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Geografia d'Europa: textos de suport |
General situation
For the purpose of this report the North Atlantic Ocean is defined as the area of the ocean lying between Europe's western seaboard and 42°W, and between 36°N and the Arctic Circle. It therefore includes the southeast coastline of Greenland, the coast of Iceland, the Faeroe Islands and the western coasts of the UK, Ireland, France, Spain and Portugal. It also includes the relatively shallow coastal waters of the semi-enclosed Irish Sea, the Celtic Sea, the Bay of Biscay and the Gulf of Cadiz. The Irish Sea in particular is subject to extensive monitoring by the UK and Irish authorities, and an overview of its current status has been recently compiled by the Irish Sea Study Group (ISSG, 1990).
The continental shelf ends at varying distances off the mainland, being relatively close to the northern shores of Spain and Portugal (less than 10 km in places) and more distant from the southwest peninsula of England (300 km). Within the continental shelf, water depths are generally less than 200 m; within the main body of the ocean, depths of 5700 m are found.
Major rivers discharging into the North Atlantic include: the Guadiana and Guadalquivir in southwest Spain (Gulf of Cadiz); the Douro and Tajo on the west coast of Portugal; the Garonne and Loire on the west coast of France (Bay of Biscay); the Severn on the southwest coast of Britain (Celtic Sea); the Mersey (Irish Sea) and Clyde on the west coast of Britain; and the Shannon on the west coast of Ireland. The rivers on the mountainous north coast of Spain have relatively small catchments, though many are heavily industrialised. Major cities along the coastal zone include: Seville, Lisbon, Oporto, Bilbao, Bordeaux, Nantes, Dublin, Liverpool and Glasgow. Many river catchments are heavily industrialised, for example the River Mersey and the River Oria, whereas others have largely rural or agricultural catchments, for example the Shannon and the Loire. Human influence on the North Atlantic is most readily detectable within the relatively shallow coastal zone.
The Oslo Convention and the Paris Convention provide the regulatory framework for the protection of the maritime area of the northeast Atlantic against pollution (for details see the article on the North Sea). As for the North Sea, those EU countries with Atlantic coastlines have to comply with the requirements of the relevant EU Directives (76/464/EEC with six 'daughter directives') that concern the discharge of dangerous substances to surface waters and the subsequent quality of those waters.
Physical features
The North Atlantic exchanges water with: the Arctic Ocean; the Norwegian Sea; the North Sea through the English Channel and to the north of the UK; and the Mediterranean through the Strait of Gibraltar. Surface water movements in the north of the ocean are dominated by the relatively warm and saline water from the main body of the Atlantic, flowing in the North Atlantic Current. This current flows on both sides of the Faeroes, directly and indirectly towards the Norwegian Sea with its lower water temperature. Here a sharp front is formed with the colder and less saline water from the East Iceland Current. Warm water moves along this front into the Norwegian Sea while the colder water from the north passes into the deep Atlantic troughs and flows southwards. Around Iceland the Arctic Current branches off from the warmer Gulf Stream. Iceland's northern and northwestern coasts are occasionally blocked by drift-ice from the north. The warmer Atlantic water moves west, beneath and outside the polar water. To the east, the Arctic water gives way to the East Iceland Current which flows in the direction of the Faeroes.
Open ocean salinity remains relatively constant at between 35 to 36 per thousand, any variation reflecting regional precipitation and evaporation patterns and the effects of ocean currents. Salinity of the water around Iceland, the UK and the Bay of Biscay is generally between 34 and 35 per thousand.
Biological features
Extensive fisheries exist in the northern part of the ocean. The waters around Iceland are particularly productive, with the total annual catch of fish being approximately 1.5 million tonnes. The most important species caught are cod (Gadus morhua), redfish (Sebastes spp), haddock (Melanogrammus aeglefinus) and saithe (also known as coalfish) (Pollachius virens). There is growing concern over the deteriorating state of fish stocks in Icelandic waters and around the Faeroes. The reasons for the deterioration are not fully understood but could include fishing and periodic climatic variability. Aquaculture (eg, of salmon, Salmo salar, and trout species), is becoming more important in Iceland. Fisheries are also generally important within the other parts of the ocean, for example off the coasts of Spain, Portugal, France and the UK.
Inputs
The atmospheric inputs of many metals (excluding mercury) to the North Sea, Baltic Sea and Mediterranean Sea are three to ten times higher than those to the open North Atlantic. In open ocean basins, however, the atmospheric flux of man-made contaminants is generally more important than the net input from rivers; this is partially because riverine materials are generally captured within the coastal zone. An estimate of the direct and riverine inputs into the northeast Atlantic Ocean during 1990 can be obtained from data submitted to the Paris Commission by the riparian Atlantic states. It was considered, however, that the loads are underestimates (OSPARCOM, 1992a) as, most significantly, there were no data from the Atlantic coasts of France and Spain, and, less significantly, none from Iceland and Greenland. (There is very little marine pollution arising from Iceland and Greenland.) Direct and riverine loads of metals to the North Atlantic are generally of the same order of magnitude as those to the North Sea, although the impact of the loads away from the coastal zone is generally very small compared with that of other seas, because of the North Atlantic's very large volume and energetic mixing and dispersion characteristics.
The disposal of solid, low-level radioactive waste in the North Atlantic ceased in 1982, by which time some 54 PBq (1.5 million Curies) of radioactive waste in 140 000 tonnes of packaged material had been disposed of at ten sites around 46°N, 17°W (Camplin and Aarkrog, 1989). The present and future risk to individuals from past oceanic dumping is considered to be extremely small. Nuclear waste reprocessingplants are sited at Sellafield (UK, on the Irish Sea) and Cap de la Hague (France, on the English Channel). In 1990, it was reported that 71 TBq (1900 Curies) of total beta radioactivity, 2 TBq of total alpha and 23 TBq of Cs-137 were discharged from the Sellafield site (Kershaw et al, 1992). Discharges from Sellafield have decreased significantly in recent years because of a progressive introduction of more advanced treatment measures (ISSG, 1990). Overall, this trend is expected to continue into the future. In addition, there are a total of ten nuclear power stations bordering the northeast Atlantic: one in France and nine in the UK. They generally discharge smaller amounts (than Sellafield) of radioactivity into the Atlantic.
Contaminant levels
The areas considered by the Oslo and Paris Commissions in their 1990 quality report (OSPARCOM, 1992a) to be generally more contaminated were: the Gironde and Loire estuaries (France); the Spanish coast near Santander, La Coruna and the Navia estuary; and the Portuguese rias de Arosa, Pontevedra and Vigo.
The results from the 1990 supplementary baseline study of contaminants in fish and shellfish (OSPARCOM, 1992a) identified the following areas with 'elevated' concentrations of metals:
Chlorinated hydrocarbon concentrations in water are in the order of a few ng/l and are fairly uniformly distributed at all depths (UNEP, 1990), but with highest concentrations occurring in surface microlayers naturally enriched with lipids.
It has been estimated that the average annual radiation dose arising from natural sources in the UK is 1.87 mSv a year (Hughes and Roberts, 1984). As a comparison, the radiation doses to the most exposed groups of people are estimated to be: from six times less to twice 'natural' levels for the Sellafield discharges; 20 times less for Cap de la Hague; from 20 000 times less to six times less from the other nuclear sites; and 90 000 times less from the previous solid waste disposal in the northeast Atlantic Ocean (Camplin and Aarkrog, 1989).
Biological effects
Most of the North Atlantic ecosystem is relatively unpolluted, and any adverse biological effects are limited to the coastal zones, often in semi-enclosed areas and bays, and estuaries bordering the main ocean. For example, there are concerns about the possible effects of eutrophication along the Irish Sea coasts, and the destruction of coastal habitats along the eastern coasts of the Atlantic.
Conclusions
Fuente:
European Environment Agency
The Dobris Assessment
Chapter 06: Seas