Ocean Conservation Image Gallery
Ocean Conservation Image Gallery

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. See more ocean conservation pictures.

Introduction to the Dead Zone

Every spring, a vast area of the northern Gulf of Mexico loses most of its oxygen and becomes deadly to marine life. The "dead zone," 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. Oxygen levels become depleted to the point that the area cannot support marine life, and sea creatures must swim to other waters or die. Beside being inhospitable to most sea life, algal blooms also cause dead zone waters to turn brown.

Ocean Conservation Image Gallery

What causes the algal blooms? In part, it's a natural phenomenon, but they've been significantly boosted by fertilizer, sewage and other pollutants entering the Gulf of Mexico from the Mississippi and Atchafalaya Rivers, both of which are fed by bodies of water from around the country. These pollutants contain phosphorus and nitrogen, which are excellent food for algae. When springtime comes and the snows melt, increased water levels bring more nutrients for the algae, which also thrive in warm water. The dead zone peaks around early August and then recedes in the fall, when nitrogen levels in water diminish.

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: CNN]. The 2006 dead zone was 6,662 square miles [Source: BBC], while the one in 2002, the largest on record so far, measured 8,495 square miles [Source: Reuters].

In 2007, the level of algae-boosting nutrients entering the Gulf of Mexico represented a 300 percent increase over levels of a half century before, when dead zones were an infrequent occurrence [Source: BBC]. 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 [Source: BBC].

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.

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 in 2003 said that the number of seasonal dead zones around the world had doubled every 10 years since the 1960s [Source: BBC]. A NOAA report cites low oxygen levels as a major problem in shallow waterways and coastal areas worldwide.

T­here are other problems for fishing communities, too. Harmful algal blooms (HABs), like red tide and golden algae, produce toxins when they decay, killing marine life and making the creatures poisonous to humans. But HABs shouldn't be confused with the algal blooms described above. While human pollution contributes to the Gulf of Mexico's dead zone and other hypoxic zones, scientists have yet to establish a connection between pollution and HABs, which occur naturally.

On the next page, we'll take a closer look at the causes of the dead zone and what's being done to prevent it from growing even larger.

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

Causes of Dead Zones

The dead zone occurs naturally, but human activity is making it much worse by allowing tributaries to become overfilled with some nutrients while those tributaries lack in other key nutrients. Nitrogen (in saltwater) and phosphorus (in freshwater) 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 the practice most 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 through creating animal waste and fertilizer runoff, using dangerous pesticides, contributing to animal disease and providing inhumane conditions for livestock. 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 regulation.

Some scientists cite the ethanol craze as a contributor to the dead zone. 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: The Herald Tribune].

Loss of wetlands has drastically reduced the ability of regional ecosystems to remove nitrogen from local waters. Seven Midwestern states lost 35 million acres of wetlands over the last 200 years, while 50 percent of wetlands are gone from Tennessee, Louisiana, Mississippi and Arkansas [Source: NOAA].

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 examining the research and working to contain 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.

Ironically, the dead zone could be positively affected by an active hurricane season. A major contributing factor to dead zones is when water becomes stratified -- warm, fresh water settles on top of colder, saltier water. This stratification limits the aeration of deeper waters as algal blooms settle to the bottom and decay. A hurricane could stir up the Gulf waters, dispersing some of the algae and partially replenishing oxygen levels. The NOAA predicts seven to 10 hurricanes for 2007, with three to five of them qualifying as "major hurricanes" [Source: NOAA]. While these storms may stir up the dead zone and possibly increase the brown shrimp catch, they will come at a time when Gulf Coast communities are still recovering from the devastation of hurricanes Katrina and Rita.

For more information about dead zones, algal blooms and other related topics, check out the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

Sources
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  • "Distribution of HABs in the US." The Harmful Algae Page. http://www.whoi.edu/redtide/HABdistribution/HABmap.html
  • "Gulf dead zone to be biggest ever." BBC News. July 18, 2007. http://news.bbc.co.uk/2/hi/science/nature/6904249.stm
  • "Harmful Algal Blooms." Texas Parks and Wildlife. Feb. 9, 2007. http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/
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