Continental shelves and pollution
Continental shelves are nearly flat land platforms that extend into the sea at the edges of continents, playing a crucial role as productive marine ecosystems. However, these regions are increasingly threatened by various forms of pollution stemming from human activities. Direct pollution sources include agricultural runoff containing chemical fertilizers and industrial waste, which can lead to hypoxic zones—areas low in oxygen where marine life struggles to survive. Indirectly, construction projects and waste dumping are altering seabeds and degrading habitats critical to biodiversity. The degradation of coral reefs along these shelves, exacerbated by pollution and destructive practices, further highlights the environmental challenges faced. Notable incidents, such as the near extinction of the brown pelican due to pesticide contamination, underscore the severe consequences of pollution on marine species. Additionally, oil spills from offshore drilling pose significant threats, impacting vast areas of the ocean and coastlines. Efforts to legislate protections, such as the National Marine Sanctuaries Program in the United States, reflect growing recognition of the need to preserve these vital ecosystems. Understanding the complex interactions between human activities and continental shelf health is crucial for future marine conservation efforts.
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Subject Terms
Continental shelves and pollution
DEFINITION: Nearly flat platforms of land that extend into the seas at the margins of continents
Human activities have had many negative environmental impacts on the world’s continental shelves, both directly and indirectly. These impacts include pollution of the waters of the shelves’ ecosystems through runoff from land, the direct dumping of wastes, and oil extraction; the construction of artificial reefs and breakwaters; and the dredging of sand and gravel.
Continental shelves constitute the parts of the oceans most utilized by humans, but much of the environmental degradation that the shelves have undergone has resulted indirectly from human activities on land. The waters of the continental shelves in many parts of the world have become polluted by chemical fertilizers, industrial wastes, and released into the seas from rivers and storm drains. Such threatens the plants and animals present in coastal ecosystems.
During the early 1960s, for example, the brown pelican, a seabird species native to the coasts of the Americas, almost became extinct owing to coastal pollution. Scientists found that the brown pelican was dying out because the birds were laying eggs with extremely thin shells that would break in the nest, so few hatchlings survived. The cause was determined to be the presence of the Dichloro-diphenyl-trichloroethane (DDT) in coastal waters, which came from agricultural that was deposited in the Gulf of Mexico by the Mississippi River. DDT had accumulated in the bodies of the fish on which the pelicans fed. After the dangers of DDT were recognized, use of the pesticide was eventually banned in the United States.
Chemical pollution of coastal waters remains a problem, however. In some areas, pollution of the continental shelves by agricultural runoff containing fertilizers has led to the creation of hypoxic (oxygen-deprived) areas known as dead zones. The nitrogen and in fertilizers are nutrients that cause to grow at accelerated rates. When the algae die and to the bottom, their decay depletes the dissolved oxygen in the water, and the fish and other marine life die. A seasonal hypoxic zone the size of the state of New Jersey forms during the summer months at the mouth of the Mississippi River in the Gulf of Mexico, and similar dead zones are found on continental shelves around the world.
Coral reefs, many of which develop along the edges of continental shelves, have been increasingly damaged by human activities, both indirectly and directly. Chemical pollution is thought to contribute to the discoloration of these reefs known as bleaching and white-band disease, and serious damage to reefs can result from the dragging of anchors across them and from shipwrecks. Other damaging human activities include the use of harmful chemicals and even dynamite to drive reef organisms out of their hiding places, and the mining of reefs to collect coral for ornamental purposes. Policymakers and legislators have responded to increasing recognition among scientists that the ecosystems of coral reefs need to be protected. In the United States, for instance, the National Marine Sanctuaries Program protects the reefs of the Florida Keys as well as thirteen other pristine marine sites around North America, Hawaii, and American Samoa.
A 2024 study published in Nature Communications indicated that high levels of greenhouse gas emissions from human activities could damage the continental shelves in the Antarctic Marine Protected Areas (MPA) by making them more acidic. This can severely harm marine ecosystem. The study predicted that MPAs could experience severe ocean acidification by 2100, causing the death of organisms such as sea snails and sea slugs.
Dumping of Wastes
Various kinds of environmental damage to the continental shelves have resulted directly from human actions. For example, humans have intentionally changed coastal seabeds—and thus destroyed marine habitats and disrupted ecosystems—by building breakwaters, pilings for bridges, artificial reefs, and wind farms; in addition, in some cases, these areas have been used for the disposal of construction debris, municipal solid waste, and outdated military hardware.
An extreme example of the dumping of on a continental shelf is provided by New York City, which has always faced difficulty in the disposal of construction debris. Manhattan Island is only 21.6 kilometers (13.4 miles) long and 3.7 kilometers (2.3 miles) wide, and it represents some of the most expensive real estate in the world. In 1890 builders in Manhattan began the practice of dumping construction debris and other waste in the New York Bight, an indentation in the coastline between New York State and New Jersey. The mound formed on the continental shelf in this area was soon so large that it was jokingly called an underwater Mount Everest. Amendments made in 1986 to the Marine Protection, Research, and Sanctuaries Act of 1972 (also known as the Ocean Dumping Act) prohibited dumping in this area, and a new site was opened 170 kilometers (106 miles) offshore. Since that time, an even larger mound of waste materials has been accumulating on the seabed at a depth of 2,500 meters (8,200 feet).
The dumping of obsolete military hardware in coastal waters creates special problems because such hardware may contain materials that are hazardous. Two crewmen on a clam boat off the shore of Massachusetts had to be hospitalized in June 2010, for example, after they hauled up World War I-era military shells filled with mustard gas along with a load of clams. It turned out that the US military had been using the area as a dumping ground for munitions from after World War II through 1970.
Dredging and
In addition to building and dumping things on continental shelves, humans also take things out. Sand is routinely dredged from the continental shelf off southern Florida, for example, to “nourish” beaches that have lost their sand due to erosion, and some cities that are heavily dependent on tourist revenue, such as Delray Beach and Palm Beach, have had their beaches rebuilt several times. In countries that are highly industrialized and densely populated, continental shelves may be mined for the sand and gravel required for construction. It has been estimated that 25 percent of the sand and gravel needed for construction in southeastern England comes from offshore; the proportion is believed to be even higher for Japan. In addition to exploiting continental shelves for sand and gravel, humans also mine deposits of valuable minerals, such as diamonds and gold, on some of the world’s continental shelves.
Oil spills represent another way in which humans degrade the shelves. Production platforms and drilling rigs jut up offshore in areas where oil and gas deposits are found, and the oil that can leak or spill from these rigs has the potential to foul the ocean and seabed for hundreds of miles around. Two examples of disastrous spills that resulted from such offshore oil extraction have taken place in the Gulf of Mexico. In 1979 a blowout in an exploratory well drilled by Petróleos Mexicanos (PEMEX), the Mexican state-owned oil company, in 50 meters (164 feet) of water off the coast of Mexico caused a spillage of oil that fouled the seabed and beaches as far away as Texas. An even greater tragedy was the blowout of BP’s Deepwater Horizon rig off the coast of Louisiana in 2010. Heavy reddish-brown oil from this spill fouled the continental shelf and beaches as far away as Florida, even though the well was drilled 72 kilometers (45 miles) offshore in water 1,524 meters (5,000 feet) deep.
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