Sewage, Animal Waste, and Fertilizers
Sewage, animal waste, and chemical fertilizers all have a high content of nitrogen and phosphorus. Artiﬁcially high levels of these substances in the water promote excessive growth of microscopic or macroscopic plants, in a process called eutrophication. When these plants accumulate, die, and decay, they cause low oxygen content in the water. Even if sewage is treated to remove solids, the liquid discharged contains high levels of nitrogen and phosphorus.
Intensive cultivation of animals in feedlots, or application of more fertilizer than a crop can absorb, also cause runoff rich in nitrogen and phosphorus that ﬁnd their way into rivers and estuaries. Vehicle exhausts and industrial chimneys are large sources of nitrogen compounds that are transported in the atmosphere and deposited in coastal waters.
On a global scale, agricultural runoff is the most important source of eutrophication, but atmospheric deposition is the fastest-growing source. It is the largest source of nitrogen off the coast of the northeastern United States, in the western Baltic Sea, and in the western Mediterranean Sea. International agencies consider that, worldwide, eutrophication is the most serious pollution problem in coastal waters.
For example, in the Gulf of Mexico, off the mouth of the Mississippi River, water near the bottom has severely reduced oxygen content over a very large area, sixteen thousand square kilometers (6,200 square miles) by 1998. Mobile animals such as ﬁsh and shrimp leave the hypoxic area, but sedentary animals such as clams and worms are killed in large numbers.
A classic example of eutrophication and its treatment occurred in the estuary of the River Thames, near London, England. In the 1950s the water was severely hypoxic for thirty-ﬁve kilometers (twenty-two miles) below London Bridge. After several sewage treatment plants were built, the water returned to a well-oxygenated state and migratory ﬁsh such as salmon once again ascend the river.
In the case of the Mississippi River, treatment of the eutrophication is more difﬁcult because runoff from agricultural land is the major cause of the problem, and more than half of the agricultural land in the United States drains into the Mississippi basin. Cleaning up the pollution would involve changes in farming methods on a national scale.
Eutrophication has important indirect effects. The plants known as sea grasses, which grow in the shallow water of estuaries, provide food and shelter for a wide range of animals, including geese, turtles, manatees, and ﬁsh. In eutrophicated water, the dense microscopic plant life signiﬁcantly reduces the penetration of light and smothers the sea grasses.
In Chesapeake Bay, Maryland, eutrophication caused an area of sea grasses to decrease by two-thirds between 1960 and 1980, and there was a corresponding decrease in landings of ﬁsh and crabs. Similar effects have been observed in Australia.
Red tides, or harmful algal blooms, are associated with eutrophication. Single species of phytoplankton multiply at the expense of all other species and become so abundant that the water is discolored. Many bloom species produce toxic substances. During the 1990s in estuaries located in the south-eastern United States, there were numerous cases of blooms of Pﬁesteria piscida, a dinoﬂagellate that produced a toxin which killed thousands of ﬁsh.
The source of the nutrients support Pﬁesteria is believed to be agricultural runoff or sewage discharge. Other types of blooms are ingested by shellﬁsh, which become toxic for humans who consume them, causing partial paralysis, memory loss, or even death. Toxic blooms have been reported much more frequently in the 1990s than in the past, and the spread of eutrophication is believed to be a contributing factor.
Pollution and Coral Reefs
On coral reefs, eutrophication causes seaweed to grow and smother the corals. Several kinds of environmental problems interact with eutrophication to cause the deterioration of coral reefs. Overharvesting of the ﬁsh and invertebrates that eat seaweed accelerates the smothering.
Careless development along coastlines and in river basins leads to soil erosion and the transport of heavy loads of silt and clay, which settle on the corals and smother them. Oil spills also take their toll. When corals are exposed to abnormally high water temperature, they respond by discharging the microscopic algae living within their tissues. Sometimes they recover, but often they die.
These episodes, called coral bleaching, became much more frequent during the 1990s and are believed to be caused by global warming. The result of pollution and global warming is that at least half of the area of coral reefs in south-east Asia is in poor condition, and in parts of the Caribbean Sea only 5 percent of the reef area consists of living coral.
Metals and Organic Contaminants
Industrial efﬂuents often contain metallic compounds. For example, Halifax, a small city in eastern Canada, discharged into its harbor during the 1990s about thirty-three tons of zinc and thirty-one tons of lead per year, with lesser amounts of copper and other metals. These metals are held in the sediment in a relatively inert form, but if stirred up into the water column, they become oxygenated and toxic.
Tin is another common pollutant in harbors. It occurs as tributyltin (TBT), which is used as a component of antifouling paints on the undersides of ships. When taken up by shellﬁsh, it accumulates in their tissues and has proved toxic to the shellﬁsh and to organisms that consume them. The United States began to phase out TBT in 1988, and it will be banned internationally beginning in 2008.
Industry also produces organic compounds such as polychlorinated biphenyls (PCBs) and various pesticides. These accumulate in the fatty tissue of plants and animals low in the food chain, and as they pass through the food web to larger and long-lived animals, there is an increase in concentration of the substances in their fat, a process known as bioaccumulation. The St. Lawrence River, which drains the Great Lakes, has accumulated large amounts of organochlorines, which have amassed in the tissues of Beluga whales.
During the 1990s, the level of this pollution was much reduced, and the whales have been protected from hunting, but their population fails to increase. Many animals have tumors and disease. There is mounting evidence that chronic exposure to contaminants causes suppression of the immune responses of marine mammals. Similar problems have occurred with seals in the Baltic Sea.
Marine beaches serve as natural traps for marine debris. Globally, the most common materials are plastics, followed by glass and metal. The chief dangers to marine life result from the ingestion of these fragments, which may block the gut, and from entangling, which may cause suffocation or prevent locomotion and feeding.
In a survey of U.S. beaches close to urban centers, cigarette butts were the most abundant debris, followed by packaging items (boxes, bags, caps, lids), medical waste, and sewage. A high proportion of this material reached the sea by way of sewers. Even street litter can be washed into surface drains and then to the sea. The dumping of sewage and waste by ships is another source.
Public revulsion at the state of U.S. beaches was a key factor in the enactment of stronger environmental protection laws, like the Ocean Dumping Ban Act of 1988 that prohibited the dumping of sewage into the ocean. On sites more remote from cities, pieces of rope and netting are the most common types of marine debris.