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.
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 (Salm 1998). 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 (World Bank 1996a). Petrochemical complexes add to the problem. For example, 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 (World Bank 1995c). Similar situations exist in Egypt, Libya, Morocco and Tunisia.
2. Many marine scientists argue that even if hydrocarbon concentrations are low, the contamination of the sea is increasing, that chronic effects could appear slowly but then be virtually irreversible, and that the most stringent precautions are therefore essential.