Downsizing The Gulf Of Mexico’s Dead Zone

By Valerie Brown
21 July 2010

The Gulf of Mexico has taken some hard hits recently, including Hurricane Katrina and the Deepwater Horizon disaster. But it has also had a chronic and worsening affliction: an oxygen-depleted “dead zone” that stretches along the coast from just west of the Mississippi River delta to Texas. Now new research (Environ. Sci. Technol., DOI: 10.1021/es903521n) suggests that shrinking this blight may require a dramatically larger reduction in the nitrogen pollution that fuels it than scientists previously thought.

Pollution such as agricultural runoff in the Mississippi River’s drainage creates regions of low oxygen content or hypoxia in the Gulf. Nitrogen and phosphorus compounds in this drainage stimulate algal blooms. Eventually the algae die and sink to the ocean bottom, where bacteria feed on them and then deplete the water of oxygen. When oxygen concentrations reach 2.0 ppb or less, marine life either leaves the area or dies.

Scientists first detected the Gulf’s dead zone in the early 1970s, and in the last 25 years it has grown alarmingly, sometimes topping 8,000 sq miles in size. Further, in the early 1990s the zone underwent an abrupt change in its sensitivity to pollution: Now a dose of these chemicals makes the zone grow larger than the same amount had done in previous years.

University of Michigan ecologist Donald Scavia and colleagues wanted to analyze how this sensitivity shift changed the way nitrogen compounds influenced the dead zone’s growth. In a previous study (Environ. Sci. Technol., 2007, 41, 8111), Scavia and his team modified a hypoxia model originally designed for river systems and found that nitrogen is a stronger driver of the dead zone than phosphorus.

In the present study, the researchers further adjusted this model to address uncertain data on the factors that control the dead zone’s growth, such as how much oxygen reaches the hypoxic region from freshwater inputs and how much seafloor organisms influence nitrogen recycling in the Gulf. The scientists used a statistical algorithm to find the most likely values for these variables and then calculated average dissolved oxygen concentrations in the Gulf.

According to this model, if the zone’s sensitivity remains unchanged, nitrogen inputs must decline by about 70% to shrink it to an average size of 1,930 square miles by 2015, the target set by the Environmental Protection Agency’s 2008 Gulf Hypoxia Action Plan. That plan called for a 45% reduction in nitrogen pollution.

Texas A&M University oceanographer Steven DiMarco, who also models Gulf hypoxia and was not involved in the current study, believes Scavia’s model “doesn’t take into account all the physical processes that are present in the Gulf.” A better model would include the effects of tides, winds and currents, DiMarco says.

And Virginia Dale, an ecologist at the Oak Ridge National Laboratory in Oak Ridge, Tennessee and chairwoman of the EPA’s Hypoxia Advisory Panel in 2007, thinks that the model misses another important factor influencing Gulf hypoxia: phosphorus. But, she says, the Scavia model has “done a good job of looking at sensitivity of nitrogen loads.”