The world's oceans can be divided into two zones: coastal and open. The coastal zone constitutes about 10% of the total ocean area and is the receptacle for wastes from rivers, direct run-off from the land, oil spills, and ocean dumping. When pollution occurs in marine waters, it is most likely to be observed first in the coastal waters, and its effects are likely to be serious there. The deep waters of the open ocean, usually 300 feet (91 meters) or more in depth, are mostly out of contact with coastal water and surface waters. The mixing of the deep and surface waters may take hundreds, even thousands of years, depending on the basin involved. Much of the manmade wastes dispersed to the open ocean: litter, plastics and oil slicks, are still in surface waters; but DDT and other chlorinated hydrocarbons are found in open ocean organisms; and radioactive isotopes of strontium and caesium from nuclear bomb detonations can now be found at depths of 1,000 meters.
Atmospheric transport of pollutants is part of the transfer process from land to sea, and a significant contributor to marine pollution. There is the movement of fission products from nuclear weapons' tests through the stratosphere, with fallout of these radionuclides on virtually all parts of the globe, demonstrated how widely dispersed substances released into the atmosphere can become. Then the DDT problem, with virtually every marine animal found anywhere near the sea surface exhibiting measurable quantities of DDT. This widespread distribution of a substance, used mainly in certain agricultural and forested parts of the world, was shown to be a result of rapid dissemination through the atmosphere. The entry of about 20,000 tonnes of lead into the sea from automobile emissions is clearly an atmospheric transport and washout phenomenon. There is some evidence that other metals, such as mercury and cadmium, are transported around the globe by atmospheric currents and may be washed out into the sea with rain. About 5,000 tonnes of mercury enter the sea each year, mostly as a result of industrial dumping. This about equals the natural contribution from the land. Mercury can be highly toxic, but it depends on the concentrations and the organisms involved. Sometimes inorganic mercury is altered by natural processes to become poisonous methyl mercury. The case of the Minamata deaths in Japan is an example. Sulphur dioxide emissions from coal-burning thermal plants and petroleum refineries in the UK, Germany, and other parts of western Europe have allegedly given rise to acidic rain and a low pH in the soft waters of some Scandinavian lakes and rivers. Although pH shift is not likely to become a problem in highly-buffered sea water, the acidic rain situation does demonstrate what can happen through atmospheric processes.
Marine contamination occurs mainly in the Northern Hemisphere, in estuaries, bays and land-locked seas, such as the Mediterranean, Baltic and North Seas and certain estuaries in North America. In the Southern Hemisphere, Port Philip Bay in Australia and various coastal waters around southern Africa and South America have been reported to be contaminated.
Contamination reaches the sea through rivers, direct coastal outfalls, drainage from human settlements and agricultural land, and deposition from the atmosphere. Some pollutants are discharged from shipping and offshore structures such as oil rigs. Persistent solid wastes also enter the sea from urban areas, and 6.4 million tons are dumped annually from shipping: of this the proportion of plastic is currently low (under one percent) but is expected to increase unless controls are applied.
Acute damage has occurred in areas around oil refineries and in industrialized estuaries, bays and coastal zones, where fish numbers have been reduced and many species eliminated. Oil pollution is a nuisance, a bird-killer and a threat to coastal shellfish and tourism, it is still on the increase but cannot be proved to have had yet any serious impacts on a wide scale.
Much of the suspended particulate matter in the sea originates from the land, and the particle-rich plumes of great rivers like the Amazon can be traced for as much as 2000 km. Particles of biological origin generally disintegrate in the top 500 m, but some detritus and faecal pellets along with part of the atmospheric fall-out reach the deep ocean floor. Such sediments are important transporters of pollutants, especially metals. The amount of iron, manganese, copper, zinc, lead, tin and antimony reaching the sea today through river discharges is of an order of magnitude greater than would be supplied by natural geological processes. Smelting and other industries may also contribute substantial quantities via the atmospheric pathways.
Even when there is no apparent mortality due to a pollutant, its chronic effects may eliminate a population of organisms in the long-term. Marine mammals (seals and porpoises) in the Baltic Sea have exhibited reproductive failure associated with high DDT and PCB concentrations in their tissues.
Sewage released from ships can cause microbiological contamination of water and shellfish. Release of ballast water (i.e. water which fills empty oil tanks) leads to contamination of water by hydrocarbons.
In the Indian Ocean there are increasing risks of pollution from oil spills because this is the main transportation artery for oil from the Middle East to Europe and America, with an estimated 470 million tonnes transported annually (1998). According to a report of the World Bank published in 1996, similar risks apply in Northern Africa as more than 100 million tonnes of oil are transported through the Red Sea annually with insufficient maritime traffic regulations). Petrochemical complexes add to the problem. For example, the World Bank reported in 1995 that three major complexes at Annaba, Arzew and Skikda in Algeria discharge large quantities of chromium, mercury, oils, phenols, acids, chlorine and urea into the sea. Similar situations exist in Egypt, Libya, Morocco and Tunisia.