Dead zones
Dead zones refer to aquatic environments that cannot sustain life due to severely depleted oxygen levels. These areas have been increasing in frequency, particularly in coastal and lakeshore regions heavily influenced by human activity since the late 20th century. The primary cause of dead zones is eutrophication, which occurs when excess nutrients, mainly nitrogen and phosphorus from agricultural fertilizers and urban runoff, promote uncontrolled algae growth. When these algae die and decompose, the process consumes oxygen, leading to hypoxic conditions that can suffocate marine organisms.
More than 140 dead zones have been identified globally, with notable examples including the large dead zone in the Baltic Sea and a seasonal hypoxic zone in the Gulf of Mexico at the mouth of the Mississippi River. These dead zones significantly impact local fisheries and coastal economies by diminishing fish populations. However, some dead zones can recover, as demonstrated by the Black Sea, which saw dramatic improvements after a reduction in fertilizer use following the collapse of the Soviet Union. Addressing the issue of dead zones requires coordinated political action to manage nutrient inputs and promote sustainable practices to prevent further environmental degradation.
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Subject Terms
Dead zones
DEFINITION: Aquatic environments incapable of sustaining life
The occurrence of dead zones in waters along heavily inhabited lakeshores and coastlines has increased drastically since the end of the twentieth century. Human nutrient inputs into these ecosystems from fertilizer use and pollution have contributed to more frequent eutrophication events, fueling this source of economic and ecological devastation.
In aquatic ecosystems, just as in all others, there are certain nutrients whose limited presence or availability prevents the unrestrained growth of particular species. In nearshore marine waters and lakes these so-called limiting nutrients are nitrogen and phosphorus. Their restricted availability keeps microscopic organisms called algae from reaching unsustainable and destructive levels.
Normally these algae form the base of the food web, utilizing the sun’s energy to convert carbon dioxide into oxygen and food. However, in the presence of excess nitrogen and phosphorus, the algae can reproduce uncontrollably in a process called eutrophication, resulting in an algal bloom. As the algae population peaks and begins its decline, microbes begin decomposing the dead algae, consuming oxygen in the process. Eventually the dissolved oxygen in the aquatic environment is depleted, resulting in the death of all organisms in the ecosystem. At this stage, the environment is deemed a hypoxic or dead zone.
In some cases, an influx of nitrogen and phosphorus into an aquatic ecosystem can be the result of natural processes. In certain areas on the western coasts of the continents, for example, a process called upwelling occurs, in which prevailing winds and currents bring nutrients up from the ocean depths and to the surface, where algae live. Humankind’s interference with the natural cycling of nitrogen and phosphorus, however, has led to significant inputs of these nutrients into aquatic ecosystems as well. Nitrogen and phosphorus are mass-produced for use as fertilizers. After the fertilizers are sprayed on crop fields, rainwater and carries these nutrients into rivers and streams, which in turn feed estuarine and marine habitats. Likewise, inadequately treated and laden with nutrients from cities and towns is often pumped into aquatic habitats.
More than 140 dead zones have been documented around the world, and the number continues to rise. The largest known dead zone is located in the Baltic Sea, where agricultural runoff and poorly treated sewage contribute to a dead zone tens of thousands of square kilometers in size. Similarly, in the United States a seasonal hypoxic zone the size of New Jersey forms during the summer months at the mouth of the Mississippi River in the Gulf of Mexico. Fishery yields there and in other locations affected by dead zones have sharply declined, sapping the lifeblood of coastal economies and cutting into marine food production.
While ominous, dead zones are not necessarily permanent. The infamous Black Sea dead zone was once the largest in the world, covering an area of approximately 40,000 square kilometers (15,000 square miles). When the Soviet Union collapsed in 1991, the industrialized agricultural economy of the region also collapsed, and the resulting drop in fertilizer use cut nitrogen and phosphorus input into the Black Sea by more than half. Within about a decade, the dead zone virtually disappeared. Although this example provides hope, the ecosystem of the Black Sea has yet to recover fully, as it supports limited marine life. To combat dead zones efficiently, political action through management of fertilizer use and will be needed, not only to rein in current dead zones but also to prevent future outbreaks entirely. As of 2024, the Arabian Sea was the largest dead zone in the world, followed by a hypoxic region of the Gulf of Mexico.
Bibliography
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Vernberg, F. John, and Winona B. Vernberg. The Coastal Zone: Past, Present, and Future. Columbia: University of South Carolina Press, 2001.
"What Is a Dead Zone?" National Oceanic Service, oceanservice.noaa.gov/facts/deadzone.html. Accessed 17 July 2024.