.

Learn More: EPA's Wetland Management Research

"The goal is to reduce the dead zone several-fold," said John Lehrter, research ecologist at the EPA's lab in Gulf Breeze, Fla. "We want to know to what degree do we need to reduce nutrients from the Mississippi River basin. The model and field observations are about describing the mechanisms - now we are putting it into this spatial context. With the model results, we can go back to decision-makers and show them what degree of nutrient reduction is required. It needs to be pretty specific and robust in its scientific realism."

Lehrter and his colleagues throughout EPA developed the model in conjunction with physical oceanographers at the U.S. Naval Research Laboratory in Stennis, Miss., who provided data about the swirling currents where the Mississippi River meets the Gulf of Mexico.

Lehrter said that hypoxia is determined by vertical separation, or stratification of the water, which occurs during the spring and summer as a layer of less-dense freshwater originating from the Mississippi River spreads out across the more-dense Gulf water.

"Once you have stratification, the trigger for hypoxia is excess organic matter that sinks down to the bottom. The next question is how it gets consumed by microbes and how much oxygen is drawn down in the process," Lehrter said.

Previous computer models of dissolved oxygen content and hypoxia have not taken into account the changing physical characteristics such as ocean currents, winds, and temperatures that can affect the development of hypoxia. As a result, the new model is more dynamic and more complex—so complex that the model has to be run on EPA's super-computer at Research Triangle Park, North Carolina.

Hypoxic zones have been found in 400 locations across the world along shallow coastal waters. The Louisiana/Texas dead zone is the second largest in the world behind the Baltic Sea dead zone.

As coastal areas become more crowded with people, these hypoxic areas are both expanding and getting worse. Still, Lehrter explained ecological impacts are not well understood. "We know from laboratory experiments that hypoxia is not a good thing, but we don't know in an open system like the Gulf of Mexico, whether mobile organisms could just escape laterally or move up in the water-column. Population level impacts are still an open question, and this is an area of very active research by community and fishery biologists."

Jacques Oliver, a biologist in the EPA's Office of Water in Washington, DC said the new model will help managers better predict how and when hypoxic conditions form. "It's important to understand what the drivers of hypoxia are and learn how to better manage those drivers."

Because of the potential economic effects of limiting nutrients from human activity, Oliver says it is vital to make sure the model is accurate before deploying it.

"Being able to use a water quality model for management purposes is really important. The science has to be first-rate. It has to be peer reviewed. That's what Lehrter's group is focusing on," Oliver explained.

Lehrter says the hypoxia model is being tested this year and will be ready to churn out decision-making products by 2013. Those products will help environmental managers make decisions about how to control the nutrient flows upstream that cause this problem in the first place.

Learn More

EPA Safe and Sustainable Water Research

Mississippi River, Gulf of Mexico Watershed Nutrient Task Force

EPA Website on Nutrient Pollution

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