Marine pollution is a major threat to any organism living in or depending upon the ocean. Human impact on coastal and open ocean habitats comes in many forms: nutrient loading from agricultural runoff and sewage discharges, toxic chemical inputs from industry and agriculture, petroleum spills, and inert solid wastes. While there has been some recognition of the destruction of marine systems from pollution, regulations are often weak or are not enforced. Recent efforts have lead to slow recovery of some coastal areas, but many of the detrimental practices continue and some systems may never recover.
Nutrient loading is perhaps the most well-studied form of pollution, and its biological consequences have been observed and documented. Algal blooms, including red tides, have been attributed to elevated nutrient levels in coastal systems. These blooms, through their respiration and decomposition , can deplete the levels of dissolved oxygen in the waters to almost zero, killing zooplankton , fish, and shellfish. These nutrients often come from runoff of agricultural fertilizers, the use of which has increased sevenfold since 1950. It is estimated that approximately 25 percent of the 46.8 million metric tons of fertilizer used in the United States annually enters rivers and coastal waters.
Another source of nutrients is the discharge of sewage. The United States and England are the only countries that dump sludge into the oceans, averaging 17 million metric tons annually between them. Throughout the world, however, raw sewage is released into rivers and coastal habitats by many countries, leading to algal blooms and increased biochemical oxygen demand . BOD, the rate at which oxygen disappears from a sample of water, increases dramatically with loading of organic material such as sewage, and results in the lowering of dissolved oxygen levels. The wastes of domesticated animals may also have a major impact in some systems. A single cow produces approximately 31 lb (14 kg) of waste per day, the equivalent of ten people. When discharged into rivers or coastal waters, wastes produced by large herds or feedlots may have substantial effects.
Boston Harbor is considered to be one of the most polluted harbors in the United States. Since Europeans settled in the Boston area, domestic wastes have been discharged directly into the harbor. As Boston's population grew, so did the dumping, a problem exacerbated by the growth of industry in the late nineteenth and early twentieth centuries. Fish and shellfish in the harbor contain toxic levels of polychlorinated biphenyl (PCB) and heavy metals , BOD levels are astronomical, and dissolved oxygen levels are low. In 1984, the Massachusetts Water Resources Authority (MWRA) was formed and has managed to improve water quality of the harbor slightly. Recommendations of the MWRA have been hard to enact, however. Sewage treatment modifications have been met with Environmental Protection Agency (EPA) and State objections, shelving the project while courts decide the fate of environmental quality in Boston Harbor.
Toxic substances are introduced into the marine environment from various sources, some of which may be hundreds of kilometers away. Heavy metals, pesticides, and acid rain threaten not only coastal and estuarine systems, but also life in the open ocean. Heavy metals occur in many forms, some of which are soluble in seawater. These soluble compounds may not be the forms in which they were originally released into the environment, often making their sources difficult to determine. Many heavy metals are released in industrial effluents, especially from chemical plants, smelters, and mining runoff. These compounds may affect humans directly through contact, or indirectly, from the consumption of fish and shellfish, where metals often accumulate in tissues.
Between the late 1930s and the mid-1950s, a Japanese chemical company manufacturing acetaldehyde discharged mercury into Minamata Bay, where it formed a soluble compound that accumulated in fish. It was not until 50 people died of Minamata disease and hundreds were left with debilitating nervous disorders from eating poisoned fish that environmental studies were initiated. Finally, in 1969, the plant was closed. Other metals, such as zinc, cadmium , copper , and silver, are commonly discharged into marine systems by industry. Not only are these metals toxic by themselves, but synergistic effects compound their toxicity.
Acid precipitation is rain, snow, or fog that has a lower pH than normal and is caused by inputs of nitric and sulfuric acid into the atmosphere from manufacturing and the burning of fossil fuels , as in automobiles. These causes may be far from the area of impact since prevailing winds can carry pollutants considerable distances. The ocean has a high buffer capacity, that is, the ability to neutralize many of the acid inputs. Therefore, most of the severe effects of acid rain are observed in freshwater lakes and rivers. Some estuaries, however, may also be seriously impacted, as has been observed in the upper Chesapeake Bay . The pH of a river feeding the Bay dropped from 6.3 to 5.8 between 1972 and 1978. Juvenile and spawning striped bass may not be able to tolerate such high acidity. Precipitation of nitrogen-based acid also increases nutrient loading in aquatic systems.
Toxic organic compounds, especially pesticides and a family of chemicals known as PCBs, have been shown to have serious effects on marine systems. Runoff has introduced considerable amounts of PCBs, DDT, and many other synthetic organic compounds into coastal areas. These compounds may persist for many years. In 1976, Congress banned the manufacture of PCBs, but they are still found today as coolants in older transformers and buried in sediments. It is estimated that 1 percent of PCBs used have reached the ocean. In 1987–1988, 700 bottlenose dolphins washed up on the U.S. Atlantic coast and some were found to have elevated levels of PCBs and DDE (a form of DDT). Biologists claim that these compounds inhibited the dolphins' immune systems, making them vulnerable to infections. In 1975, workers at a chemical plant on the Chesapeake were poisoned when the insecticide Kepone was leaked or dumped into the bay. The Kepone spread downstream where it poisoned fish and shellfish.
One of the most publicized sources of marine pollution is that caused by petroleum products. While large oil spills can devastate a local area, equally important is the discharge of crude oil while cleaning bilges and emptying tanks at sea. Over the past decade, an average of approximately 32 million gallons (120 million l) of oil have been spilled annually. The Gulf of Mexico has recorded the most spills, while spills in the Persian Gulf average 2 million gallons (7 million l) per year. Since tanker ports and refineries are, by necessity, located on the coast, these sensitive areas receive considerable damage from the spills. The damage to marine life is staggering. Seabirds are killed by the hundreds of thousands annually, their oil-matted plumage making flight impossible and exposing them to hypothermia. Oil-soaked fur of marine mammals loses its water repellency, also leading to death by hypothermia. Ingestion of oil by fishes, birds, and mammals also may result in death.
Hundreds of tons of inert solid wastes are dumped into the oceans from ships annually. Of these, plastics and polystyrene (styrofoam) are deadly to marine life. Often floating for hundreds of miles and lasting for many years, plastics are frequently mistaken for food by fishes, turtles, and mammals, and proceed to either interfere with subsequent feeding or strangle the consumer. It has been estimated that plastics and discarded fishing gear, such as monofilament line and discarded nets, kill one million seabirds and 150,000 marine mammals each year.
Beginning with the Refuse Act of 1899 and the Water Pollution Act of 1948, there have been efforts to remedy the polluted oceans. Not until 1972, however, was legislation drafted that was powerful enough, and sufficient monies appropriated, to effect any change. With the Federal Water Pollution Act, or Clean Water Act , improvement of water quality in the United States began. By 1985, five of the goals of the 1972 Act were reached, and since then, the Clean Water Act has been rewritten to reflect increasing national concerns.
Pollution of the world's oceans had become so pervasive that an international convention was convened in 1973 to establish laws governing the discharge of all wastes into the ocean. The International Convention for the Prevention of Pollution from Ships (1978), commonly known as MARPOL, and subsequent annexes to the convention, covers all garbage discharged by ships at sea beyond three miles from shore. While it is too soon to tell, adherence to the MARPOL agreement should significantly reduce the amount of solid wastes polluting the world's oceans.
Boston Harbor, the Chesapeake Bay, and the Mediterranean Sea are the most publicized examples of marine pollution. However, the problem is widespread. Many estuaries in Southeast Asia, many coastal areas in Japan, and the waters off Rio de Janeiro are also areas where historical and current use of the waters as dumping grounds has left them in disastrous conditions. The Golden Horn estuary around Istanbul, Turkey, has been declared dead, containing no living organisms and posing a threat to surrounding waters at the eastern end of the Mediterranean.
Since humans have been living along the coasts and using the seas as waste dumping grounds for thousands of years, these areas are the most heavily impacted. What makes matters worse is that coastal waters are vital spawning and nursery grounds for most of the commercially harvested fish and shellfish in marine systems. Destruction of these areas has significant nutritional and economic repercussions. Examples of the world-wide destruction of marine habitats due to pollution are numerous, but, with increased international efforts to limit inputs and rehabilitate damaged areas, improvements in the health of marine systems may be seen.
[William G. Ambrose Jr. and Paul E. Renaud ]
Nybakken, J. W. Marine Biology: An Ecological Approach. 2nd ed. New York: Harper and Row, 1988.
Dolin, E. J. "Boston's Murky Political Waters." Environment 34 (1992): 6–33.