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Geografia d'Europa: textos de suport |
General Situation
The Norwegian Sea is bordered to the east by the coast of Norway between 61°N and North Cape (of Norway), 25° 45'E, to the northeast along a line from North Cape over Bear Island to the south point of West Svalbard. Its western limit extends from this point along a line to Jan Mayen and further to Gerpir, the easternmost point of Iceland. The southern limit extends from there to the Faeroe Islands and on to the position 61°N, 00° 53'W. The coastal areas of the sea are scantily populated and the major occupations are fishing, aquaculture, mining and some heavy industry. The port of Narvik is especially subject to shipping of ore vessels. Freight vessels and fishing boats are also important in the area, as is transit traffic from the Russian harbours in the north. The Norwegian Sea continental shelf is a site of oil and gas exploration activities.
Norway is a signatory to the Paris Convention, which covers the Norwegian Sea. The part of the Norwegian Sea north of the Arctic Circle is also included in the Arctic Monitoring and Assessment Programme (AMAP), to which Norway is a signatory (for more details on AMAP see the article on the White Sea).
This section on the Norwegian Sea has been compiled largely from the following references (specific points are referenced as appropriate): Aksnes et al (1993), Blindheim (1989) and Northern Europe's seas (1989).
Physical features
The Norwegian basin has two main depressions separated by a ridge between the Voring Plateau and Jan Mayen, through which only a narrow passage is deeper than 3000 m. The larger basin to the south contains wide areas with depths between 3500 and 4000 m. The Norwegian Sea is separated from the North Atlantic Ocean by the ScotlandIceland ridge: the deepest passage is the channel to the southwest of the Faeroes with a depth of 850 m; the channel between the Faeroes and Iceland is shallower than 500 m. To the north the connection to the Greenland Sea is deeper than 2500 m off the Barents Sea shelf, which forms the northeastern margin of the basin.
There are four major water masses within the Norwegian Sea. The warm and saline Atlantic water (temperature higher than 8°C and salinity greater than 35 per thousand) flowing in from the North Atlantic is confined to the upper layers over most of the area. Arctic water from the east Icelandic current (temperature less than 3°C and salinity between 34.7 and 34.9 per thousand) occupies upper layers in the southwestern part of the sea: at intermediate depths its distribution is much wider. Bottom water (salinity close to 34.92 per thousand and temperature below 0°C) originating from the Greenland Sea and the Arctic Ocean fills the deeper layers and represents the largest water volume. Within the area these three different water masses are continuously being mixed and modified.
Coastal water along the coast of Norway is the fourth major water mass in the Norwegian Sea. It is made up of a mixture of freshwater from land and more oceanic water, and is generally of low salinity. Locally salinity can be very variable, reaching a minimum in spring when runoff is at a maximum due to snow-melt. As a result the coastal waters are strongly stratified, and their temperature may be subject to large seasonal variations, particularly in surface layers. Most of the Norwegian Sea is ice-free during winter, with sea-ice occurring only in the border areas between Bear Island and West Svalbard, and along the borderline between West Svalbard and the island of Jan Mayen.
Biological features
In the Norwegian Sea winter convection currents extend to about 300 m depth, varying somewhat in different areas. Thus waters with high concentrations of nutrients are brought into the surface layer. In early spring the waters of this layer are rich in nutrients and have a large potential for primary production. As a result the Norwegian Sea is amongst the most productive seas in the world. Phytoplankton production starts along the Norwegian coast, where the wide salinity range in the coastal waters maintains some stratification throughout the year. In the open sea the mixed layer in early spring is too deep to support primary production and consequently the spring bloom occurs one to two months later than in coastal waters. The spring bloom consists mainly of diatoms but there are considerable yearly variations in the qualitative and quantitative aspects of the bloom in all parts of the Norwegian Sea. Summer production is relatively poor but there is more diversity with regard to species.
Herring (Clupea harengus) has traditionally been the most commercially important pelagic fish in the Norwegian Sea. Herring spawn along the Norwegian coast, mainly in March. The hatched larvae are carried northward by the coastal current and most are transported into the Barents Sea, which is the main nursery area. The herring return to the Norwegian Sea when sexually mature. By the end of the 1960s the herring stock was almost depleted. Since then the blue whiting (Micromesistius poutassou), which feeds in the Norwegian Sea and spawns west of the British Isles, has formed the largest pelagic plankton-consuming fish population in the area. Three species of redfish (Sebastes marinus, Sebastes mentella and Sebastes viviparus) are also found in the Norwegian Sea, the first two having commercial value. In addition, cod (Gadus morhua), saithe (Pollachius virens) and haddock (Melanogrammus aeglefinus) are found along the continental shelf.
Seals and whales are important components of high latitude ecosystems. Bowhead (Balaena mysticetus), blue (Balaenoptera musculus), fin (Balaenoptera physalus), sei (Balaenoptera borealis), humpback (Megaptera novaeangliae), sperm (Physeter catodon), and minke (Balaenoptera acutorostrata) whales all use the Norwegian Sea as an important feeding area. The bowhead, blue, sei and minke whales have a more or less clear seasonal migration pattern with a northward movement in the spring and a return to more southern areas in autumn. It has been estimated that up to 10 000 individual whales live in the area during the summer season, with fin and sperm whales being the most common. The world population of minke whale has been estimated to represent about 86 000 individuals.
Hundreds of fish farms are located along the Norwegian coasts between Sogn og Fjordane and Tromsø. These farms have produced increasing quantities of organic matter, resulting in local nutrient and eutrophication problems.
Inputs
Norway submits data to the Paris Commission on direct and riverine inputs into the Norwegian Sea. It is considered that discharges to the Norwegian Sea from riverine and direct sources are amongst the lowest when compared with those to other seas in the Convention's area (OSPARCOM, 1992a). For example, in 1990, cadmium loads to the Norwegian Sea were estimated to be 8.9 tonnes, compared with 55 tonnes to the North Sea and 70 tonnes to the North Atlantic. In the case of total nitrogen the loads were 31 kilotonnes (kt), 920 kt and 314 kt, respectively. No data were available on the contribution of other sources of contaminants, for example atmospheric deposition. There are also inputs of contaminants via ocean currents flowing into the Norwegian Sea.
The largest scale salt-water fish farming industry in the Nordic region is located in the fjords of the west coast of Norway, including the Norwegian Sea coastline. Such operations have marked effects on the nutrient status of the waters where exchange with the open sea is limited (eg, in some fjords). It has been estimated that at present aquaculture releases some 13.5 kt of nitrogen and 2.7 kt of phosphorus into Norwegian coastal waters annually, equivalent to the nutrient load in untreated sewage from a population of 2 million people (Bernes, 1993).
Contaminant levels
The concentrations of trace metals in sea water in the Norwegian Sea are reported to range from 12 to 23 ng/l cadmium, 88 to 120 ng/l copper, 187 to 195 ng/l nickel and 25 to 35 ng/l lead. There are very limited data available on trace organic compound concentrations in sea water. For example, mean concentrations of *HCH and *HCH in the Norwegian Sea were reported to be 0.2 to 1.0 ng/l, respectively. Levels of synthetic surface active substances and hydrocarbons in sea water were reported to be very low, and concentrations of chlorinated hydrocarbons did not exceed 1 ng/l (Izrael and Tsyban, 1992).
Data were submitted from two coastal sites along the Norwegian Sea for inclusion in the Oslo and Paris Commissions' 1990 supplementary baseline study of contaminants in fish and shellfish. Neither showed 'elevated concentrations' of metals in tissues or 'higher concentrations' of organochlorines in one or more species (OSPARCOM, 1992a). The concentration of PCBs in zooplankton in the southern part of the sea have been found to be up to 55.6 µg/kg, and HCH levels were between 0.3 and 1 µg/kg in molluscs and 2.2 µg/kg in zooplankton these concentrations were considered to be very low (Izrael and Tsyban, 1992).
Biological effects
It has been postulated that the ecosystem of the Norwegian Sea has been affected by climatic variability and, in particular, variability in temperature and salinity (Blindheim, 1989). For example, during the years 196568 waters of below 0°C in the East Iceland current extended further eastward into the Norwegian Sea than previously observed. This change is believed to have brought about a complete alteration in the feeding migrations of herring. In addition, a succession of several poor year classes of blue whiting during the late 1970s is also a possible effect. Even though the herring stock was subject to heavy fishing pressure in the 1960s, it is likely that the climatic variability was also an important factor for the depletion of stock around 1970. The biomass of the herring is at present estimated to be 2 million tonnes compared with 1950s values of 10 million tonnes (Aksnes et al, 1993).
Conclusions
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