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.