Ocean Pollution: Sources, Types, and Impacts on Marine Ecosystems

Unquestionably, pollution—from a myriad of sources—has affected every marine system on Earth. Indigenous human populations in Arctic areas have the highest contaminant levels of any people on Earth, a result of ingesting marine fish and mammals, which bioaccumulate toxins due to their high trophic levels. Approximately 90 percent of pollution entering the ocean is derived from land-based sources; the remainder is a result of deliberate or accidental dumping. It has been assumed that land-based sources of pollutants reaching the oceans are primarily via rivers; however, transport of pollutants through the atmosphere to the oceans is more significant. Sources of pollution in the ocean may be point source (from a single source, such as an industrial facility or sewage discharge) or nonpoint source (where the exact source is unknown, e.g., agricultural runoff).

Most researchers recognize the primary types of pollution as biological (pathogens), toxic substances (contaminants), nutrients, marine debris, hydrocarbon spills, and underwater noise. Certain scientists also recognize ocean acidification as a result of climate change as a type of pollution.

Biological pollution (pathogens) includes bacteria, viruses, and parasites, either occurring naturally or as a result of human activities. They may reside in seawater, the seabed, or live on or within marine organisms. Anthropogenic (human) sources of pathogens in the ocean include sewage and agricultural runoff. Pathogens affect human health through direct ingestion (e.g., shellfish, seawater when swimming), skin contact, or inhalation. Shellfish are known to transmit gastroenteritis and hepatitis, as well as cholera and typhoid fever. Marine mammals are known to have been infected by terrestrial animal diseases, likely transmitted from freshwater into the ocean. Climate change may exacerbate the effects of certain marine pathogens, mainly through increased temperatures providing optimal growth conditions.

A man scavenges among the plastic litter on a beach in Senegal’s Petite Cote, Western Africa (Juan Vilata/Dreamstime.com)

Over the past 100 years of industrial chemistry, hundreds of synthetic compounds have found their way into the world’s oceans. Thousands of new compounds are created each year with sometimes unknown implications should the compound be released into the environment and without a regulatory regime to apply precaution to their use. Synthetic compounds have been categorized in many ways, but generally, they can be separated into persistent organic pollutants (POPs), pesticides, pharmaceuticals, flame retardants, and other substances captured under the general heading of “contaminants of emerging concern” (or CECs). Unlike pathogens, the consequences of synthetic compounds on the environment and human health are more subtle, including long-term bioaccumulative effects leading to potential chronic illness and cumulative/delayed effects.

POPs capture a large group of chemicals that are similar in that they are halogenated (containing fluorine [F], chlorine [C], bromine [Br], iodine [I], and astatine [At]) organic compounds that degrade very slowly in the environment, bioaccumulate in fatty tissues, and are highly toxic. POPs have a wide range of effects, including changes in behavior, development, growth, and reproductive capacity.

Uses of POPs have varied since their introduction, but common uses have included pesticides (e.g., dichloro-diphenyl-trichloroethane, better known as the acronym DDT), industrial chemicals (e.g., polychlorinated biphenyls better known as PCBs), solvents, and pharmaceuticals. Over time, the use of the more persistent POPs has slowly been phased out through international agreements or individual nations passing bans, resulting in a gradual decrease of these substances in the marine food chain. However, although the compounds replacing the 20+ banned POPs are certainly less persistent and toxic, they continue to affect marine food webs, often in new and unknown ways.

“First-generation” pesticides, such as DDT, were banned by many countries in the 1970s and internationally in 2001 due to persistence and toxicity. “Second-generation” pesticides consist primarily of organophosphates and carbamates, which are cholinesterase inhibitors, meaning they inactivate a key enzyme necessary for nervous function. These compounds break down much more quickly in the environment; however, they can still be toxic to marine life. “Third-generation” pesticides work either by mimicking juvenile hormones so the insect cannot reach adulthood or by preventing insect larvae from molting and are the safest pesticides for marine environments.

The effects of pharmaceuticals used to treat or prevent human disease on marine environments are a relatively new branch of study. Humans use over 6,000 different prescription and nonprescription drugs that enter the marine environment through either direct disposal (e.g., flushing unused medicines) or human wastes.

Approximately 90 percent of oral drugs pass through the body and in turn pass unaffected through sewage treatment systems, ultimately enter the oceans. Although there have been demonstrated effects on marine life from certain pharmaceuticals, such as birth control pills reducing reproductive success in invertebrates due to endocrine disruption or the development of antibiotic resistance in marine microbes, the overall impacts of drug inputs into the marine environment are poorly understood. However, early evidence suggests that many drugs are biologically active in marine environments at very low concentrations and have the capacity to affect the behavior and the reproductive success of marine biota that reside near human settlements. In addition to pharmaceuticals, personal care products such as cosmetics, antibacterial soaps, lotions, and insect repellents are not removed in the sewage treatment process and they exhibit many of the same properties as prescription drugs.

Lastly, CECs include a range of compounds such as flame retardants used in furniture, carpets, clothing, and toys; plasticizers that increase the flexibility or fluidity of materials; solvents capable of dissolving other substances; surfactants that break down the interface between water and oils and therefore aid cleaning substances; and nanoparticles, which are substances smaller than 100 nm used in a range of industrial processes and consumer products.

Metals are naturally occurring elements that, when increased above background levels by human activities, may have significant impacts on environmental and human health. Certain metals such as copper (Cu) and zinc (Zn) are necessary to sustain life; however, they can be toxic at higher concentrations. Other metals, including cadmium (Cd), chromium (Cr), lead (Pb), and mercury (Hg), have no role in biological processes but they are highly toxic. Metals are introduced into the ocean via land-based processes (mining, industrial processes), the atmosphere (e.g., coal combustion releasing mercury), and antifouling compounds used to keep vessels and other marine structures free of marine plants and invertebrates.

Metals affect the marine environment and humans in a number of ways. First, metals— like certain other pollutants—bioaccumulate up the food chain and concentrate in top predators (e.g., tuna, swordfish, marine mammals, seabirds). Metals may bind to small clay and silt particles that are ingested by benthic (bottom) animals, which are in turn consumed by other animals, or stored in near-shore plant and marine algae tissues and in turn consumed. At certain concentrations, metals may affect respiration, digestion, nervous, and reproductive system functions, leading to changes in behavior, slower growth, and reproductive failure.

Although copper, cadmium, chromium, and lead are generally found in localized areas (e.g., marinas, industrial areas) with localized impacts on environmental and human health, mercury is a global problem. Mercury is a potent neurotoxin that easily bioaccumulates, which is why most countries now recommend limits on tuna and swordfish consumption for children. In 2013, the global Minamata Convention on Mercury came into force, given the rapid rise in mercury in marine biota and corresponding human health issues.

Nutrients include human inputs of nitrogen (N) and phosphorus (P) that exceed natural background levels, resulting in algal blooms. Aside from upsetting the natural state of the food web, these algal blooms may also be toxic (“harmful algal blooms” or HABs) to marine animals (fish and shellfish) or humans or consume available dissolved oxygen, creating “dead zones” (hypoxia or anoxia) due to a lack of oxygen for other species, a process termed “eutrophication.” Harmful algal blooms have expanded in recent decades, primarily due to increasing coastal human populations and the resultant creation of N from agriculture (animal wastes and fertilizers), sewage, and the deposit of airborne N from fossil fuel combustion into rivers and oceans. Climate change may also be exacerbating HABs due to increases in water temperatures, fostering the increased growth of primary producers such as phytoplankton.

Marine debris is any intentionally or unintentionally discarded solid, manufactured item in the marine environment. The National Oceanic and Atmospheric Administration estimates that approximately 600 million kilograms of trash enter the ocean each year, 60 to 80 percent of which is plastics. Polystyrene foam, or Styrofoam, is another major source of marine debris that tends to float, whereas the majority of plastics sink. The majority of marine debris (70 percent) is found on the seabed, and the remainder is split evenly between floating on the ocean surface and found on beaches.

The major types of marine debris are domestic materials (e.g., shopping bags, plastic bottles), industrial materials, and fishing gear. Plastics are durable and may not break down for hundreds of years. Additionally, plastics absorb and concentrate hydrophobic organic pollutants, thus acting as reservoirs and transport mechanisms for these substances. Approximately 80 percent of marine debris comes from land-based sources.

Marine debris affects the marine environment in a number of ways. Ingestion and entanglement are the primary effects on marine mammals, seabirds, sea turtles, and fish. In highly populated coastal areas, plastics may affect plants and animals living on or in the seabed through accumulation, resulting in the degradation of habitat through smothering and toxicity. It is estimated that marine debris is responsible for up to 100,000 marine mammal deaths annually, and 90 percent of seabirds have plastics in their bodies, likely contributing to the 67 percent decline in global seabird populations between 1950 and 2010. Ingested plastics inhibit growth and reproduction by lacerating or displacing food in animals’ stomachs or transferring toxic substances to the host. Lost or abandoned fishing gear can continue to “ghost fish” where fish persist in getting caught in nets that may have been adrift for many years.

Hydrocarbon (oil) spills are a unique type of pollution. Hydrocarbon spills occur as a result of the transport of different forms of oil or the exploration and development of offshore oil resources. Approximately half of the world’s oil production (approximately 1.6 billion tons) is transported by sea. Much of this transport is over long distances in large tankers that pass through straits and transit along coastlines. This means of transport has resulted in many serious accidents and spills over the past several decades; however, due to improvements in safety, currently about 12 percent of oil entering the ocean is a result of transportation accidents. Although the average number of major oil spills per year has dropped from twenty-five in the 1970s to three today, spills continue to affect fisheries, tourism, and coastal economic activities.

Marine environments also currently produce one-third of the world’s hydrocarbons and have the potential for significant expansion. The shallow marine (<400 m) and deep marine (>400 m) environments are estimated to contain 36 percent and 11 percent of the global distribution of oil-bearing reservoir rocks, respectively. Hydrocarbon-bearing rocks are found in the deep marine environment as a result of avalanches that move sediments into the deep ocean. Early efforts to develop offshore hydrocarbons were undertaken in the 1920s and 1930s in Azerbaijan, the Gulf of Mexico, and Venezuela. However, it was not until the 1950s that offshore oil and gas extraction became economically and technically feasible in the Gulf of Mexico. Globally, 500 billion barrels of oil have been discovered in offshore areas, of which 200 billion barrels have been extracted. It is estimated that a further 300 billion barrels may yet be discovered in offshore areas.

In addition, a significant amount of oil is transported short distances by seabed pipelines from production wells to offshore or onshore facilities.