16.8: Chapter Summary

Assimilative Capacity.

The oceans can safely assimilate wastes if the rate at which the waste is introduced does not exceed the assimilative capacity, which is the point at which toxic or other adverse effects occur. Assimilative capacity varies by waste and location because residence time determines the concentration of contaminants, and each contaminant has a different concentration–effect relationship. Release of a specific type and amount of contaminant that causes pollution in one location may not do so elsewhere.

Adverse Effects of Human Activities.

Increased turbidity can reduce primary productivity. Excessive nutrient inputs can alter phytoplankton species composition and can cause blooms, eutrophication, and hypoxic or anoxic bottom waters.

Habitat can be altered by development and by discharges of wastes. Many areas of mangrove swamps and coastal wetlands have been filled and developed. Damming of rivers has prevented or inhibited anadromous and catadromous fish migrations and has altered salinity regimes and circulation in estuaries. Dredging, anchoring, fishing, and other activities disturb and alter seafloor habitats.

Community structures within ocean ecosystems can be altered when human activities affect habitat or affect some species but not others. Selective harvesting of fish, invertebrates, and marine mammal species has caused major changes in communities in some areas.

Toxic substances released to the oceans can be concentrated in seafood and present a human health risk or result in economic loss if the resource is unfit to eat. Contamination of seafood, particularly shellfish, has caused many shellfisheries to be closed and sometimes has caused human disease outbreaks.

Coasts have been marred and altered in many locations by structures. Many beaches are periodically closed because of contamination with improperly treated sewage or medical wastes.

Toxicity.

With the possible exception of carcinogens, teratogens, and mutagens, toxic substances are toxic only above a critical concentration that varies by substance, species, and even life stage within a species. The toxicity of a waste can be evaluated by comparing contaminated sites with control sites or by performing laboratory bioassays. Both approaches are difficult, and their results must be carefully interpreted. Many toxic substances are bioaccumulated by most species. Certain toxic substances are biomagnified, so their concentration increases at each trophic level.

Petroleum.

Natural inputs from seeps are the largest source of most oil released to the oceans. Stormwater runoff and other discharges from the everyday uses of petroleum are also a very large source. Oil tanker accidents are a far smaller source, and releases from offshore platforms are even smaller.

When oil is spilled, a slick forms and the volatile compounds evaporate to the atmosphere or dissolve, leaving lumps of tar or, if seas are rough, a gummy suspension called “chocolate mousse.” If the slick reaches shore, oil coats any substrate but is eventually decomposed by bacteria.

Oil spill events damage the ecology of shores reached by the oil and kill seabirds and marine mammals in the spill area. Some damaged areas recover naturally within a few years. Wetlands and other low-energy coastal habitats recover more slowly than high-energy rocky shores or beaches. Some spill cleanup efforts may have little or no benefit and may even retard natural recovery. Long-term effects may persist in some spill areas, but they are difficult to detect and document.

Sewage.

Sewage is discharged to the marine environment to protect public health. Sewage is a natural material, composed mostly of water and particulate and dissolved organic matter that is contaminated with chemicals from household wastes and industry. Sewage also contains pathogens that can be classified as pollution because the pathogens can infect humans through water contact or seafood consumption. Organic matter and nutrients, especially nitrogen, can cause algal blooms and anoxia, and toxic compounds can accumulate in discharge-area ecosystems. Sewage treatment removes floatables, particulates, and some dissolved organic matter, pathogens, and some potentially toxic chemicals. Treatment has substantially reduced the incidence of anoxia in rivers and estuaries.

Particulate organic matter from sewage outfalls can accumulate in sediments around the outfall and cause alteration of benthic infaunal communities. If outfalls discharge large amounts of sewage, the benthic infauna is degraded in a zone around the outfall, but the effects decrease with distance. If the same amount of sewage were discharged from several small outfalls instead of a single large one, no areas would be degraded, and the benthic infauna around each outfall would be enhanced.

Urban and Agricultural Runoff.

Runoff from streets and agricultural land contains toxic chemicals, nutrients, particles from paints, solvents, oil, and other substances deposited by vehicles, combustion products, pesticides, fertilizers, and many other sources. In estuaries, these nonpoint source inputs are often much larger than sewage or industrial inputs.

Industrial Effluents.

Industrial discharges of toxic substances and other contaminants have been considerably reduced in recent decades. However, violations of regulations and illegal discharges to sewers and streams are common. The industrial effluents of a chemical company in Minamata, Japan, in the 1950s caused the only documented case of human deaths due to toxic substances (methylmercury) discharged to the marine environment. DDT discharges have caused ecological damage in a number of coastal ocean areas.

Dredged Material.

Navigation channels that are dredged to maintain navigation depths are often sites of accumulation of past and present contamination. Dredging damages the benthos at the dredging site and at the dump site, which is usually nearby in the estuary or coastal ocean. Toxic contaminants in dredged material are preferentially released to the suspended sediment during dumping and may be bioavailable. Most of the fine-grained, contaminant-rich dredged material dumped in estuaries or just outside estuary mouths is transported back into the estuary by the estuarine circulation and may accumulate in channels to be dredged again.

Plastics and Trash. 

Trash, particularly floating trash, is dumped into rivers and the ocean and can cause aesthetic problems when washed up on beaches or accumulated in areas visited by divers. Plastics are a problem because they degrade very slowly, and they may strangle or choke birds and marine organisms or accumulate in the gut until the animals starve to death.

Antifouling Paints.

To be effective, antifouling paints must be toxic and must be slowly released to solution. Tributyltin, used in some paints, is very persistent and highly toxic. It has accumulated in some estuaries to toxic levels, drastically reducing populations of commercially valuable shellfish and other invertebrates.

Radionuclides.

The oceans have been contaminated by radionuclides from testing of nuclear weapons, liquid waste discharges, and dumping of radioactive wastes. Other than at a few nuclear bomb test sites, radionuclide concentrations in the oceans do not present a significant human health or ecological risk. However, the former Soviet Union disposed of nuclear submarine reactors and large quantities of radioactive wastes in the Arctic Ocean and the Sea of Japan, and there is concern that these materials may eventually release radionuclides to the food web.

Noise.

The oceans are filled with numerous sources of natural sound. However, additional noise is contributed by ships, resource extraction activities, and military and civilian use of sonar. Intense sounds at certain frequencies have been found to be harmful to marine mammals. As a result, there is considerable concern that the increased anthropogenic sound in the oceans may cause harm to marine species.

Nonindigenous Species.

Species are carried in ships, in ballast water, and attached to hulls and are inadvertently introduced to ecosystems where they do not occur naturally. Species are also deliberately introduced. Many introduced species do not thrive, but some outcompete important species in their new habitat and may totally disrupt natural food webs. Estuaries have been particularly severely damaged by nonindigenous species.

Habitat Alteration.

Alteration or destruction of habitat, especially filling of wetlands, is among the most damaging forms of pollution, especially in estuaries and the coastal zone. Habitat can also be altered in many other ways, such as by dredging, by the construction of structures that affect circulation and erosion, and by the reduction of river flow rates that results from the withdrawal of freshwater for human uses.

Fishing.

Many ocean fisheries, perhaps most, are overfished. Many fish stocks have collapsed due to overfishing, and some do not recover even if the fishing pressure is relieved. Fishing also contributes to ocean pollution in many other ways, especially through loss of fishing equipment, much of which is now made of synthetic materials that are highly resistant to decomposition, and bottom trawling that destroys benthic habitat.

Climate Change.

Global climate change has caused the oceans to warm. The warming has caused changes in a number of ocean processes that have resulted in adverse impacts on marine ecosystems and humans, some of which are likely to become greater and many of which will continue to drive change in the oceans for centuries to come. Impacts include changes in ocean circulation and evaporation/precipitation patterns; increases in wave and storm energy affecting coastlines; melting of sea ice and glaciers; rising sea level; and impacts on marine biological systems, including coral bleaching due to warmer water. The rate of change of ocean temperature, acidity, oxygen levels, and other ocean conditions due to climate change is faster than has occurred at any time in Earth’s recent history, and these changes are expected to result in species extinctions and continuing disruption of marine species life cycles and distribution. 

Acidification.

Anthropogenic releases of carbon dioxide cause carbon dioxide to be taken up by seawater, where a series of chemical reactions causes the acidity of ocean water to rise. Adverse effects of elevated acidity have already been observed in several areas of the world’s oceans involving a variety of marine species. Most at risk are coral reefs and marine species that use calcium carbonate to construct their skeletal material, such as oysters and pteropods. 

Deoxygenation.

Elevated ocean temperatures due to climate change have reduced the solubility of oxygen and, therefore, the concentration of dissolved oxygen in ocean waters. Lower oxygen concentrations in water sinking to form deep ocean waters, together with changes to stratification and primary production caused by anthropogenic releases of carbon dioxide and nutrients, especially nitrogen, have expanded the area of open ocean oxygen minimum zones in which hypoxia and anoxia are present. Upwelling has caused some of this water to be transported onto the continental shelf off Oregon, killing marine species and causing what is known as a dead zone. Similar processes have also led to the development of hypoxia and anoxia in more than 500 coastal areas that have resulted in dead zones and the deaths of marine organisms that are unable to avoid these zones. 

Climate Change Summary.

The release of anthropogenic carbon dioxide, together with anthropogenic releases of nutrients, especially nitrogen, causes multiple impacts on the oceans. These impacts include sea level rise, loss of sea ice, coral bleaching, ocean acidification, and deoxygenation due to the warming of ocean water. These anthropogenic releases also cause changes in ocean stratification and primary production that also cause deoxygenation. The combination of elevated carbon dioxide in the atmosphere, warming and acidification of ocean water, and deoxygenation of the oceans has been associated with at least some of Earth’s five mass extinctions. 

Risk Assessment and Management.

Scientific risk assessment must consider both the probability that a feared impact might occur and the severity of the impacts if they do occur. The consequences of climate change/acidification/deoxygenation could include mass extinctions and other global and long-lasting environmental and economic impacts. These potential consequences make this the most important ocean pollution issue.