Global overview

The most visible and familiar impact of human activity on the marine environment is oil pollution caused by tanker accidents. 
Yet despite the scale and visibility of their impacts, the total quantities of pollutants entering the sea from shipwrecks are dwarfed by those of pollutants introduced from other sources, like domestic sewage, industrial discharges, urban and industrial run-off, spillage, explosions, sea dumping operations, mining, agricultural nutrients and pesticides, waste heat sources, radioactive discharges and natural oil seepages.
Land based sources are estimated to account for around 44% of the pollutants entering the sea and atmospheric inputs account for an estimated 33%. By contrast, shipping and accidental spills account only for around 12%, ocean dumping for 10% and offshore mining for 1%.

By the year 2050 it is estimated that the world's population could have increased to around 9 billion. Of these, some 60% will live within 60 km of the sea. The agricultural and industrial activities required to support this population will increase the already significant pressures on coastal areas.
Europe counts with 70,000 km of coastline. In Europe, maritime regions represent about 40% of total population and count for 40% of European economy.

The 20 unforgettable oil spills - Source: Cedre, ITOPF

In 2002, the wreckage of the Prestige spilled 64,000 tons of oil off the Galician coast. The spill polluted hundreds of kilometers of coastline and more than one thousand beaches on the Spanish and French coasts. This resulted in huge economic losses and caused great damage to the local fishing industry -Source: Archive Photo EFE

Pollution from ships

Europe is the world's largest market in crude oil imports, representing about one third of the world total.
90% of oil and refined products are transported to and from Europe by sea.

Whether by accident or normal ship operation, the marine environment is degraded. Accidents resulting in massive spill provide gripping illustrations of the problem of vessel pollution.

While being toxic to marine life, polycyclic aromatic hydrocarbons (PAHs), one of the components in crude oil, are very difficult to clean up, and last for years in the sediment and marine environment. Large oil spills at sea constitute a threat to the environment, placing enormous demands on the national authorities responsible for response and clean-up operations.

A bird covered by thick fuel oil sits helplessly in clumps of oil mixed with sand and seaweed on the shore near Port Kavkaz, on the Russian Black Sea (November 2007). Source : Sergei Grits/AP

Besides accidental pollution, caused by ships in distress, there are three types of routine ship operations which pollute the sea: ballast water (the water used to fill tanks when empty in order to maintain its stability at sea), tank washings and engine room effluent discharges. Due to these operations large amounts of oil are pumped deliberately from ships every day, along almost all of Europe's coastline. This is the greatest source of marine pollution by ships, and the one that poses an insidious long term threat to the marine and coastal environment.

Ships also create noise pollution that disturbs natural wildlife, and water from ballast tanks can spread harmful algae and other invasive species.

Toxic chemicals

The input of man-made chemicals to the oceans potentially involves a huge number of substances. 63,000 different chemicals are thought to be in use worldwide with 3,000 accounting for 90% of the total production tonnage. Each year, up to 1,000 new chemicals may be brought onto the market.

Of all these chemicals some 4,500 - including PCBs (Polychlorinated Biphenyls), DDT, pesticides, furans, dioxins and phenols - fall into the category of persistent organic pollutants (POPs). They're resistant to breakdown and have the potential to accumulate in the tissues of many species of aquatic life in a process called bioaccumulation, causing hormone disruption. They are also known to accumulate in benthic environments, such as estuaries and bay muds.
Scarily, seafood consumed by people living in temperate regions is also affected by POPs. Oily fish tend to accumulate POPs in their bodies and these can be passed to human consumers.

Marine debris

Marine debris (or marine litter) is human-created waste that has deliberately or accidentally become afloat in a lake, sea, ocean or waterway. 80% of marine debris reaches the sea via rivers. Ocean dumping, disposal of objects and wind-blown landfill waste are all contributing to this problem.

80% of marine debris is plastic, most of which is non-biodegradable. Waterborne plastic is both unsightly and dangerous, and poses a serious threat to fish, seabirds, marine reptiles, and marine mammals, as well as to boats and coastal habitations.

Trace metal pollution

Apart from chemicals and plastic, heavy metals - such as mercury, lead, nickel, arsenic and cadmium - are also compounds which do not disintegrate rapidly in the marine environment and can accumulate in living organisms. Heavy metals have a relatively high density and are toxic or poisonous at low concentrations.

Trace metal pollution from metal mining, production and processing industries can damage the health of marine plants and animals and render some seafood unfit for human consumption. The contribution of human activities can be very significant: the amount of mercury introduced to the environment by industrial activities is around four times the amount released through natural processes such as weathering and erosion.

Diagram illustrating eutrophication. Source: EPA, US.
















 

Phaeocystis blooms can produce unpleasant foam which often accumulates on beaches - Source

Eutrophication

Eutrophication is an increase in chemical nutrients, typically compounds containing nitrogen or phosphorus, in an ecosystem. It can result in an increase in the ecosystem's primary productivity (excessive plant growth and decay), and further effects including lack of oxygen and severe reductions in water quality, fish, and other animal populations.

The biggest culprits are rivers which empty into the ocean runoff waters containing many chemicals used as fertilizers as well as waste from livestock and humans. Nutrient pollution from sewage discharges and agriculture can result in unsightly and possibly dangerous "blooms" of algae in coastal waters. As these blooms die and decay they use up the oxygen in the water.
This leads, in some areas, to dead zones where oxygen dissolved in the water falls to levels unable to sustain marine life.

Estuaries tend to be naturally eutrophic because land-derived nutrients are concentrated where runoff enters the marine environment in a confined channel. The World Resources Institute has identified 375 hypoxic coastal zones around the world, concentrated in coastal areas in Western Europe, in the USA and in East Asia.

In addition to land runoff, atmospheric anthropogenic nitrogen can also enter the open ocean.

Radioactive contamination and thermal pollution

According to Greenpeace, normal operation of nuclear power stations can pollute the sea, but by far the biggest point-sources of man-made radioactive elements in the sea are the nuclear fuel reprocessing plants at La Hague in France and at Sellafield in the UK. These discharges have resulted in the widespread contamination of living marine resources over a wide area; radioactive elements traceable to reprocessing can be found in seaweeds as far away as the West Greenland Coast and along the coast of Norway.

A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers. Water used as a coolant is returned to the natural environment at a higher temperature. The change in temperature impacts aquatic organisms by decreasing oxygen supply and affecting ecosystem composition.

Acidification

The oceans are normally a natural carbon sink, absorbing carbon dioxide from the atmosphere. Because the levels of atmospheric carbon dioxide are increasing, the oceans are becoming more acidic. The potential consequences of ocean acidification are not fully understood, but there are concerns that structures made of calcium carbonate may become vulnerable to dissolution, affecting corals and the ability of shellfish to form shells.

Sources
Defending our Oceans/Pollution - Greenpeace
Prestige oil tanker, Spain - UNEP Emergency Response Service 
Prestige, Spain, 13 November 2002 - International Oil Pollution Compensation Funds
The Community framework for cooperation in the field of accidental or deliberate Marine Pollution - European civil Protection
Wikipedia Marine debris - Marine pollution - Thermal pollution

Links
Educative website Understanding black tides
Fish out of water — marine management in a changing climate
European Environment Agency:
Marine biodiversity: life in seas under threat
10 messages for 2010 - Marine ecosystems