No Oxygen, No Life: The Gulf of Mexico’s ‘Dead Zone’

The words “dead zone” evoke sci-fi movie scenes of a Martian landscape or the aftermath of a nuclear blast. Bleak places where life is...

The words “dead zone” evoke sci-fi movie scenes of a Martian landscape or the aftermath of a nuclear blast. Bleak places where life is absent and things go bump in the dark. In the Gulf of Mexico, a real dead zone exists, and the suffocating effects to the area’s inhabitants are as frightening as a horror film.

The Gulf of Mexico dead zone is an oxygen-starved area devoid of most marine life. It is off the Louisiana coast where the Mississippi River empties into the Gulf of Mexico. In 2003, this killer swath of ocean reached a record size of 8,492 square miles (22,079 sq km), larger than the state of Massachusetts.

Seasonal Phenomenon

The dead zone is a seasonal phenomenon that forms each spring as excess nutrients, primarily nitrogen, wash down the Mississippi River and empty into the Gulf of Mexico. An overabundance of nitrogen stimulates the growth of phytoplankton (microscopic algae and other plants) at the water’s surface. These phytoplankton produce oxygen, which remains near the surface. The phytoplankton eventually die or are eaten by floating, often microscopic animals. The byproducts (dead plant matter and fecal waste) from this plentiful feast fall to the ocean floor where they are consumed by bacteria. As colonies of bacteria feed, they deplete oxygen from the ocean bottom.

This isn’t necessarily a problem as long as oxygen-rich surface water can reach and replenish the oxygen-deprived bottom water. But in late spring and early summer, storm frequency decreases, resulting in calmer water. In addition, surface temperatures rise. And the light warm fresh water flowing into the gulf from the Mississippi forms a separate layer above the heavier, colder salt water. Without significant storms to churn the layers together, the bottom of the gulf has oxygen levels too low to support marine life — a condition called “hypoxia.”

On the surface, hypoxic waters can appear normal, but the bottom is eerily devoid of any living animal and littered with the casualties of suffocation. In extreme cases, the sediment turns black.

Fish can swim out of the hypoxic waters, or dead zone, to areas with life-sustaining oxygen levels, but other marine animals such as mollusks, anemones and worms cannot and die. Hypoxic waters kill high levels of aquatic species, disrupting the food chain and habitat. In addition to wreaking ecological havoc, the dead zone can be a financial drain on coastal communities. Changes in the distribution of fish, shrimp and oyster beds can translate into economic losses for the Gulf of Mexico’s multibillion dollar seafood industry. And recreational fishermen go elsewhere, taking their wallets with them.

Hypoxia affects many of the world’s coastal areas including Chesapeake Bay, and the Baltic and Black seas. But the Gulf of Mexico dead zone holds the unfortunate title of the largest area of oxygen-depleted coastal water in the United States and the entire western Atlantic Ocean. The dead zone varies in size every year depending on river flow, wind and other environmental factors, but it has increased since 1985 when scientists first began taking regular measurements. Since 1993, it has exceeded almost 4,000 square miles (10,400 sq km). The dead zone reaches its peak in early summer, and is broken up in late August or September by cooler surface temperatures and tropical storms or hurricanes.

Foul Waters

The primary culprit responsible for the growing size of the dead zone is an increasing supply of nitrogen dumped into the Gulf of Mexico from the Mississippi River. Nitrogen levels discharged from the river have nearly tripled since the 1950s, with an annual average since 1980 of 1.6 million metric tons. Sources of nitrogen include fertilizers, animal manure, sewage runoff, industrial waste and atmospheric pollutants.

The Mississippi River basin, at 1.2 million square miles (3.1 million sq km), is the largest river basin in North America and the third-largest in the world. It includes 40 percent of the United States, drains 30 states, and is home to 70 million people or nearly one-quarter of the country’s population. It extends from the Appalachian to the Rocky mountains, and from southern Canada to the Gulf of Mexico. The majority of all agricultural chemicals used in the United States are dumped on the Mississippi River basin’s cropland, which comprises 58 percent of the area. According to a U.S. Geological Survey study, more than half of the nitrogen entering the gulf from the Mississippi appears to come from agricultural sources. Lands in southern Minnesota, Iowa, Illinois, Indiana and Ohio contribute the bulk of the basin’s nitrogen.

Taming a River

In addition to increased nitrogen levels from the Mississippi River basin, other factors contribute to the dead zone. Before humans tinkered with the mighty Mississippi in an attempt to control flooding, the river had the ability to spill over its banks when it got too full. The excess water would seep into belts of natural vegetation along the river, called riparian zones, and into wetlands. Both riparian zones and wetlands act as filters, cleansing flood water of excess nutrients.

Landscape changes in the basin have hampered the river’s ability to rid itself of excess nutrients. The river has been diverted, straightened and levied in many places. Also, most of the original freshwater wetlands and riparian zones in the basin are gone. Ohio, Indiana, Illinois and Iowa have drained more than 80 percent of their wetlands. Indiana, Illinois, Iowa, Minnesota, Missouri and Wisconsin collectively have lost 35 million acres of wetlands over the past 200 years.

Reducing the Dead Zone

Can the Gulf of Mexico dead zone be eliminated? Probably not, given the country’s current standard of living with its accompanying forms of pollution — fertilizers, pesticides, sewage, industrial waste, etc. But according to an Environmental Protection Agency action plan developed by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, the dead zone can be reduced.

The plan is the result of a multiyear study on the dead zone conducted by leading scientists in the field. A major goal of the plan is to reduce the size of the dead zone to less than 1,930 square miles (5,018 sq km), or about half of the average by the year 2015. The best way to accomplish this is to reduce the amounts of nutrients, particularly nitrogen, entering the gulf from the Mississippi River. A 20 percent to 30 percent reduction of nitrogen may be necessary to have a significant positive effect on marine life. And a 40 percent reduction is required to return nitrogen levels to those during 1955-1970 when hypoxia was less of a problem.

On paper, the plan sounds simple. In practice, significant changes will need to take place. A combination of activities will be necessary, including reducing fertilizer use on farms, establishing wetlands and riparian zones near streams to soak up excess nitrogen, and reducing discharges from sewage treatment and industrial plants. The size of the dead zone every year will indicate whether the plan works.

Making the Connection

The dead zone is, in part, the result of human-caused factors. It is also proof that everything is connected — a Minnesota farm and a Wisconsin sewage plant can affect marine life a thousand miles away. In the horror movies, people flee from killer forces or work together to combat them. In the case of the Gulf of Mexico dead zone, the choice is clear.

On the surface, hypoxic waters can appear normal, but the bottom is eerily devoid of any living animal and littered with the casualties of suffocation. In extreme cases, the sediment turns black.

Resources To learn more about the Gulf of Mexico dead zone, visit:

Science Museum of Minnesota:

Web site:

Environmental Protection Agency:

Web site:


By Amy Gulick