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Geografia d'Europa: textos de suport |
General situation
The Mediterranean Sea is the largest of the semi-enclosed European seas. It is surrounded by 18 countries and has shores on three continents (Europe, Africa and Asia) with a combined population of 129 million people in the catchment draining into sea, and sharing a coastline of 46 000 km. It is one of the leading tourist areas in the world, hosting 100 million visitors every year. This influx of people increases the waste discharges from domestic and industrial sources.
The Mediterranean Sea has an average depth of 1.5 km, though more than 20 per cent of the total area is covered by water less than 200 m deep (UNEP, 1989). The sea consists of two major basins, the eastern and the western. There are also smaller regional seas within the Mediterranean: the Ligurian, Tyrrhenian, Adriatic and Aegean seas. It is linked to the Atlantic by the Strait of Gibraltar, with the Black Sea and Sea of Azov by the Dardanelles, the Sea of Marmara and the Bosporus, and with the Red Sea by the Suez Canal. The Mediterranean Sea is characterised by low precipitation, high evaporation, high salinity, low tidal action and relatively low nutrient concentrations outside the inner coastal zone and parts of some regional seas.
The coasts of the northwestern Mediterranean are the most affected by pollution because of the concentration of urban populations, industrial activities and discharges of major rivers including the Ebro and the Rhone. The Adriatic receives the discharge of the River Po. The North African coast, in contrast, is for most part arid with little urbanisation or industrialisation. Pressures on the marine environment therefore vary widely depending on the local or regional situation.
Countries bordering the Mediterranean Sea met in Barcelona in 1975, under the auspices of UNEP, to draw up a programme of action to protect the Mediterranean this became the Barcelona Convention for the Protection of the Mediterranean Sea against Pollution. The UNEP Regional Seas programme had been initiated in 1974 when the Mediterranean had been selected as a 'concentration' area where coastal states would be assisted in the implementation of an action plan.
The Mediterranean Action Plan (MAP) consists of three main components: legal; environmental assessment; and environmental management. The legal component is contained within the Barcelona Convention. The long-term Mediterranean Pollution Monitoring and Research Programme (MEDPOL) was launched in 1975 under MAP, Phase 1 lasting until 1980. Phase II, endorsed in 1981, foresees monitoring at four levels: sources of pollution; nearshore areas; offshore areas; and transport of pollutants from the atmosphere. MEDPOL monitoring started in 1983 through the implementation of National Monitoring Programmes and at present 16 countries have ongoing programmes and are submitting data. The Blue Plan, which forms a part of the environmental management component of MAP, was launched in 1979 to assist the Mediterranean countries in making appropriate practical decisions for the protection of their marine and coastal environment.
Physical features
Surface water, entering the Mediterranean from the Atlantic, migrates generally towards the east. Evaporation processes transform this surface water into denser, deep water which flows east to west back into the Atlantic. In fact, water loss by evaporation exceeds the water input from runoff and precipitation, resulting in the Mediterranean's characteristic high salinity (average 38.5 per thousand, ranging from 37 in the west to 39 in the east). Strong vertical currents in winter ensure mixing of the water column and oxygenation of the deep waters. It takes on average about 80 years for the water in the Mediterranean to be completely exchanged.
Biological features
The Mediterranean Sea has a high species diversity but its biological productivity, while being extremely varied, is among the lowest in the world due to extremely low nutrient concentrations (UNEP, 1989). The total number of species of animals and plants has been estimated to be around 10 000 (Boudouresque, 1993). Its fauna includes many endemic species and is notably richer than the fauna of the Atlantic coasts. The eastern and western Mediterranean, separated by the relatively shallow straits between Sicily and Tunisia, show differences in resident fauna and flora, indicating a degree of isolation between the two regions (Clark, 1986). The biodiversity of the western Mediterranean is also greater than that of the eastern (Boudouresque, 1993).
The most threatened species in the Mediterranean is the monk seal, Monachus monachus, which is one of the ten most threatened species of mammals in the world (IUCN, 1988).
The loggerhead turtle, Caretta caretta, and the green turtle, Chelonia mydas, nest regularly and in significant numbers in the Mediterranean (COE, 1990a) both are recognised by the IUCN as globally threatened species, the former being ranked as 'vulnerable' and the latter as 'endangered' (IUCN, 1988). There are on average 2000 female loggerhead turtles nesting annually in the Mediterranean, the majority in Greece and Turkey. Green turtles, as far as is known, nest only in the extreme southeast of Turkey and in Cyprus (COE, 1990a).
There are around nine species of whales and dolphins regularly found in the Mediterranean (Tethys Research Institute, 1991). The biggest populations are found in the particularly rich pelagic zone of the western Ligurian Sea.
The extensive seagrass beds of Posidonia oceanica are an important part of the Mediterranean marine ecosystem, often occupying a considerable part of the littoral zone. A characteristic feature is the formation of Posidonia seagrass beds parallel to the shore in sheltered and shallow bays, isolating a coastal lagoon (Augier, 1982). These beds have suffered greatly from physical modifications of the coast. Posidonia plays an important role in the ecosystem: through the production of organic material at the base of the food-chain; as a primary oxygenproducer; as a feeding and nursery area for numerous species of fish (many commercially important); through the stabilisation of sediments; and through attenuation of wave and swell (protection of beaches) (Boudouresque, 1993).
A tropical, non-indigenous alga, Caulerpa taxifolia, has been observed in the Mediterranean since 1984. First seen in the Monaco area (perhaps as a result of an accidental release from an aquarium in Monaco (Meinesz and Hesse, 1991)), this alga had, by 1990, been found up to 150 km from Monaco at Toulon. At some locations it inhabits a wide range of substrates, including rock, mud and sand, and a wide range of depths, 3 to 35 m, and has achieved 100 per cent coverage in some places. Wherever it becomes established it considerably modifies the vegetal communities in the infralittoral zone. It also contains a toxin which may inhibit some other organisms such as grazers, epiphytes and competitors. It appears to be consumed by only a few fish species.
There are important fisheries in the Mediterranean, with fish such as mullet (Mugilidae) and hake (Merluccius spp) being in most demand. Other fish such as anchovy (Engraulis encrasicolus), sardines (Sardina pilchardus) and mackerel (Trachurus spp) in the northwest are also intensively fished. Oil pollution in some parts of the sea has led to tainting of a variety of fish and bivalves.
Inputs
Contaminants enter the Mediterranean from rivers, direct discharges (land-based and offshore), atmospheric deposition, and through water exchange primarily with the Atlantic Ocean and Black Sea. A major assessment of land-based inputs into the Mediterranean, reported by UNEP et al (1984), relied upon data collected by national monitoring networks. More recent work, such as that undertaken within the CEC EROS 2000 project, has indicated that these earlier estimates are too high (eg, CEC, 1992; Martin et al, 1989 and Dorten et al, 1991). In particular, when non-conservative processes, which reduce gross loads passing through estuaries, are taken into account, and when the partitioning of contaminants between the dissolved and particulate phase are considered, the net riverine loads of metals, such as mercury, copper, lead and zinc, entering the main body of the sea are now calculated (Dorten et al, 1991; Martin et al, 1989) to be much lower than estimated in the earlier compilation.
The Mediterranean basin is unusual in that it is rich in mercury deposits, for example cinnabar and metallic mercury (Zafiropoulos, 1986) 65 per cent of the world's mercury mineral resources are located in the region (Scoullos, 1993). Natural inputs of mercury (via rivers) can be locally very significant, for example into the Tyrrhenian Sea (Baldi, 1986), compared to anthropogenic sources. Dissolvedmercury riverine inputs to the Mediterranean are now estimated to be between 1.7 and 20 tonnes per year (Dorten et al, 1991), compared with the original estimate of 120 tonnes per year (UNEP et al, 1984) from rivers.
Recent work in the northwestern basin has assessed the relative contributions of atmospheric and river inputs of about 40 elements including heavy metals, radionuclides and nutrients. For the majority of the elements analysed the proportion of atmospheric deposition relative to total deposition (from rivers and atmosphere) did not exceed 20 per cent (CEC, 1992). However, for heavy metals atmospheric input generally appears to predominate: this derives from both the heavily industrialised northern boundary and dust loads originating in the Sahara region (Dorten et al, 1991). For cadmium, lead and copper (in the dissolved phase) atmospheric inputs are much greater than riverine (50, 200 and 5 times respectively) (Martin et al, 1989). For inorganic nitrogen, atmospheric and riverine inputs are roughly equivalent, and in the case of inorganic phosphorus, riverine inputs are most important. Major anthropogenic contaminant loads are discharged from the rivers Nile, Rhone, Ebro and Po.
There is also an exchange of dissolved trace metals between the Mediterranean and the Atlantic Ocean through the Strait of Gibraltar, and also with the Black Sea via the Bosporus. Recent mass balance calculations (Martin et al, 1993) indicate that in the case of dissolved copper, nickel and cadmium there may be a net export to the Atlantic.
Inputs of oil to the sea are estimated at 635 000 tonnes per year (UNEP, 1989). Of this, around half is spilt from tankers during ballasting and loading operations and during the washing of the bilge and tanks. Land-based sources account for 42 per cent of total oil loads, and the atmosphere accounts for 6 per cent. Natural seeps have also existed over geological time-scales, particularly in the northeastern parts of the sea (UNEP, 1989). The coastal area off Libya receives the highest oil input. Oil spills are also a problem, but are infrequent and generally localised.
Nuclear power stations are operating in three countries on the Mediterranean: Spain, France and Slovenia. However, it is the nuclear reprocessing plant at Marcoule in France that represents the most important source of artificial radioactivity in the sea. It is also apparent that atmospheric inputs are a significant proportion of the total input load of some radionuclides such as Cs-137 (Martin et al, 1989).
Contaminant levels
In the Mediterranean Action Plan status report of 1989, it was reported that the concentrations of trace metals and chlorinated hydrocarbons in sea water and sediment should be considered (at that time) with caution, and typical concentrations could not be identified this was because of inadequate analytical quality control and different analytical methods. However, concentrations in biota were considered to be more reliable because extensive laboratory intercalibration had been carried out (UNEP, 1989). Over the last five years the CEC has funded research through a number of projects implemented in the western Mediterranean and some regional seas (eg, EROS 2000) as reported by Scoullos (1993).
Elevated concentrations of mercury, cadmium, zinc and lead in sediments are found at 'hot-spots', which are generally in the coastal zones receiving industrial effluents, solid waste and domestic sewage. For example, concentrations of up to 37 mg/kg dry weight of mercury have been reported, compared with a typical background of 0.05 to 0.1 mg/kg dry weight (Fowler, 1990). Zinc concentrations are reported to be as high as 6480 µg/g, 5930 µg/g and 2550 µg/g (dry weight) at 'hot spots' along the coasts of Spain, at Venice and at Marseilles, respectively (Scoullos, 1993).
The MEDPOL programme has used two indicator species for monitoring contaminants, the mussel (Mytilus galloprovincialis) and the red mullet (Mullus barbatus). Mercury has been given special attention because recent data show that Mediterranean fish (eg, bluefin tuna, Thunnus thynnus, sardines, Sardina pilchardus, anchovy, Engraulis encrasicolus, and scads, Trachurus spp) and other marine animals generally have higher levels than those of the North Atlantic (FAO, 1986). However, results are very variable and are available mainly for the northern parts of the sea.
The limited number of measurements of organochlorines in sediments in the Mediterranean show a few 'hot spots'. For example, elevated sediment concentrations of PCBs have been found near the Athens sewage outfall; in the Bay of Naples; near the Marseilles outfall; and offshore from Nice (UNEP, 1989).
Measurements on biota are scattered and variable, but again the industrialised regions and major estuaries stand out. Mussels containing elevated levels of PCBs have been observed at Toulon and Marseilles near the Rhone estuary (UNEP, 1989). Levels of PCBs and other organochlorines in red mullet and mussels show a general decrease from the more northern parts of the sea to the south and east. There are as yet not enough reliable data for trend analysis.
The amounts of petroleum hydrocarbons in marine organisms and sediments in the area are poorly known. The available data cover mainly the coastal zone, and thus the contamination of the open waters is less well known. Results also show an increased level of petroleum hydrocarbons in sediments compared with the concentrations in water, which indicates that they may be accumulating in the sediments. Aliphatic and aromatic petroleum hydrocarbon concentrations in sediments range from 1 to 62 µg/g, and 2 to 66 µg/g, respectively, along the Spanish coast outside harbours, oil terminals and river mouths (Scoullos, 1993). Polycyclic aromatic hydrocarbons (PAHs) in the northwest Mediterranean range from 0.4 to 0.7 µg/g in the deep sea basin, from 0.3 to 0.5 µg/g on the continental shelf, and from 0.4 to 5 µg/g off Barcelona (Tolosa et al, 1993).
Almost no observations exist about the effect of petroleum hydrocarbons on Mediterranean marine organisms (UNEP 1989).
Biological effects
The Mediterranean Sea is generally considered to be oligotrophic (poor in nutrients) and does not have widespread problems arising from nutrient enrichment. However, coastal areas which receive anthropogenically enhanced nutrient loads from rivers and the direct discharge of untreated domestic and industrial wastewater are most susceptible to eutrophication. Examples can be observed in many coastal lagoons, estuaries and semi-enclosed bays, particularly in northern areas (eg, bays of the Ebro delta, the Albufera of Valencia, the coastal lagoons of southeastern France, the lagoon of Tunis, the Kastela Bay in Croatia and the Izmir Bay in Turkey (Estrada, 1993).
Two areas show extensive cultural eutrophication: the Gulf of Lions and the Northern Adriatic Sea. In the Northern Adriatic extensive dinoflagellate and diatom blooms occur in the spring and autumn: on occasions massive quantities of mucilage are produced (Estrada, 1993). However, the species responsible for this mucilage or gel production or the trigger mechanism is not yet fully known (Barth and Fegan, 1990). The key features of the North Adriatic which make it vulnerable to gel production are believed to be: shallowness (under 35 m); low turbulence during the summer and the high riverine nutrient input (Estrada, 1993). Studies in the Emilia-Romagna coastal waters of the northwest Adriatic (to the south of the River Po delta) have indicated that phosphorus rather than nitrogen is the prevailing limiting nutrient (Vollenweider et al, 1992). However, in some circumstances in this area, such as in the summer and when nitrogen supply is low, nitrogen may also become limiting.
The most severe eutrophic conditions are restricted to semi-enclosed bays and to the areas within the estuarine plume of the Po, which dominates the freshwater inputs. Numerous point sources are also important nutrient inputs. In response to the increased nutrient load, primary productivity increases with a maximum in the Emilia-Romagna area. During summer this area has suffered from persistent heavy algal blooms whose eventual decomposition causes anoxic conditions and mass kills of fish and benthic fauna. In 1990 the accumulation of gelatinous material produced by Phaeocystis spp on the beaches of Benicasim (Spain) had an adverse effect on the tourist trade.
'Red' tides also occur in the Mediterranean, often associated with the dinoflagellate Noctiluca scintillans (eg, frequently along the Catalan coast since the 1970s) (Estrada, 1993). Toxic algal episodes are also reported, for example, paralytic shellfish poisoning (from Alexandrum minutum and Gymnodinium catenatum) and diarrhetic shellfish poisoning (from Dinophysis spp). There is increasing evidence to suggest that mass development of toxin-producing unicellular algae is related to eutrophication. However, a cause-effect relationship between increased productivity and toxin production has not been proven (Barth and Fegan, 1990).
There has been a marked reduction in the Posidonia oceanica (seagrass) beds over the last decades. The reduction has been particularly noticeable around large industrial ports (eg, Barcelona, Marseilles, Toulon, Nice, Genoa, Naples, Athens and Algiers). The depths at which Posidonia is able to grow has also decreased. This has been associated with an increase in water turbidity reducing the amount of light exposure on the sea bed. Because of the role Posidonia plays in the Mediterranean ecosystem (see above, on biological features), its reduction is considered to have severe economic consequences (Boudouresque, 1993).
Conclusions
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