Geografia d'Europa: material de suport

Geografia d'Europa: textos de suport

COASTAL ZONE PROCESSES AND PROBLEMS

Being the interface between sea and land (or between salt water and freshwater), the coastal zone acts as a very important buffer influencing the fate of riverine contaminants and direct discharges as they are transported from land-based sources to the sea (UNESCO, 1987). The gross flux of contaminants through the land/sea boundary is predominantly influenced by the geological, biogeochemical and landuse characteristics of the given catchment and its hydrology. Biogeochemical cycles in the seaward part of the coastal zone are controlled mainly by interactions between dissolved and particulate phases. Therefore, the processes that control net fluxes of contaminants traversing the boundary between coastal zone and ocean are basically those that determine whether a contaminant is dissolved or adsorbed on particles or taken up by living organisms during transport in the coastal zone. Long-range transportation of nitrogen and persistent pollutants seems to be an increasing threat to the environmental conditions of coastal waters.

Sediments are an important component of the coastal environment. Only a small proportion of sediments coming from the continents reaches the open ocean, most being deposited in the shallow and calmer waters of the coastal zone, including estuaries, bays, fjords, and also in tidal mudflats. Occasionally resuspension and water circulation patterns transport sediments to other areas along the coast. This balance between sedimentation and resuspension is treated further below under 'coastal erosion'.

Many environmental contaminants (eg, heavy metals, synthetic organic compounds, phosphorus, radionuclides) are adsorbed on suspended and settled particles. Sedimentation thus acts as a sink for these substances, if only temporarily, and plays an important role in determining the environmental impact of the contaminants. Sedimentation can lead to elevated concentrations of contaminants in the sediments of sheltered bays, harbours and tidal mudflats by accumulation. The water column may then become degraded by the release of sediment-bound contaminants over time, even when inputs from contaminant sources have stopped. The partitioning of chemicals between water, particles and settled sediment is therefore of critical importance in assessing or understanding the environmental behaviour of contaminants in the coastal zone, particularly at, and under the influence of, the salt water/freshwater boundary.

Although the seaward part of the coastal zone occupies only a small part of the world's oceans' water volume and surface area (0.5 per cent and 10 per cent, respectively), this zone (in particular the estuarine section) is functionally of great importance, with rates of production and consumption comparable with those found only in intensive agriculture and tropical and subtropical rainforests (Odum, 1971). It has been estimated that the coastal zone accounts for 30 per cent of ocean biological production (Mantoura et al, 1991) because of direct riverine nutrient input and upwelling along the shelf-edge. Most of the world's fish catch (70 to 80 per cent) comes from this zone.

THE PROBLEMS: CAUSES AND CONSEQUENCES

The most important underlying sources of pressure and theresulting problems (including physical, chemical and biological modifications) that these give rise to for the coastal environment are summarised in Table 1.

Table 1 - Relationships between human activities and coastal zone problems

Human activity	Agents/consequences	Coastal zone degradation problems
Urbanisation and transport	Landuse changes (eg, for ports, airports); road, rail and air congestion; dredging and disposal of harbour sediments; spills at sea (oil, sewage garbage); water abstraction; wastewater and waste disposal	Loss of habitats and species diversity; visual intrusion; lowering of groundwater table; salt water intrusion; water pollution; human health risks; eutrophication; introduction of alien species
Agriculture	Land reclamation; fertiliser and pesticide use; livestock densities; water abstraction; river channelisation	Loss of habitats and species diversity; water pollution; eutrophication; reduction of freshwater inputs to coastal waters
Tourism, recreation and hunting	Development and landuse changes (eg, golf courses); road, rail and air congestion; ports and marinas; water abstraction; wastewater and waste disposal	Loss of habitats and species diversity; disturbance; visual intrusion; lowering of groundwater table; salt water intrusion in aquifers; water pollution; eutrophication; human health risks
Fisheries and aquaculture	Port construction; fish processing facilities; fishing gear; fish farm effluents	Overfishing; impacts on non-target species; litter and oil on beaches; water pollution; eutrophication; introduction of alien species; habitat damage and change in marine communities
Industry (including energy production)	Landuse changes; power stations; extraction of natural resources; process effluents; cooling water; windmills; river impoundment; tidal barrages	Loss of habitats and species diversity; water pollution; eutrophication; thermal pollution; visual intrusion; decreased input of freshwater and sediment to coastal zones; coastal erosion

The assessment of the state of the marine and coastal environment shows that, in all of Europe's seas, marine pollution of the coastal zone is a significant problem, whether locally (for example, in individual estuaries or bays) or regionally, such as in large enclosed or semi-enclosed seas. A similar conclusion emphasising marine pollution as an important coastal issue was made by the European Coastal Conservation Conference (ECCC, 1991). In particular, the enrichment of natural waters by nutrients, primarily nitrogen in marine waters but also phosphorus in low salinity waters, has been associated with increased primary productivity and nuisance algal growth (eutrophication) in the coastal zones and semi-enclosed and enclosed areas of seas. As well as being the major problem in coastal zones, eutrophication can also be a more widespread and general problem in the more enclosed seas of Europe, for example the Black Sea and Baltic Sea.

Phisical modifications and habitat loss

In many European seas the coastal zone is an important area for human habitation, industry, location of centres of energy production, military activities, fisheries, bird life and recreation. This inevitably leads to a conflict of use over resources such as water (eg, for bathing and shellfisheries) but also for landuse (eg, harbours and marinas), which can have adverse effects on the quality of the coastal environment. The lack of effective coastal zone management can lead to the loss of important components of the ecosystem and habitats (eg, dunes and wetlands). Physical modifications, such as the construction of ports and tourist facilities, result in the loss of habitats. The damming of rivers alters the hydrological regime, resulting in a reduction of freshwater flow. This can have serious consequences on the coast. Reducing freshwater flow generally means a reduced sediment load, which may induce coastal erosion.

Coastal erosion

Erosion of the shoreline and dunes, caused by ocean currents, tidal movements and wave and wind action, is a common natural phenomenon along many parts of the European coast. The result is the movement of vast amounts of sediment (mud, silt and sand). At present, approximately 70 per cent of all sandy coasts in the world are subject to coastal erosion (Hoogland, 1992). An inventory of coastal evolution in the EU undertaken within the EC CORINE programme, showed 55 per cent of the coastline (total length of 56 000 km) to be stable, 19 per cent to be suffering from erosion problems, and 8 per cent to be depositional.

The natural balance between erosion and sedimentation can be affected by those human activities which decrease the amount of sand available for erosion. Extraction of sand close to the coast weakens the coast locally, and dredging and construction of reservoirs on many rivers have drastically decreased the riverine sand load to coastal waters, increasing the in situ demand for sand. Increased coastal erosion due to lack of riverine sand is reported to present major problems along the Romanian Black Sea coast and in the Caspian Sea. To protect the coast from erosion, various types of sea defences have been constructed in many countries with erosion problems.

For example, in The Netherlands, where the largest proportion of the coast consists of dunes, 40 per cent of the coast is reinforced by defence works such as groynes, pile groynes and defences at the foot of dunes to protect them against the North Sea (Ministry of Transport and Public Works, 1990) and to avoid disastrous floods like that of 1953. Caution needs to be exercised in constructing defences designed to protect coasts since, by interfering in the natural erosion/deposition balance, these can create their own unwanted knock-on effects in neighbouring localities, leading to increased erosion or silting.

Habitat loss and degradation

Other physical modifications affect mainly the landward part of the coastal zone. The most important are those resulting from space-consuming developments of settlements, industries, agriculture (eg, reclamation of arable land), ports, military installations, tourism and recreation (eg, golf courses). Among other effects, these can lead to the creation of vast built-up areas at the expense of natural habitats (eg, dunes, saltmarshes) and, as a result, damage or destroy a substantial part of the natural coastline's habitats (CEC, 1991). In France, for example, 15 per cent of natural areas on the coast have disappeared since 1976 and are continuing to do so at the rate of 1 per cent a year. Italy, which had around 700 000 hectares of coastal marshes at the end of the last century, had no more than 192 000 hectares in 1972 and has fewer than 100 000 hectares today. Some estimates suggest that about one third of the coastal dunes in northwestern Europe and three quarters in the western Mediterranean have disappeared (Gehu, 1985). Such large-scale habitat destruction will inevitably lead to a decline in species distribution and abundance.

Contamination and coastal pollution

Excessive loads of contaminants are a direct result of humans being unable, or unwilling, to control or limit their polluting activities effectively. Pollution of the coastal zone is primarily a result of the contaminant load being discharged into receiving waters, resulting in such deleterious effects as harm to plants and animals, hazards to human health, hindrance to marine activities (including fishing), impairment of quality for use of sea water and reduction of amenities.

Type and origins of contaminants

Despite the coastal zone's ability to reduce the harmful effects of some contaminants, coasts are also vulnerable to pollution since wastewater is often discharged directly or indirectly into sheltered and shallow coastal waters with poor mixing. In such areas, large quantities are not needed to overwhelm the local receiving waters. The sea is often the ultimate recipient of many contaminants from many sources:

directly from rivers bearing contaminants flowing into the sea;
through direct discharges from outfalls or runoff from land into estuaries and coastal waters;
through the dumping of waste such as sewage sludge, chemical waste and dredged spoil;
from atmospheric deposition, which, in quantitative terms, can be as significant as that from other sources;.
discharges from ships.

The most important contaminants in the coastal zone are organic matter, synthetic organic compounds (eg, PCBs and pesticides such as DDT and residues), microbial organisms, nutrients (mainly nitrogen and phosphorus), oil, litter, and, generally to a lesser extent, heavy metals (eg, cadmium, mercury and lead) and radionuclides.

Routine operations of ships, as well as shipping accidents, are regular, and often major, point sources of oil and risks for the introduction of unwanted organisms from ballast water. The contribution from oil and gas exploration and the routine operation of rigs is generally a minor source of contaminants in the coastal zone. While loads of contaminants from atmospheric deposition can be very significant over the whole surface area of individual seas (for example, inorganic nitrogen and some heavy metals, such as cadmium, in the Mediterranean and northern seas), their proportion of the total load to the coastal zone from all sources is likely to be relatively small.

Catchment management

In much of Europe there is a lack of effective catchment management, control and regulation through which a reduction of coastal zone pollution could be achieved. Many of Europe's seas have large multinational catchments, which may include states with no coast. Contaminants within the seas may therefore originate hundreds or even thousands of kilometres away from the sea. Hence, effective catchment management and pollution control measures require concerted international efforts and cooperation. This relates not only to water quality but also to water quantity, since the manipulation of river flows can have secondary effects on the receiving seas. There is also exchange of water, and hence contaminants, between many of the seas.

It is clear, therefore, that a significant contributory cause of coastal zone pollution in many of Europe's seas is inadequate control of diffuse sources of contaminants. Diffuse sources also include discharges of contaminants in industrial fume stacks and atmospheric emissions from some agricultural practices, such as ammonia arising from intensive livestock farming, which occur well inland of the coast.

Wastewater disposal

In many sea catchments there is inadequate treatment of domestic sewage, with crude, untreated sewage often being discharged directly to coastal waters, estuaries and rivers. In many of Europe's seas there are also uncontrolled and/or inadequately treated discharges of industrial wastes which can significantly add to the total loads of contaminants. Due to the breakdown of organic matter, such discharges can cause immediate problems to marine life from high oxygen demands in the water column and sediments and through toxic effects of ammonia. The introduction of sanitary litter to coastal waters with wastewater discharges often decreases considerably the amenity value of impacted areas.

Of immediate concern to humans is the presence of large numbers of pathogens that can cause illness and disease (eg, hepatitis and typhoid). Also associated with the presence of pathogens is the potential contamination of seafood, particularly shellfish, which in some seas can be commercially important as well as a source of food for fish and birds. There are many examples where the beaches of Europe's seas have been closed to bathers because of contamination by human pathogens arising from inadequately treated sewage. At present EU standards for bathing waters relate to contamination levels of indicator faecal bacteria (E coli) and other pathogens (Salmonella and entero-viruses) (CEC, 1993).

Sewage is also an important source of nutrients, and is therefore implicated in the problem of eutrophication. However, the major input of nutrients to coastal zones is often from tributary rivers. Nutrients in rivers arise from the application of nitrogen and phosphorus in fertilisers used in agriculture; phosphates are also used as 'builders' in detergents. Intensive farming practices and spreading of animal slurry contribute significant inputs of nutrients into rivers. Atmospheric inputs, soil erosion, release of nutrients from sediments and organic plant wastes also represent diffuse sources of nutrients. In urban catchments, point sources of nutrient input will be more important, and derive from sewage treatment works and industrial wastes. Another important point source of nutrients into the coastal zone of some seas is from fish-farm sites (eg, in the Baltic and Norwegian seas).

The fate and impact of nutrients

The environmental impacts of anthropogenic nutrients entering the sea have recently been subject to much public interest and scientific debate. Nutrients, however, are not pollutants per se, but stimulate and control plant growth when other environmental factors are optimal. Much evidence has accumulated to suggest that input of combined nitrogen (predominantly as nitrate) to the sea is the single most important factor contributing to 'cultural eutrophication', being the enrichment of a waterbody by nutrients of anthropogenic origin causing an accelerated production of plant material which during its presence and decay produces a series of undesirable phenomena that restrict the use of the water resource. Nitrogen is generally considered to be the limiting nutrient for primary productivity and biomass of algae in waters outside estuaries and inner coastal waters (Codispoti, 1989; Jickells et al, 1991). In estuaries, phosphorus, nitrogen or both can temporarily limit production.

There is scientific evidence to support the view that eutrophication-related phenomena in nearshore waters appear more frequently and are more serious than in the past, for example: the Adriatic Sea (Marchetti, 1990); Swedish west coast (Lundälv, 1990); Danish waters (Miljøstyrelsen, 1991); Black Sea (Mee, 1991); and in many areas of the Baltic Sea. The Paris Commission has also reported eutrophication problem areas within the North Sea, mainly along the eastern seaboard, such as within Belgian and Dutch coastal waters, and the German Bight (OSPARCOM, 1992). This is associated with increased inputs of anthropogenic nutrients from land into coastal waters.

Eutrophication has been associated with a number of effects, such as an increased prevalence of nuisance and toxic algal blooms within coastal zones. Physico-chemical factors other than nutrients (eg, salinity, temperature, light conditions and turbulence) can be involved in bloom formation, as can biological factors such as decreased zooplankton grazing on the algae. A recent study (Larsen et al, 1993) showed that zooplankton growth rates, and thereby the grazing potential, were markedly reduced by toxins released during an algal bloom, leading to self-propagation of the bloom. Dense blooms of colonial or chain-forming species (eg, species of Nodularia, Phaeocystis and Chaetocerus) can result in drifts of cells on the sea surface or on the beach, or slimy deposits on fish nets. Once extensive phytoplanktonic blooms die, they fall to the sea bed, where the resultant microbial degradation results in massive deoxygenation of the bottom waters causing wide-scale benthic mortality.

Some species of several phytoplankton groups (eg, diatoms, dinoflagellates, cyanobacteria) produce toxins, which are known to cause the death of marine invertebrates, for example mussels, that filter them from the water. In other cases the accumulated toxin may represent a hazard to organisms further up the food-chain (eg, fish, sea-birds, marine mammals and humans). The most widespread algal toxins are those causing paralysis (paralytic shellfish poisoning, PSP), diarrhoea (diarrhoeic shellfish poisoning, DSP), amnesia (amnesic shellfish poisoning, ASP), haemolysis and dermatitis. Although not yet documented for Europe, there are good reasons for awareness about ASP since, like the common PSP, it can cause very dangerous intoxications of humans. Amnesic shellfish poisoning was first discovered in Canada in 1987, where more than 150 people became seriously ill (with three deaths) as a result of eating infected mussels, Mytilus edulis (Quilliam et al, 1989).

There are also often changes in the composition of macroalgal community structure, from the perennial slower growing brown algae to the more ephemeral green algae such as Enteromorpha species.

The clouds of algal slime that occasionally arise in the northern Adriatic Sea (in particular, during the summers of 1988 and 1989, when they were washed up along tourist beaches) have received considerable public attention and are of great concern to the tourist industry. At present the underlying mechanisms forming and maintaining such large masses of slime are unknown and no rigorous hypothesis has yet been presented explaining this peculiar phenomenon (Battaglia, 1990). Eutrophication does not seem to be the prime factor involved and it is hypothesised that thermal anomalies following high insolation and calm wind conditions in the preceding spring may be of importance.

Factors affecting the fate of the riverine flux of dissolved nitrogen (mostly as nitrate) into the coastal zone are therefore very important for determining the eutrophic state of these waters. These factors include a series of complex biological and biochemical reactions of which denitrification is a very important process, transforming nitrate under anoxic conditions into the non-eutrophying gases N₂O and N₂. Estimates by Seitzinger (1988) suggest that about 45 per cent of the gross flux of total nitrogen to European estuaries is being lost in denitrification, thus reducing the risk of marine eutrophication. Denitrification in ocean shelf and estuarine systems of the world has been estimated by Christensen et al (1987) at 62 million tonnes of nitrogen per year. Although no estimate is available for the European coastal zone, it is clear that a significant part of an estimated European riverine gross flux of nitrogen (between 2.5 and 6.5 million tonnes per year) is lost to the atmosphere from the coastal zone before reaching the open oceans. However, this large loss of nitrogen, of primarily agricultural origin, does not stop many European coastal waters suffering from eutrophication.

Environmental impact of other contaminants

Synthetic organic compounds reach the coastal zone from a variety of sources, but in general riverine loads are higher than inputs from other sources. Marine organisms have the ability to accumulate some contaminants discharged into marine waters. In particular, some synthetic organic compounds such as organochlorine pesticides and PCBs have a high bioaccumulation potential and, though present at low concentrations in the water column, they can build up through the food-chain to form elevated concentrations in animals. It may be possible, therefore, for some contaminants to reach such high levels in food types as to pose a threat to human health. In some seas (eg, the Black Sea) the presence of pollutants has been linked with a decline in populations of commercial fish species, seals, porpoises, dolphins and whales. For example, studies on the three Baltic Sea seal species have shown pathological changes in the female reproductive tract, in adrenals, bone tissue and the alimentary tract of the animals (Olsson et al, 1992). On the basis of contaminant concentrations in the Baltic Sea, PCBs were suspected of being responsible for the occurrence of sterility among the seals. A similar suspicion has been reported for the observed decrease of the seal population and its reproductive failure in the Dutch Wadden Sea (Reijnders 1980; 1986). However, although elevated levels of chlorinated organics have been found in marine mammals in the North Sea, there is no direct evidence to relate levels found to any potential harm (Zabel and Miller, 1992).

Sediment may also be an important physical contaminant (eg, in the disposal of dredged spoil) through the restriction of light to, and the blanketing of, benthic communities.

Oil is a very noticeable contaminant which can adversely affect the quality of the coastal zone, in terms of both amenity and effects on organisms. Oil discharges from anthropogenic sources to the seas represent a significant coastal pollution problem despite the fact that global oil discharges decreased from an estimated 3.2 million tonnes in 1981 to 2.35 million tonnes in 1990 (GESAMP, 1993).

This estimate is highly influenced by numbers and size of tanker accidents each year. The observed general decrease in the number of oil spills and the quantity of oil spilled is due mainly to measures taken against oil spills from shipping by international conventions, whereas inputs of oil spills from land-based sources, in particular spills of petroleum hydrocarbons, are of increasing concern, causing sub-lethal effects in, for example, crustaceans and echinoderms. Sea-birds, mammals (eg, sea otters and polar bears) and caged fish in aquaculture farms are often the most conspicuous victims of oil pollutions.

Metals also enter the marine environment from a variety of sources, with rivers generally predominating. Heavy metals occur naturally in sea water and sediments at levels reflecting local geology and geochemistry, and are generally present in such low concentrations in the sea that they do not constitute a major threat to marine organisms except at some contaminated sites.

Nuclear weapons testing, the dumping of radioactive waste, and discharges from nuclear power stations and reprocessing plants add to the radioactivity of the sea caused by naturally occurring radionuclides. The latter two artificial sources are the most important to the coastal zone. Anthropogenic inputs of radioactivity are generally considered to be less than 1 per cent of natural radioactivity, although the radionuclides involved are mostly of a different nature.

Litter

The Coastwatch Europe surveys (Coastwatch, 1994) have demonstrated that litter is a major problem within the coastal zone of all the seas surveyed. Litter, in particular that made of synthetic materials such as ropes, nets, plastic bags and packaging rings and straps, is increasingly being introduced into the marine environment, and is particularly noticeable in the coastal zone, where it tends to accumulate from the action of wind and waves. Such litter often floats in the sea, entangling and killing fish, birds and marine mammals. The presence of some forms of litter, such as sanitary,medical and glass, particularly on beaches, poses a direct threat to human users. The resultant decrease in amenity value of beaches also decreases the recreational attractiveness of the areas to both local people and visitors.

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Fuente:
European Environment Agency
The Dobris Assessment
Chapter 35: Coastal Zone Threats and Management

Última actualització: 1 de setembre de 2000