What is a dead zone?

A dead zone is an area where no life exists. In marine ecosystems, dead zones are caused by excess nutrients that lower the available oxygen for fish and other marine life, creating a zone where no life can exist. Dead zones can also be caused by the input of hot wastewater into a marine ecosystem. Habitats that would normally be teeming with life are transformed into biological deserts.

An increase in chemical nutrients such as nitrogen and phosphorous are the major contributing factor to dead zones. These nutrients are the fundamental building blocks of certain types algae, including Cyanobacteria, Green Algae, Dinoflagellates, Coccolithophores, and Diatom Algae. The increase in these nutrients leads to a rapid increase in the density of these organisms, creating an algal bloom. This is called eutrophication, which is defined as an excess of nutrients in a body of water due to runoff from the land that increases plant production and therefore causes death of species due to lack of oxygen. Algae blooms result from the inputs of nitrogen and phosphorous, especially Cyanobacteria, but these Cyanobacteria are not good food for zooplankton and fish, so they accumulate in the water, die, and then decompose. The process of decomposition leaves the water with uninhabitable oxygen levels for most fish and other species.

The use of chemical fertilizers, mainly by big agriculture, is the main cause attributed to eutrophication in inland and coastal waters. Runoff from sewage, urban land use, and fertilizers used in home lawns and gardens also contribute to dead zones, but while this is still a very important piece to the puzzle, the levels contributed by big agriculture are much higher than domestic contributions. Natural causes can include coastal upwelling and changes in wind and water circulation patterns.

Oceanographers began noticing increased cases of dead zones during the 1970s. The UN Environment Programme published its first Global Environment Outlook year Book in 2004, and in it they reported 146 dead zones in the world’s oceans. These dead zones were directly attributed to depleted oxygen levels. The largest dead zone was 70,000 square kilometers, or 27,000 square miles. In 2008, there were 405 known dead zones worldwide.

While dead zones, also known as hypoxic zones, do occur naturally, scientists are concerned with the ones that are created by anthropocentric activities. Dead zones are most present in the United States on the East Coast and the Gulf of Mexico, as well as the Great Lakes. The second largest hypoxic zone in the world is in the northern Gulf of Mexico.

A 2013 map of the dissolved oxygen levels in Gulf of Mexico 

The dead zone in the Gulf of Mexico is ever-expanding, according to scientists. As of August 2015, the National Oceanic and Atmospheric Administration measured the hypoxic zone measured 6,474 square miles, larger than the measurement taken by NOAA in June of this year. The increase in size was likely caused by a wet summer in the eastern half of the country along the Mississippi watershed. Last year, the dead zone was measured at 5,052 square miles. The Gulf of Mexico/Mississippi Watershed Nutrient Task Force has a target of 1,900 square miles for this dead zone. 

The impact that dead zones have on ecosystems is tremendous. The massive loss of life that we are seeing can have a chain effect, creating holes in food chains that cause problems in ecosystems all over the world. We do not yet know the full extent of the effects. Dead zones also have detrimental impacts on human life. An area that is unable to support marine life is an area that is unable to produce natural resources and food resources for the human population, effecting the economy and peoples ability to reap benefits from our environment, from the place we call home. 

Other sources: 

National Ocean Service

NASA Earth Observatory