Threats to Aquatic Environments - Water Pollution Poses A Threat

oil marine fish mercury

Water pollution poses a considerable threat to many aquatic species. Contaminants include chemicals, biological substances (such as manure and sewage), solid waste materials, and dirt and soil eroded from riverbanks or nearby lands. In general, aquatic creatures are not killed outright by water contamination. A major exception is an oil spill, which can kill many creatures through direct contact. The more widespread and common threat is overall degradation of water quality and habitats due to pollutants. Exposure to contaminants can weaken the immune systems of aquatic animals and make them more susceptible to disease and other health and reproductive problems.


Pesticides are chemicals used to kill insects that feed on crops and vegetation. The first documented use of pesticides was by the ancient Greeks. Pliny the Elder (23-79) reported using common compounds such as arsenic, sulfur, caustic soda, and olive oil to protect crops. The Chinese later used similar substances to retard infestation by insects and fungi. In the 1800s Europeans used heavy metal salts such as copper sulfate and iron sulfate as weed killers.

The invention of DDT (dichloro-diphenyl-trichlor-oethane) in 1939 marked a revolution in the war against pests. DDT was effective, relatively cheap, and apparently safe for people—on the face of it, a miracle chemical that promised a world with unprecedented crop yields. Its discoverer, Paul Muller, received a Nobel Prize for discovering the high efficiency of DDT as a contact poison against pests. In the United States, pesticide use in agriculture nearly tripled after 1965, as farmers began to use DDT and other pesticides, herbicides, and fungicides intensively and began to accept these chemicals as essential to agriculture.

For many years, it was thought that if pesticides were properly used, the risk of harm to humans and wildlife was slight. As the boom in pesticide use continued, however, it eventually became apparent that pesticides were not safe after all. The fundamental reason that pesticides are dangerous is that they are poisons purposely designed to kill living organisms. Part of the problem is biomagni-fication—a predator that eats organisms with pesticides in their bodies ends up concentrating all those pesticides in its own tissues. Eventually, the concentration of pesticides causes serious problems. DDT was eventually shown to have harmed numerous bird species, particularly those high in the food chain, such as bald eagles and peregrine falcons. DDT caused the production of eggs with shells so thin they could not protect the developing chick.

As the dangers of pesticides became more apparent in the 1960s and 1970s, some of the most dangerous, like DDT, were banned in the United States. However, the use of other chemical pesticides increased until the 1980s. Use levels have generally held steady since then. The primary reason that pesticide use has leveled off in recent decades is not concern regarding its safety, but declines in its effectiveness. This is due to the fact that pest species quickly evolve resistance to pesticides. Worldwide the number of resistant pests continues to climb. Unfortunately increased resistance has only created a demand for new and more powerful chemicals.

Nutrient Pollution

Nutrient pollution is primarily composed of nitrogen and phosphorus. These chemicals are commonly used in inorganic fertilizers and are naturally found in biological wastes, such as manure and sewage. In addition, nitrogen compounds are emitted into the air during the combustion of fossil fuels, such as oil and coals. All of these factors combine to produce a heavy load of nutrients that enter waterbodies via land runoff, direct discharges, and atmospheric deposition.

While nutrients are not poisonous by nature, large quantities of them can cause serious health problems in aquatic animals. Nutrients also encourage the growth of FIGURE 4.5 Harmful effects of land-based nutrients to marine environment "Figure 14.2. Nutrient Pollution in Coastal Waters," in An Ocean Blueprint for the 21st Century, Final Report, U.S. Commission on Ocean Policy, 2004, (assessed February 1, 2006)aquatic plant life, disrupting food webs and biological communities. Aquatic plants and algae may grow so rapidly that they block sunlight or deplete oxygen essential to other species. (See Figure 4.5.)


Oil enters waterbodies through a variety of means, including natural and anthropogenic (human-related) sources. Figure 4.6 shows the sources of oil input to the North American marine (oceanic) environment in 2002. Natural seepage accounts for 63% of marine oil exposure. Anthropogenic activities are blamed for the other 37%. Runoff of oil in municipal and industrial waste discharges is the primary source of oil exposure attributed to human activities. Smaller amounts are blamed on atmospheric fallout from additional anthropogenic sources, including air pollution (8%), marine transportation vehicles (3%), recreational marine vehicles (2%), and offshore oil and gas development (2%).

Oil spills are devastating accidents to aquatic life. Oil spilled into the ocean floats on the water surface, cutting off oxygen to the sea life below and killing mammals, birds, fish, and other animals. The dangers presented by oil spills have grown worse over the years. In 1945 the largest tanker held 16,500 tons of oil. Now, supertankers FIGURE 4.6 Sources of oil released into North American waters, 2002 "Figure 24.3. Oil Inputs to the North American Marine Environment," in An Ocean Blueprint for the 21st Century, Final Report, U.S. Commission on Ocean Policy, 2004, (accessed February 1, 2006)the length of several football fields regularly carry more than 550,000 tons.

In 1989 the tanker Exxon Valdez ran aground on the pristine Alaskan coastline, spilling eleven million gallons of oil into Prince William Sound and killing millions of animals. In 1994 a federal jury assessed $5 billion in punitive damages and $3.5 billion in criminal fines and cleanup costs against Exxon. The Valdez spill led to additional safety requirements for tankers, including double hulls. Larger oil spills than the Valdez have occurred both before and since, but the incident alerted many people to the damage that can be done to marine habitats. Many species affected by the spill, particularly seabird species, had yet to recover more than a decade later.

In January 2001 the tanker Jessica released 150,000 gallons of fuel near the Galapagos Islands, a biologically rich area harboring numerous unique species including Darwin's famous finches, marine iguanas, and a tropical penguin population. There was widespread relief when winds blew the oil slick seaward rather than towards the islands. Sea bird and sea lion deaths numbered in the dozens, and it was believed that a true catastrophe had been avoided. Ongoing studies of the Galapagos' unique marine iguanas, however, revealed in June 2002 that numerous iguanas likely died due to oil-related injuries after the spill. In particular, 60% of the marine iguanas on Santa Fe Island died in 2001, despite the fact that oil contamination was relatively low, with only about one quart of oil per yard of shoreline. Similar deaths were not found on another island where there was no contamination. Scientists believe that the deaths occurred when oil contamination killed the iguanas' gut bacteria, making them unable to digest seaweed and causing them to starve. Marine iguanas have no natural predators and generally die either of starvation or old age.

The U.S. National Research Council warns that, even without large catastrophic oil spills, many marine habitats are regularly exposed to oil pollution. Harbors and aquatic habitats near developed areas are in particular jeopardy. In 2003 the agency estimated that each year approximately twenty-nine million gallons of petroleum enter North American ocean waters due to human activities. Nearly 85% of this amount is attributed to street runoff, polluted rivers, and losses from airplanes, small boats, and jet skis. As little as one part of oil per million parts of water can be detrimental to the reproduction and growth of fish, crustaceans, and plankton.

Ocean Dumping and Debris

Ocean debris comes from many sources and affects diverse marine species. Waterborne litter entangles wildlife, masquerades as a food source, smothers beach and bottom-dwelling plants, provides a means for small organisms to invade nonnative areas, and contributes to toxic water pollution. Records of interactions between ocean debris and wildlife date back to the first half of the twentieth century. Northern fur seals entangled in debris were spotted as early as the 1930s. In the 1960s various seabirds were found to have plastic in their stomachs. By the early twenty-first century, a total of 255 species were documented to have become entangled in marine debris or to have ingested it.

Some scientists once thought it was safe to dump garbage into the oceans, believing the oceans were large enough to absorb sludge without harmful effects. Other scientists argued dumping would eventually lead to the pollution of the oceans. Metropolitan centers such as New York City once loaded their sludge and debris onto barges, took the vessels out to sea, and dumped the refuse, in a practice called ocean dumping. Problems with ocean dumping were not fully recognized until floating plastic particles were found throughout the Atlantic and Pacific Oceans.

The perils of ocean dumping and debris struck home both literally and figuratively in the summer of 1988, when debris from the ocean, including sewage, garbage, and biohazards from medical waste, washed up on the Atlantic seaboard, forcing an unprecedented 803 beach closures. In some cases authorities were alerted to beach TABLE 4.1 How long does marine debris stay in the environment? Adapted from "Marine Debris Timeline," U.S. Environmental Protection Agency, Gulf of Mexico Program, October 9, 2003, (accessed April 4, 2006)wash-ups when children turned up hypodermic needles in the sand. Aquatic species also faced serious dangers from these materials, including absorbing or ingesting hazardous waste substances, and ingesting needles, forceps, and other dangerous solid debris. In 1994 hundreds of dead dolphins washed up on Mediterranean beaches, killed by a virus linked to water pollution. Scientists pointed to this event as an indication of what may happen to other marine animals (and humans) if pollution continues.

How long does marine debris stay in the environment?
SOURCE: Adapted from "Marine Debris Timeline," U.S. Environmental Protection Agency, Gulf of Mexico Program, October 9, 2003, (accessed April 4, 2006)
Cardboard box 2 weeks
Paper towels 2-4 weeks
Newspaper 6 week
Cotton glove 1-5 months
Apple core 2 months
Waxed milk carton 3 months
Cotton rope 3-14 months
Photodegradable 6-pack ring 6 months
Biodegradable diaper 1 years
Wool glove 1 years
Plywood 1-3 years
Painted wooden stick 13 years
Foam cup 50 years
Tin can 50 years
Styrofoam buoy 80 years
Aluminum Can 200 years
Plastic 6-pack ring 400 years
Disposable diapers 450 years
Plastic bottles 450 years
Microfilament fishing line 600 years
Glass bottles/jars Undetermined

At the urging of the Environmental Protection Agency (EPA), the dumping of potentially infectious medical waste into ocean waters from public vessels was prohibited in 1988. In 1992 the federal government banned ocean dumping. In 1995 the EPA stepped up efforts to educate people about the dangers of polluting coastal waters through improper disposal of trash on land, sewer overflows to rivers and streams, and dumping by ships and other vessels. The EPA further warned that marine debris poses not only a serious threat to wildlife, but remains in the environment for many years. (See Table 4.1.)

Mercury and Other Toxic Pollutants

Since 1993 the EPA has published an annual listing of all fish advisories issued around the country. These advisories are issued by states to protect residents from the adverse health risk of eating fish contaminated with certain pollutants. The EPA's 2004 National Listing of Fish Advisories (September 2005, OST/fish/advisories/fs2004.pdf) showed that 3,221 advisories were issued during 2004 affecting 35% of the FIGURE 4.7 Contaminants blamed for fish consumption advisories, 2004 Adapted from "Bioaccumulative Pollutants," in Ten Individual Slides Providing National Summaries and Statistics for the 2004 National Listing of Fish Advisories (NLFA) Database, U.S. Environmental Protection Agency, September 15, 2005, (accessed February 11, 2006) and "Other Contaminants," in 2004 National Listing of Fish Advisories, U.S. Environmental Protection Agency, Office of Water, September 2005, (accessed February 11, 2006)nation's total lake acreage and 24% of its total river miles. Nearly 14.3 million lake acres and just over 839,000 river miles were under advisory during 2004. As shown in Figure 4.7 more than two-thirds of the advisories were issued due to mercury contamination. Polychlorinated biphenyls (PCBs) accounted for another 24% of advisories. Dioxins, pesticides, and other contaminants contributed the other 9% of the total.


Mercury contamination is a problem in many of the nation's waterbodies. Scientists believe that the main source of mercury pollution is rainwater that carries mercury from coal-burning power plants, incinerators that burn garbage, and smelters that make metals. Because mercury becomes concentrated in organic tissues like DDT, even small concentrations of mercury in the water can be harmful to health. (See Figure 4.8.)

Mercury can cause brain damage and other serious health problems in wild species and in humans. During the 1990s scientists began to report widespread mercury contamination in fish, including those inhabiting remote lakes that were assumed pristine. As a result, many states now warn people against eating certain types of fish.

FIGURE 4.8 Biomagnification of mercury in the food chain David P. Krabbenhoft and David A. Rickert, "Figure 4. Mercury (Hg) Biomagnifies from the Bottom to the Top of the Food Chain," in Mercury Contamination of Aquatic Ecosystems, U.S. Geological Survey, 1995, (accessed April 4, 2006)

The Environmental Protection Agency's National Fish and Wildlife Contamination Program reported that in 2004 mercury was the cause of 2,436 fish and wildlife consumption advisories. Figure 4.9 shows the advisories in effect as of December 2004. Most states across the nation's northern border and in the Northeast had statewide advisories in effect for all their rivers and lakes, as did Florida. All of the Gulf Coast, Hawaiian coast, and much of the eastern seaboard were under advisory for mercury contamination in marine fish.

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