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Dead zone

Dead zone

Dead zone

Encyclopedic entry. Dead zones are low-oxygen, or hypoxic, areas in the world’s oceans and lakes. Because most organisms need oxygen to live, few organisms can survive in hypoxic conditions.

Dead zones are low-oxygen, or hypoxic, areas in the world’s oceans and lakes. Because most organisms need oxygen to live, few organisms can survive in hypoxic conditions. That is why these areas are called dead zones.

Dead zones occur because of a process called eutrophication, which happens when a body of water gets too many nutrients, such as phosphorus and nitrogen. At normal levels, these nutrients feed the growth of an organism called cyanobacteria, or blue-green algae. With too many nutrients, however, cyanobacteria grows out of control, which can be harmful. Human activities are the main cause of these excess nutrients being washed into the ocean. For this reason, dead zones are often located near inhabited coastlines.

Understanding the eutrophication process provides the clearest picture of how and why dead zones develop.

Causes of Eutrophication

Eutrophic events have increased because of the rapid rise in intensive agricultural practices, industrial activities, and population growth. These three processes emit large amounts of nitrogen and phosphorous. These nutrients enter our air, soil, and water. Human activities have emitted nearly twice as much nitrogen and three times as much phosphorus as natural emissions.

Different regions of the world emit different levels of these nutrients. In developed countries, such as the United States and nations in the European Union, heavy use of animal manure and commercial fertilizers in agriculture are the main contributors to eutrophication. Runoff from large agricultural fields enters creeks and bays because of rain or irrigation practices.

In developing countries of Latin America, Asia, and Africa, untreated wastewater from sewage and industry mainly contribute to eutrophication. Factories and sewage facilities are less regulated than they are in developed countries, and sometimes wastewater is simply dumped into creeks, rivers, lakes, or the ocean.

Atmospheric sources of nitrogen also contribute to eutrophication in some areas of the world. Fossil fuels and fertilizers release nitrogen into the atmosphere. This atmospheric nitrogen is then redeposited on land and water through the water cycle—rain and snow.