Dead zone is a region of water in which oxygen levels are too low to support most animal life. Dead zones can occur in rivers, lakes, oceans, and other bodies of water. Dead zones can form naturally, but today most such areas form as a result of human activities.
Dead zones usually are caused by a process called eutrophication. Eutrophication may begin when water becomes enriched with excessive nutrients (nourishing substances). The nutrients promote the rapid growth of tiny algae. Algae are plantlike organisms that make their own food using sunlight in a process called photosynthesis. Algae that reproduce quickly form vast growths called algal blooms. While some of these algae are eaten and support life higher in the food chain, many eventually die. Then their remains are decomposed by other microbes. Through respiration, this process uses up oxygen in the water. As oxygen levels drop, animals and other living things in the water may die. The microbes feed on their remains, further reducing oxygen levels.
The excessive nutrients that lead to eutrophication can come from natural sources, but they usually accumulate through pollution created by human beings. The runoff of fertilizers used in agriculture ranks as one important source of nutrient pollution. Sewage is another major source. Most dead zones form in coastal waters near rivers, where nutrient pollution collects from sources upstream.
The formation or spread of a dead zone can do great harm to living things. Sea grasses and seaweeds may die. Fish may die or may flee to other areas. Such animals as clams and oysters that cannot relocate are likely to die. Living things that do not die may become stressed, harming their growth and reproduction. Larvae (young) can be especially vulnerable.
Dead zones have greatly increased in number and size since the mid-1900’s, mostly because of increasing pollution. While some dead zones continue the year around, hundreds of dead zones now form in coastal areas of the oceans each year on a seasonal basis. These dead zones can span tens of thousands of square miles or kilometers. The largest dead zones include zones in the Gulf of Mexico, the Baltic Sea, and the East China Sea.
Reducing water pollution can limit the size and occurrence of dead zones. However, nutrient pollution of rivers and coastal waters is generally increasing. Global warming will likely worsen the problem, because warmer waters cannot hold as much oxygen and warmer temperatures also speed the growth of algae.
