A dead zone isn't quite like a zombie apocalypse, but it's nearly as bad. Simply put, a dead zone is an underwater area where most plants and animals have almost no chance of surviving because of the lack of dissolved oxygen in the water.
What causes a dead zone, and is there any hope of restoring an area that's been classified as one? Here's a basic introduction to dead zones around the world, including America's own Gulf of Mexico dead zone.
Welcome to the Dead Zone
In order to understand dead zones, it helps to get a little information on basic biology. First, remember that algae, phytoplankton and a host of other microorganisms depend on nutrients in the water to survive. When there aren't enough of these nutrients -- mainly phosphorous and nitrogen -- these organisms don't survive. But when there are plenty of these nutrients, the organisms flourish and their populations skyrocket.
But this population increase isn't all good news: Massive amounts of nutrients create huge "algae blooms" that can, in severe cases, cause the water to change color to that of the organisms that are thriving, usually green or red. This process, also called eutrophication, results in billions of organisms reproducing, living and eventually dying. Once dead, their bodies sink and decompose.
The microbes responsible for the decomposition process use oxygen, and this is what causes the lack of dissolved oxygen in the water of a dead zone. When there's no oxygen in the water -- a condition called hypoxia -- plants and animals living on the floor of the ocean or lake can't survive.
What Causes a Dead Zone?
A dead zone is created in a lake or ocean when huge amounts of phosphorous or nitrogen are flushed into the body of water. This sometimes happens naturally after a large flood, but in most cases today, the nutrients come from farmland that's been fertilized with phosphorous- or nitrogen-rich fertilizers.
As modern agriculture strives to increase crop yields by applying enormous amounts of fertilizers to farmland, dead zones worldwide are growing in number and in size. At the mouths of just about any large river (especially in a developed or developing country), there's a sizable dead zone.
Perhaps the largest dead zones in the world are found around the Baltic Sea in Northern Europe, home to seven dead zones. Other dead zones are located off the coast of New Zealand, China and Japan. There are also large dead zones in the Black Sea, the Adriatic Sea and Chesapeake Bay. The size of these dead zones varies from year to year, as the amount of rainfall and fertilizer use can change from one season to the next.
The Gulf of Mexico Dead Zone
The second largest dead zone in the world is found in the Gulf of Mexico, where the Mississippi River flows into the gulf. Draining an area over 1,245,000 square miles, the Mississippi watershed extends from New York State west to Idaho, and as far north as Canada.
This huge watershed carries not only industrial toxins but many tons of nutrient-rich fertilizer into the gulf, where it fuels immense algal growth. As a result, the gulf dead zone can grow as large as it did in 2002 -- 8,400 square miles, larger than the state of Massachusetts.
The recent drought in the Midwest, however, has had the unusual effect of shrinking the size of the gulf dead zone. An article in The New York Times finds that in 2012, the dead zone shrunk to less than 3,000 square miles -- still a large area, but not as large as it's 2002 record.
As worldwide agricultural practices encourage the massive use of fertilizers, many scientists fear that dead zones worldwide will continue to grow, threatening marine life and fisheries around the globe. According to Carlton College researchers, "The Gulf of Mexico is a major source area for the seafood industry. The Gulf supplies 72 percent of U.S. harvested shrimp, 66 percent of harvested oysters, and 16 percent of commercial fish. Consequently, if the hypoxic zone [dead zone] continues or worsens, fishermen and coastal state economies will be greatly impacted."
Fortunately, the growth of dead zones can be reversed. In addition to limiting sewage runoff and the use of fertilizers, increasing the size and diversity in wetlands in rivers and along coastlines will mitigate the amounts of nitrogen and phosphorus that creates dead zones.