Should we be worried about the dead zone in the Gulf of Mexico?

By: Jacob Silverman  | 
Photo courtesy of NASA/Goddard Space Flight Center Scientific Visualization Studio The above map shows concentrations of phytoplankton, the algal blooms that contribute to dead zones, in Gulf Coast waters.

Every spring, a vast area of the northern Gulf of Mexico loses most of its oxygen and becomes deadly to marine life. The Gulf of Mexico dead zone, which is also called a hypoxic zone, is caused by the growth of massive quantities of algae known as algal blooms. As algae die, bacteria feed on them and, in the process, suck up the water's available oxygen. The resulting low oxygen levels in the water kill fish and other marine life.


What Causes the Algal Blooms?

Algal blooms occur when the surface waters contain excess nutrients, particularly nitrogen and phosphorus, fostering rapid algae growth. These harmful blooms partly arise from natural processes, but human activities have exacerbated their frequency and intensity. The Mississippi River and the Atchafalaya River discharge into the Gulf of Mexico, carrying with them pollutants like fertilizer and sewage from the expansive Mississippi River Basin. The Basin collects water from various tributaries around the country, contributing to the nutrient runoff entering the Gulf. With springtime's arrival and snowmelt, these tributaries transport an even greater amount of nutrients, creating a fertile environment for algae growth, particularly in warm waters.


Dead Zones and Their Impact

The 2007 dead zone is one of the largest since measurements began in 1985. It was mapped at around 7,900 square miles — bigger than several U.S. states [Source: NBC News]. The 2006 dead zone was 6,662 square miles [Source: BBC], while the one in 2002 measured 8,495 square miles [Source: Reuters]. The dead zone was 8,776 square miles in 2018 — the largest we've seen yet [Source: The Progressive Farmer].

In 2007, the level of algae-boosting nutrients entering the Gulf of Mexico was triple the levels of a half-century before, when dead zones were an infrequent occurrence. A scientist from Louisiana State University attributed the change to an increase in intensive farming, which generally employs lots of nitrogen-rich fertilizers, combined with effects from the weather.


The National Oceanic and Atmospheric Administration (NOAA), which monitors the dead zone, said the area presents a danger to the $2.8 billion-per-year fishing industry that operates along the Texas and Louisiana coasts [Source: NOAA]. Millions of pounds of brown shrimp are caught every year in these waters, but over the last decade, fishermen have reported declining brown shrimp catches. Shrimp may be dying or simply swimming to other, more breathable waters.

Is the Gulf of Mexico the Only Hypoxic Zone?

The Gulf of Mexico dead zone isn't the world's only hypoxic zone. For years, Lake Erie has suffered from a recurring dead zone, believed to be a result of a combination of phosphorus contamination, invasive mussel species and a warming climate. A report from the United Nations said the number of seasonal dead zones around the world had doubled every 10 years since the 1960s. A NOAA report cites low oxygen levels as a major problem in shallow waterways and coastal areas worldwide.


Causes of Dead Zones

The above picture shows some dead crabs that washed ashore on the Oregon coast, victims of a large recurring dead zone in that area.
Elizabeth Gates/Associated Press

Dead zones occur naturally, but human activity is making them much worse by allowing tributaries to become overfilled with some nutrients while those tributaries lack other key nutrients [Source: National Ocean Service]. Nitrogen and phosphorus are the nutrients that contribute most to algal blooms. A lack of silicon in the water limits the growth of diatoms, a helpful type of algae. So where's this nutrient pollution coming from? Intensive farming is a practice commonly linked to dead zones.

Intensive farming, also called intensive agriculture, uses a large investment of capital and some combination of fertilizer, pesticides, fungicides, heavy machinery, irrigation and other modern farming techniques to maximize output from a plot of land. The practice is characterized by higher productivity and requires fewer laborers than extensive agriculture.


Critics accuse intensive-farming practitioners of harming the environment by creating animal waste and fertilizer runoff, using dangerous pesticides, and contributing to animal disease. Today, intensive farming is both quite pervasive and productive, although the use of fertilizers, chemicals and safe environmental practices can vary drastically depending on the farmers and government regulations.

Some scientists cite the ethanol craze as a contributor to the dead zone [Source: University of Wisconsin–Madison]. The use of biofuel crops means more corn than ever is being planted in the United States. Corn requires a lot of fertilizer, which is full of nitrogen that seeps into groundwater and ends up in the Mississippi by way of local rivers. Nitrogen levels in the Mississippi River were up 35 percent in May 2007 compared to 2002, and the river's water levels were down more than 20 percent compared to five years prior, causing a huge influx of algal blooms [Source: Herald-Tribune].

The loss of wetlands has drastically reduced the ability of regional ecosystems to remove nitrogen from local waters. The lower 48 states have lost more than 50 percent of the wetlands they originally had. Alaska and Hawaii have also lost some wetlands [Source: DOE Office of Scientific and Technical Information].


Reducing Dead Zones

Despite the dead zone's gradual expansion, scientists argue that we have the capability to reduce it. Limiting the use of nitrogen-rich fertilizers, implementing water conservation and recycling practices, and preventing sewage leaks and runoff from waste treatment plants should all help to keep nitrogen levels down. In 1998, the U.S. Congress passed the Harmful Algal Bloom and Hypoxia Research and Control Act, which called for containing harmful algal blooms and hypoxia. Researchers at universities and the NOAA are using modeling techniques to estimate how much of certain compounds need to be removed in order to reduce the dead zone's size.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.