Introduction to Ocean Sciences

Chapter 19: Pollution

Guide to Reading and Learning

For many of you this is the most important chapter in the text, as it brings together the information learned in other chapters and applies it to an examination of the conflicts between ocean resource uses, pollution, and other adverse impacts on the ocean environment.

One of the reasons you decided to study ocean sciences may have been that you are interested in environmental issues in general and perhaps specifically in protecting ocean ecosystems. Unfortunately, you may have come equipped with only the misinformation or incomplete information that originates from the advocates of one side or another of all environmental debates. Although advocacy does have a place in our society, it is my strong belief that our society will make better choices about how to manage our environment only if our citizenry have a better understanding of what those choices really mean. Because of the barrage of advocacy-driven information and misinformation that we all receive, citizens must be able to evaluate this information for themselves. If you read this chapter you will learn the basic concepts and information you need and how to use these tools to question information sources and evaluate issues for yourself. You may not agree with all that is said in the chapter, but if you do not, I would challenge you. Use the knowledge you have gained from this text, re-examine your beliefs, critically examine the universe of data and information available to you on the internet and elsewhere, and only then decide for yourself.

Chapter 19 Essential to Know 

Critical Concepts used in this chapter

CC.8, CC.14, CC.17, CC.18

19.1 Pollution versus Contamination

  • The term pollution is often misused. If the concentration of some substance is increased in the environment by human activities, this is contamination. Ocean contamination only becomes pollution when the alteration of the environment has adverse effects on ocean ecosystems or ocean resources.
  • The broad definition of pollution includes any activity of humans that leads to a deleterious effect on ocean ecosystems or resources. Thus, for example, one of the major polluting activities is the filling of wetlands.

Assimilative Capacity

  • For all contaminants, the oceans have an assimilative capacity, which is the maximum rate of introduction of a contamination that does not lead to pollution (i.e., there is no adverse effect). Assimilative capacity can be smaller for restricted areas of the oceans with long residence times and is different for each different substance.

19.2 Adverse Effects of Human Activities

  • A variety of adverse effects on the oceans can be caused by human activities.

Interference with Photosynthesis and Respiration

  • An increase of suspended sediment concentrations can interfere with or inhibit photosynthesis.
  • Addition of nutrients can alter primary production and phytoplankton composition, which can lead to blooms, eutrophication, and oxygen depletion and anoxia.

Habitat Alteration

  • Activities such as filling wetlands, damming rivers, and altering river flows can render habitat unusable by normal indigenous species and promote the success of usually more stress tolerant and less desirable opportunistic species. Anadromous and catadromous species may be prevented from making their reproductive migrations.

Community Structure Alteration

  • Alterations of community structures can result from various human activities including introduction of nonindigenous species that outcompete resident species.

Contamination of Food Resources

  • Food resources can be contaminated with toxic substances or pathogens, rendering them unusable. Contamination by pathogens introduced in human sewage, animal wastes, and runoff can render shellfish unfit for human consumption even though the species themselves are not adversely affected.

Beach Closures and Aesthetic Losses

  • Coastal structures such as groins, piers, breakwaters, and port facilities can alter the natural form and function of the coastline and reduce its aesthetic and recreational value.
  • Aesthetic values of beaches can be adversely affected by stranding of trash and closure of beaches due to pathogen pollution.

19.3 Toxicity

  • Many toxic chemicals are present naturally in the oceans but at concentrations low enough not to have adverse consequences.
  • Toxic chemicals, even those that do not occur naturally, can be released to the oceans without causing any harm unless their concentration is high enough that the assimilative capacity is exceeded.
  • Toxic chemicals are present in low concentrations in all wastes, and recycling can never be effective in reducing these concentrations to zero, regardless of the technology or cost involved.
  • Wastes containing toxic chemicals can be disposed in secure landfills but these landfills will fail quickly within geological timescales and the toxics will be released to groundwater, freshwater, and the coastal oceans.
  • Many scientists believe dispersal of some toxic wastes in the oceans would be an environmentally preferable alternative to land disposal.

Effects of Toxic Substances on Marine Organisms

  • For all toxic substances, there is a concentration in the environment below which the chemical is nontoxic.   
  • Many toxic elements are also essential to life and may inhibit growth or other functions of some species if present below a certain concentration. The difference between this concentration and the concentration at which the substance is toxic varies for each toxic substance and species but in some instances is small, and naturally occurring concentrations are close to the toxic levels.
  • Carcinogens, teratogens, and mutagens may be exceptions in that they may have no threshold below which they do not cause an effect. However, the probability of an individual being affected decreases as concentration decreases and there is a concentration below which these substances have no measurable or significant effect.

Evaluating Toxicity

  • Toxicity of a substance is difficult to evaluate.
  • Laboratory methods are difficult because sensitive species are difficult to keep alive in the laboratory, all species and life stages cannot be tested, and stress due to laboratory conditions may confound the tests.
  • Methods that assess the effects of a substance in the marine environment itself are difficult because there are many uncontrollable factors, both natural and anthropogenic, that can cause adverse effects. It is usually not possible to separate the effects of just one toxic substance or even of one source of contamination such as sewage.

Bioaccumulation and Biomagnification     

  • Many species bioaccumulate toxic substances, i.e., the concentration in their body tissues is higher than that in the environment or food but this concentration can decrease if the environmental concentration decreases.
  • In the marine environment there are only a few organic toxic substances, primarily fat soluble substances such as PCBs, DDT, and methylmercury, that are biomagnified. Biomagnification occurs when the organism continuously concentrates the toxic substance from its food or environment and the body concentration does not decline if the environmental concentration is decreased.
  • Other than organometallic compounds such as methylmercury, toxic metals are not biomagnified in marine species.

Synthetic and Naturally Occurring Toxins

  • Some toxic substances are naturally occurring while some are synthetic. Since organisms may not have an evolved mechanism for detoxifying or decomposing synthetic compounds, these compounds should be subject to controls different from those intended for naturally occurring substances, including the possibility of banning their production.

19.4 Waste Disposal

  • The oceans have, in the past, been the ultimate repository for almost all types of wastes created by human society.
  • At present, the only wastes still discharged in large quantities are treated sewage liquid effluents and dredged material, at least in the United States.
  • Many scientists believe that ocean disposal may be the most environmentally sound means of disposing of some of society’s wastes that cannot be recycled.

19.5 Petroleum

  • Petroleum is a mixture of naturally occurring hydrocarbons and, when released to the marine environment, it is progressively decomposed to inorganic matter.

Sources of Petroleum Contamination

  • The major sources of petroleum inputs to the oceans are municipal wastes, discharges of ballast water by tankers, tanker accidents, natural sources (mostly seeps), urban runoff, and industrial discharges.
  • Offshore oil and gas operations and spills from rigs and seafloor pipelines contribute only comparatively small amounts.

Fate of Petroleum in the Oceans     

  • Oil spilled in the oceans or released from seeps spreads on the water surface to form a slick.
  • Oil is a mixture of many different hydrocarbons that vary widely in toxicity and chemical and physical characteristics.
  • Volatile components of petroleum evaporate into the atmosphere within a day or two after a spill or are dissolved in the water. What remains behind becomes more viscous and can eventually clump into tar balls. However, in rough seas the petroleum may be mixed with air and water to form a gummy suspension.
  • If it does not reach shore, the spill is eventually entirely dissipated.
  • Oil that reaches shore adheres to sand and rocks and any other hard substrate.
  • Petroleum is naturally occurring and many marine decomposers can utilize it. Thus, spilled petroleum is completely decomposed and assimilated by the oceans within a few years, even the oil that coats solid substrates on the coast. However, some oil may persist much longer if it is mixed in with sand or muddy sediments.
  • Recovery and recolonization of oiled shores tends to occur faster on high energy coastlines and is slower in colder climates.

Effects of Major Spills

  • Major oil spills from tanker accidents can, but may not, cause substantial damage to the ecology of beaches and coastal waters into which the oil is spilled. However, this damage is only temporary and most marine ecosystems have recovered almost completely within a decade after a spill.
  • Cleanup of oil spills is often very aggressively pursued after a tanker spill due to the media attention and public pressure. However, the value of more than a limited effort to clean up easily removed quantities of oil is questionable. Aggressive cleanup may cause additional unnecessary ecological damage.
  • Efforts to capture and clean marine mammals and birds after a spill appear not to be effective.

Chronic Inputs

  • Chronic inputs of oil to the marine environment, especially urban runoff and sewage inputs to estuaries, are very large and may cause more long-term and lasting damage than major spills such as tanker accidents.

Lessons Learned

  • Oil spills can severely damage coastal ecosystems but the damage last only a few years until the petroleum is decomposed and recolonization has occurred. Thus, they can be serious pollution problems but are less serious than the public believes, perhaps because they are among the few ocean pollution problems that can be seen in dramatic video images.
  • Cleanup after an oil spill must be limited since aggressive cleanup efforts often cause more harm than the spill and usually delay recovery..
  • Tankers are more likely to spill oil than offshore oil platforms or pipelines so offshore oil development may be less environmentally damaging than continuing to import oil.
  • Ecosystems are naturally variable so that, after any major disturbance including oil spills, the ecosystem will never return to a preexisting balance of species and cannot be restored to such a state.

19.6 Sewage

  • Sewage is the most fundamental and unavoidable waste produced by human populations.
  • Collection of sewage in sewers was probably one of the greatest contributions to human health in history and continues to be essential.

Nature of Sewage Wastes      

  • Sewage is essentially water with a high concentration of dissolved and particulate natural organic matter, contaminated by industrial and domestic sources of toxic substances.
  • Sewage also contains human pathogens. Sewage treatment is designed to remove suspended solids and oxidizable organic matter from the sewage.
  • Treatment also removes some pathogens, nutrients, and toxic substances, but much of these remains in the liquid effluent left after sewage treatment. This treated liquid effluent is discharged to rivers, estuaries, and the oceans. There is currently no technologically feasible alternative to such discharge.

Sewage Treatment

  • There are three levels of treatment of sewage. Primary treatment removes floating material and settled solid material. Secondary treatment reduces the organic matter content of the sewage by 90% usually through bacterial digestion. Tertiary treatment generally removes some nutrients but is very expensive and only required and performed in very few locations.
  • Sewage treatment has substantially reduced the occurrence of areas of oxygen-depleted water near discharge locations in water bodies with long residence times. However, disposal of treated sewage effluent has a number of adverse effects caused by the pathogens, toxic substances, and nutrients.
  • All sewage treatment produces an organic rich semisolid sludge that contains substantial quantities of trace metals and pathogens. This sewage sludge must be used for landfill or incinerated, both of which create environmental problems. Some sewage sludge was once disposed in the oceans.


  • Nutrients in sewage have contributed to, or caused, eutrophication in coastal waters, including changes in the dominant species of phytoplankton, an increase in nuisance blooms, and an increase in periodic anoxia caused by plankton blooms.

Toxic Substances

  • Toxic substances have, in some limited instances, reached concentrations in discharge waters that are toxic to marine organisms and have reached concentrations in biota in the discharge region that are too high to be safe for human consumption.
  • Most toxic substances in sewage now come from nonpoint sources such as people’s homes rather than from industry.


  • Many pharmaceuticals and drugs disposed in sinks or toilets or passed through the body are not removed by sewage treatment and there is growing evidence that these can cause adversely affect some marine species.


  • Pathogens in sewage have caused vast areas of shellfish beds to be unsafe and closed to fishing.

Effects on the Benthos

  • Sewage discharges have caused degradation of the benthic ecosystem surrounding some discharge outfalls.
  • The area of degraded benthic environment is greater as the volume of discharge through an outfall increases. For small discharges there is no area of degraded benthos.
  • Current regulatory philosophy is to collect sewage effluents into a few large discharges in the hopes of limiting the areas degraded, but this strategy is not supported by the scientific facts.

Lessons Learned

  • The organic matter and nutrients in sewage sludge or treated sewage dumped or discharged into the oceans can cause blooms and eutrophication but only if the discharges are large and the residence time of the water in the discharge area is long. If dispersion is adequate, sewage discharges can actually enhance the primary and secondary production of the discharge area.
  • Sewage sludge and sewage have also caused problems in the marine environment as a result of the toxic metals, organic substances, and pathogens that they contain. Sewage treatment is not designed to remove these from the sewage although some removal to the sewage sludge and some die off of pathogens does occur in the treatment process. If these toxins and pathogens were removed, sewage could be used to enhance marine ecosystem productivity if properly dispersed in appropriate locations.
  • The environmental impacts of sewage discharged to the oceans are increased rather than decreased by the current regulatory philosophy that discharges should be confined to the smallest possible number of locations in the oceans rather than in a more highly dispersed manner.

19.7 Urban and Agricultural Runoff

  • Urban and agricultural runoff are nonpoint sources that contain a very wide range of different contaminants including nutrients from fertilizer, herbicides and pesticides, and trace metals from a variety of sources.
  • These nonpoint sources contribute more of many important contaminants to most estuaries than the contribution of all point-source discharges such as industrial and sewage outfalls.
  • Nonpoint sources are more difficult to control and reduce than are the point sources, which may be one reason that less has been done to control them than point sources.

19.8 Industrial Effluents

  • Industrial effluents were major sources of contaminants and pollution several decades ago. However, enforcement of the 1970 Clean Water Act has dramatically reduced such sources.
  • The only recorded instance where humans have died as a result of toxic substances discharged to the marine environment took place in Minamata, Japan, where a number of people died or were permanently disabled or afflicted by the poisoning effects of methylmercury discharged by a chemical plant and concentrated in shellfish and fishes used for food.
  • There are a number of recorded instances of industrial discharges of toxic substances causing ecological damage in the marine environment. One of the largest and best known of such incidents was the poisoning of pelicans, sea lions, and other marine animals by DDT and its metabolites discharged off Southern California several decades ago. These populations are now steadily recovering.

19.9 Dredged Material

  • Sediments are dredged from many shipping channels and marinas to aid navigation. In some cases the volume of material dredged is very large and the dredged material is contaminated by toxic metals and organic compounds released to the estuary or ocean decades before environmental discharges were controlled by law.
  • Much of the dredged material is dumped back into the water at dumpsites that are mostly near the estuary mouth, either just inside or just outside. When dumped, the material falls to the seafloor and usually accumulates in a mound. However, much of the fine-grained material that contains most of the contaminants may be released to the suspended sediment and transported back up the estuary in the lower layer of the estuarine circulation.

19.10 Plastics and Trash

  • Plastics do not decompose readily in the oceans.
  • Plastics and other trash are dumped in the oceans, lost overboard by vessels, and washed into the oceans from rivers.
  • The plastics include fishing nets and fishing line that can entangle birds, mammals, and fishes and kill them. Plastic bags and plastic particles are ingested by some marine animals and may cause them to starve, or may cause mammals to drown.
  • Plastics and other trash wash up on beaches and create an aesthetic problem.
  • Illegal disposal of medical wastes has from time to time caused beaches to be closed to human use.

19.11 Antifouling Paints

  • Antifouling paints applied to ships’ hulls and underwater structures are effective because they are toxic. However, the toxic substances are released to the water and may cause adverse effects in harbors and marinas where the water residence time is long and concentrations of the toxic substances can be high.
  • Tributyltin-based paints, which were used extensively in the late parts of the twentieth century, were particularly damaging in the marine environment because they are very toxic and do not readily biodegrade. Use of tributyltin paints is now restricted in most countries.

19.12 Radionuclides

  • Radionuclides were released to the oceans in substantial quantities during the 1950s when atmospheric testing of nuclear weapons was practiced. During the same period large quantities of low level nuclear wastes were dumped in the oceans.
  • At present, only Russia still dumps radioactive wastes in the oceans, and the United Kingdom still discharges radioactive wastes through one ocean outfall. Both of these practices are being phased out.
  • During the cold war, the former Soviet Union disposed of large amounts of highly radioactive wastes at sites in the Pacific and Arctic Oceans. They also disposed of a number of nuclear submarines (complete with their reactors and radioactive material inside) at sites in the Arctic Ocean.
  • The total quantity of radioactive materials dumped in the oceans is small compared to the quantity released to the environment in the Chernobyl nuclear reactor accident.
  • It is believed that the radioactive materials dumped in the oceans will remain mostly trapped in the ocean sediments and will not cause any major ecological or human health problem. However, there still remains concern about the large concentrations of radioactive materials in the submarine nuclear reactors dumped by the Soviet Union.

19.13 Nonindigenous Species

  • Nonindigenous species can be introduced to ocean and estuarine ecosystems in ships’ ballast water, attached to ships’ hulls, and in many other ways.
  • Nonindigenous species have also been deliberately introduced into some estuaries and coastal waters in an effort to develop commercially or recreationally valuable populations of the species.
  • Some nonindigenous species, especially those that survive long passages in or on ships, can be very tolerant of environmental conditions and can compete effectively with indigenous species for food and/or habitat.
  • In some estuaries such as San Francisco Bay, most species are now nonindigenous and only a small percentage of the indigenous species that were present 200 years ago are still present.
  • Although introduced species may be commercially or recreationally valuable they usually are not, and the introduction of a new species often changes the balance of the ecosystem in totally unexpected and undesirable ways.

19.14 Habitat Alteration

  • Destruction of habitat, especially destruction of wetlands, has adversely affected many species that have life cycles in which the juvenile stages live in these wetlands.
  • Alteration of river flows by dams and levees has adversely affected habitat for estuarine species by reducing the area of high productivity and has drastically affected the ability of anadromous and catadromous species to complete their migrations.
  • Other habitat alterations such as construction of piers, levees, seawalls, jetties, and groins all alter circulation and sediment characteristics and, as a result, the distribution of biota.


Critical Concept Reminders:

CC.8 Residence Time (p. 551)

  • The residence time of seawater in a given segment of the oceans is the average length of time the water spends in that segment. The residence times of some coastal water masses are long, therefore some contaminants discharged to the coastal ocean can accumulate to higher levels in these long residence time regions than in areas with shorter residence times. To read CC8 go to page 19CC.

CC.14 Photosynthesis, Light, and Nutrients (p. 552)

  • Photosynthesis is the primary process by which simple chemical compounds are made into the organic compounds of living organisms. Photosynthesis depends on the availability of carbon dioxide, light, and certain dissolved nutrient elements including nitrogen, phosphorus, and iron. Alterations of the availability of any of these elements by human activity can have adverse consequences in marine ecosystems. To read CC14 go to page 46CC.

CC.17 Species Diversity and Biodiversity (p. 553)

  • Biodiversity is an expression of the range of genetic diversity; species diversity; diversity in ecological niches and types of communities of organisms (ecosystem diversity); and diversity of feeding, reproduction, and predator avoidance strategies (physiological diversity), within the ecosystem of the specified region. Species diversity is a more precisely defined term and is a measure of the species richness (number of species) and species evenness (extent to which the community has balanced populations with no dominant species). High diversity and biodiversity are generally associated with ecosystems that are resistant to change. Changes in diversity are often used as an indicator of the impacts of human activities. To read CC17 go to page 53CC.

CC.18 Toxicity (pp. 555, 556, 564, 572).

  • Many dissolved constituents of seawater become toxic to marine life at levels above their natural concentrations in seawater. Some synthetic organic chemicals are especially significant because they are persistent and may be bioaccumulated or biomagnified. To read CC18 go to page 54CC.


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