Ocean pollution

DEFINITION: Introduction by direct or indirect human action of harmful materials into the marine environment

Ocean pollution can harm marine organisms, alter or destroy marine habitats, negatively affect human health, and impede or halt fishing operations. Land-based sources account for the bulk of the pollutants that enter the marine environment (approximately 80 percent); the remainder come from maritime operations and dumping.

Five major types of pollutants can be found in the oceans. The first type, degradable waste, makes up the greatest volume of discharge and is composed of organic material that will eventually be reduced to stable inorganic compounds such as carbon dioxide, water, and ammonia through bacterial attack. Included in this category are urban wastes, agriculture wastes, food-processing wastes, brewing and distilling wastes, paper pulp mill wastes, chemical industry wastes, and oil spillage.

The second type of pollutant is fertilizer, which has an effect similar to that of organic wastes. Nitrates and phosphorus compounds are carried from agricultural lands by runoff from irrigation and rainfall into rivers and from there into the oceans. Once in the sea, they enhance phytoplankton production, sometimes to the extent that the accumulation of dead plants on the seabed produces anoxic (oxygen-deprived) conditions.

A third kind of pollutant, dissipating waste, is composed mainly of industrial wastes that lose their damaging properties after they enter the sea. Their effects are therefore confined to the immediate area of the discharge. Some examples are heat, acids, alkalis, and cyanide.

A fourth type of pollutant is particulates. These are small particles that may clog the feeding and respiratory structures of marine animals. They can also reduce plant photosynthesis by reducing light penetration; when settled on the ocean bottom, they may smother animals and change the nature of the seabed. Some examples are dredging spoil, powdered ash from coal-fired power stations, china clay waste, colliery (coal-works) waste, and clay from gravel extraction. Larger objects such as containers and plastic sheeting have impacts similar to those of particulates.

The last type of pollutant is conservative waste. Such waste is not subject to bacterial attack and is not dissipated, but it can react with plants and animals. Examples of conservative waste include heavy metals, such as mercury, lead, copper, and zinc; halogenated hydrocarbons, such as polychlorinated biphenyls (PCBs) and dioxins; and radioactive materials.

Pollution Inputs from Land

Pollutants can reach oceans in many ways. According to the United Nations Environment Programme (UNEP), land-based activities are responsible for as much as 80 percent of the pollutant load in coastal waters and deep oceans. Rivers that have been polluted by land runoff are the largest source of harmful substances. River water enters the sea carrying a staggering load of pollutants accumulated along the river's entire length. In agricultural areas, nutrients from fertilizers and livestock wastes, as well as silt from eroding fields, are common pollutants in runoff. Herbicides and pesticides that drain from fields introduce carcinogens and other harmful chemical components into receiving waters. In urban areas, runoff carries an array of pollutants. Typical among these are wastes deposited by motor vehicles, including gasoline, oil, grease, and heavy metals; sediment from construction sites; herbicides and pesticides from lawns and gardens; road salts; viruses and bacteria from inefficient septic systems; and spilled chemicals from industrial sites.

Another type of pollutant input from the land is direct outfall from pipes that empty into oceans. This pollution comes from urban and industrial wastes that are deposited directly into coastal waters. During the nineteenth century, rivers such as the Thames and the Tees in England, the Clyde in Scotland, and the Hudson in New York state became incredibly polluted by this type of input. With increasing pressure to preserve inland sources of drinking water, newer industries that need large quantities of water have sought out coastal locations, which has resulted in increased levels of ocean pollution.

Urban areas on the coast also present the problem of municipal wastes and sewage. In many coastal population centers, these wastes are treated. However, roughly 40 percent of the world's population lives on the coast, often in fertile and productive estuarine and delta areas. This makes it almost impossible to process all wastes before they enter the sea. According to a 2010 UNEP report on the state of the marine environment, more than 80 percent of wastewater that is discharged enters water bodies untreated. By 2015 the UN was urging that untreated wastewater be made a focus for policy, especially for developing and low-income countries with large urban populations but fewer resources for treatment available. Subsequent UN programs aimed at both the development of infrastructure and the regulation of wastewater discharge in developing countries.

Pollution Inputs at Sea

On the open water, ships can pollute the ocean in many ways. They often carry toxic substances such as oil, liquefied natural gas, pesticides, and industrial chemicals. Shipwrecks can lead to the release of these chemicals and result in serious damage. Notable examples include the Torrey Canyon (1967), Amoco Cadiz (1978), Exxon Valdez (1989), and Sea Empress (1996) oil spills, which fouled coastlines and caused the deaths of many seabirds and marine animals. Noxious or dangerous materials are frequently carried on deck as a safety precaution and can be lost overboard during storms. During routine operations, ships discharge oily ballast water, bilgewater, and cargo tank washings (not always legally). They also discard much of their garbage overboard. Marine dumping of any form of plastic, however, has been illegal since 1988.

Another offshore pollutant input comes from activities such as dredging spoil and sewage sludge. Dredging spoil is the waste sediment generated by dredging operations, which may be conducted to keep shipping channels and entrances to ports open, as an underwater mining method, or as a means for removing polluted bottom sediments. While dredged material is sometimes disposed of on land, especially if it is highly toxic and in need of treatment, it is often barged out to sea to be dumped. Dredging spoil from harbors contains large quantities of heavy metals and other contaminants that are transferred to ocean dumping grounds. Even clean spoil, however, can have a negative impact on marine life: Dumping it makes the water column more turbid (cloudy), which can fatally clog the respiratory systems of fish and shellfish.

Sewage from the treatment of land-based sewage has also been dumped at sea in large quantities. This sludge may be contaminated with heavy metals, oils and greases, and other substances. Additional offshore industrial activities that can pollute the ocean include oil exploration and extraction, sand and gravel extraction, and mineral extraction. Other harmful wastes that are dumped at sea include ashes from power stations and colliery waste. Until the practice was banned in 1993, containers of low-level radioactive waste were disposed of at deep ocean sites. Conventional munitions, incendiary devices, and chemical weapons were also routinely dumped at sea until the early 1970s.

Pollution Inputs from Air

The last type of input comes from the air. While this type of input is complex and challenging to quantify, scientists believe that it represents a large and significant contribution to pollution. Atmospheric contaminants may exist as gases or aerosols. They are deposited by gas exchange at the sea surface, fall out as particles (dry deposition), or are scavenged from much of the air column by precipitation within clouds (wet deposition). A nontoxic atmospheric gas of increasing concern is carbon dioxide (CO₂). The world's oceans absorb about one-fourth of the CO₂ that human activity adds to the atmosphere annually. The dissolution of CO₂ in seawater, which forms carbonic acid, is creating increasingly acidic oceans. This change in pH is impeding the ability of some marine organisms to form shells and skeletal structures.

Impacts

Nutrients from sewage, treated or untreated, are increasing the rate of plant growth in coastal waters, a phenomenon known as cultural eutrophication. Sewage and agricultural runoff are high in nitrogen and phosphorus, which encourage ocean plants to grow. This results in unsightly algal scum on beaches. When the extra algae die and sink to the bottom, the resulting bacterial decomposition uses available dissolved oxygen, causing deoxygenation on the bottom waters. In extreme cases, this can kill fish. Some algal blooms contain toxic substances. Fish and shellfish that eat these algae become unfit for human consumption.

Water-soluble compounds from crude oil and refined products include a variety of compounds toxic to a large number of marine plants and animals. Chemicals released during oil spills from tankers or from offshore drilling operations are toxic to a wide range of plankton, which is at special risk because it lives near the water's surface. The 2010 BP Deepwater Horizon explosion in the Gulf of Mexico was responsible for the largest accidental marine oil spill in the world. Salt marshes and mangrove swamps trap oil from oil spills. When the plants are in bloom, the blossoms become coated with oil and thereafter rarely produce seeds. A 2002 report from World Wildlife Fund Canada estimated that, in the waters of Atlantic Canada alone, roughly 300,000 seabirds die from operational discharges of oil at sea every winter, showcasing the threat that oil spills present in the twentieth and twenty-first century.

Antifouling paints and other coatings that discourage the growth of barnacles and other marine organisms on ships and marine structures can be highly toxic to the ocean environment. Antifouling agents leach from surfaces to which they are applied and persist in the water. Tributylin (TBT), an effective antifouling substance introduced during the 1960s, was subsequently found to cause deformations in oysters and sex changes in whelks. A complete global prohibition on the use of TBT and related compounds for ship antifouling went into effect at the beginning of 2008.

Conservative pollutants are not affected by bacteria; when consumed by animals, they cannot be excreted and thus remain in the body. This process is called bioaccumulation. Animals that eat polluted plants and animals receive an enriched diet of those conservative materials, which they also are unable to excrete. The pollutants can accumulate in their bodies over time, eventually reaching concentrations high enough to cause illness or death. This process, called biomagnification, particularly affects the ocean predators. Bioaccumulation and biomagnification can render seafood unsafe for human consumption.

Discarded plastic items can cause harm to the larger animals of the sea. They can trap mammals, diving birds, or turtles, causing them to drown. Fishing gear lost overboard can continue to drift, snaring fish and entangling seabirds. Marine animals routinely mistake plastic items for food, ingest them, and die. Oceanographers have reported the existence of at least five extensive "garbage patches" far from land where systems of ocean currents called gyres have trapped huge masses of plastic particles and intact or semi-intact plastic items. The largest and best-known patch, the Great Pacific Garbage Patch in the North Pacific Ocean, is actually comprised of two patches, the Western Garbage Patch, near Japan, and the Eastern Garbage Patch, between Hawaii and California. Other large garbage patches are located in the South Pacific, North Atlantic, South Atlantic and Indian Oceans. In 2017, additional garbage patches were located in the increasingly vulnerable Arctic Ocean. In 2019, explorers undertaking a record-breaking dive in the Mariana Trench found a plastic bag and other debris nearly eleven kilometers beneath the ocean's surface.

Measuring the extent of marine garbage patches accurately has proven challenging for several reasons. Much of the debris is too small to be seen by the naked eye, and some of it sinks several meters below the surface of the water. The ocean gyres that bound each of the patches (except for the Arctic) are millions of square miles in area, making study of an entire patch expensive and time-consuming. Scientists have measured the density of microplastic in given areas of different patches, however, although their measurements only account for debris floating on the surface. According to scientists from the Sea Education Association, in 2010 the Great Pacific Garbage Patch had up to 750,000 bits of microplastic per square kilometer, or 1.9 million bits per square mile, while a patch in the North Atlantic Ocean between 22 and 38 degrees north latitude had a density of about 200,000 bits per square kilometer (520,000 bits per square mile). In 2018, a National Aeronautics and Space Administration (NASA) Earth Observatory report revealed that 8 million tons of plastic waste enters the ocean through rivers and beaches each year. By 2022, National Geographic estimated that almost 70 percent of ocean debris ultimately settles on the ocean floor, which raised questions concerning the existence of further garbage patches located beneath the ocean's surface.

Oceanographer Andrés Cózar and colleagues reported in 2014 that plastic debris was found in 88 percent of open ocean surface samples, though the density of the debris varied considerably. The team also noted that the amount of floating debris was far less than expected given the amount of plastic produced globally, so further study was needed to determine where the missing plastic went and to assess its environmental impact. Scientists are also continuing to study the effects the increasingly small particles of plastic have on marine life and human health as they become an inescapable part of the food web. In 2017, a study found microplastic particles in 83 percent of drinking water samples collected across fourteen countries.

Mitigation and Reparation Efforts

In late 1988, a global ban went into effect prohibiting the marine dumping of plastics. However, illegal dumping, uncontrolled runoff, beach littering, and (in the case of lightweight plastic shopping bags) even wind continue to carry plastics into oceans, where they can persist for decades, if not centuries. Later efforts to stop plastic pollution specifically included stricter rules on exporting of plastic waste under the Basel Convention signed in 2019 and the creation of the UN Intergovernmental Negotiating Committee (INC) on Plastic Pollution in 2022.

A number of international agreements have tackled other aspects of ocean pollution. The 1973 International Convention for the Prevention of Pollution from Ships and the 1978 Protocol that modified it, known as MARPOL 73/78, address various marine pollution sources and the means for controlling or eliminating them. The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, also known as the London Convention, entered into force in 1975; an update, the London Protocol, entered into force in 2006. This treaty regulates ocean dumping. The United Nations Convention on the Law of the Sea, adopted in 1982, addresses economic and environmental issues on the open sea. It was a replacement for related agreements in 1958 and 1960 that were inadequate to handle modern marine pollution problems and other concerns. Agenda 21, approved by governments that attended the Earth Summit held in Rio de Janeiro in 1992, called for all coastal countries to develop integrated coastal zone management plans by the year 2000. Later Earth Summits also included discussion of ocean pollution issues. Coastal areas remain a high priority because they are most affected by pollution sources on land.

Various countries have taken measures of their own to address ocean pollution. For example, in September 2016, the US government under the administration of President Barack Obama designated its first-ever marine monument in an effort to protect an area of ocean approximately the size of the state of Connecticut off the coast of Cape Cod. The designation banned oil and gas exploration and drilling as well as most commercial fishing in the area.

Some countries, states, and municipalities have begun efforts to control pollution in their estuaries and bays, and fish and shellfish have returned to formerly polluted areas. Maritime ship-based activities have less impact on the ocean than in the past, mainly because of the introduction of international treaties limiting the discharge of wastes at sea (although such regulations are difficult to enforce and are often violated). In addition, many schools, aquariums, nonprofit groups, and governments all work on an ongoing basis to educate the public about the need to prevent ocean pollution. Additionally, companies in the United States and around the world have sought to create products that are commonly made of plastic using alternative materials that are more sustainable and eco-friendly in their decomposition. Campaigns in the twenty-first century have focused on banning plastic bags, straws, and other waste to reduce overall global pollution.

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