Coastal pollution and impacts
Coastal and estuarine ecosystems have been, and still are, heavily influenced by the human species through pollution and habitat loss throughout the world. Examples of environmental issues include the enrichment of enclosed waters with organic matter leading to eutrophication, pollution by chemicals such as oil, and sedimentation due to land-based activities or sea-level rise due to the global change. Over 80% of all marine pollution originates from land-based sources which are primarily industrial, agricultural and urban. Pollution accompanies most kinds of human activities, including offshore oil and gas production and marine oil transportation.
Contents
Background
The relative contribution of each of the channels discussed in the introduction into the combined pollution input into the sea will be different for different substances and in different situations. Quantitative estimates of these processes are difficult because of the lack of reliable data and the extreme complexity of biogeochemical cycles, especially at the sea-land and sea-atmosphere interfaces. Point and non-point source pollutions continue globally, resulting in the steady degradation of coastal and marine ecosystems. Indirect (or diffuse) inputs are usually widespread, low-level discharge often likely to result in chronic pollution. In this section, generic sources of pollution and effects (eutrophication, contamination and pollution from industry and agriculture, etc.) are considered. The sensitivity of the coastal zone to watershed impacts is examined in relation to land-derived pollution and water quality.
Contamination is usually considered to occur when an input of waste from human activities increases the concentration of a substance in seawater, sediments or an animal above the background level for that area or animal, but without any obvious effect.
Agriculture
Emissions and inputs from agriculture are a significant source of pollution to the coastal zone and to the atmosphere throughout the world. The contribution of agriculture to the pollution of coastal ecosystems affects biogeochemical cycles, notably in terms of:
- Discharge of methane and ammonia contributing to the greenhouse effect
- Use of insecticides and other pesticides affecting species, cultivated or not
- Pollution, from use of oil for instance
- Nutrients run-off from silage and slurry-manure, use of fertilizers leading to eutrophication
Eutrophication
Eutrophication results from the increase of nutritional resources to a particular water body and includes the supply of mineral nutrients (nitrogen, phosphorus, silicon, trace elements) as well as organic carbon[1]. Discharges and emissions from land-based sources (industry, households, traffic, agriculture) provide large inputs of nutrients to coastal waters via rivers, direct discharges, diffuse sources and deposition from the atmosphere. However, eutrophication cannot be defined just in terms of an increase in nutrients concentration, as its manifestations (very often harmful to ecosystems) occur due to the existence of natural conditions, such as high temperatures and calm coastal waters. In the recent past eutrophication has been most pronounced in the developed world, but it has to be expected that it will become more and more important in the developing countries of Asia, Africa and Latin America in the near future[2].
Additional atmospheric input of inorganic nitrogen has increased significantly to a level where it is already higher than the natural nitrogen supply in the North Atlantic Ocean basin. It is expected that the worldwide production of nitrogen (mainly from fertilizer industry and the burning of fossil fuel) will affect the biogeochemical cycles on a global scale. Unfortunately, the interactive effects of the altering N-cycle on the carbon cycle (including the dynamics of greenhouse gases within both cycles) are poorly understood.
The effects of eutrophication vary from increased growth of phytoplankton, benthos and fish to changed species composition at moderate eutrophication. From blooms of nuisance causing or toxic algae (Fig. 1) to mass growth of certain species and mortality of others at severe eutrophication, and ultimately to anoxic conditions and mass mortality (fishkills). An algal biomass related phenomenon such as oxygen depletion of the water column and consequent mortality of animals can be prevented by a general reduction of nutrient discharges. At present, mathematical models on ecosystem dynamics are reliable enough to estimate dose-effect relationships. Algal species related harmful effects are less predictable. There is a general consensus that there is a global increase in harmful algal blooms. Also, there are reports which suggest a link between blooms of toxic algae and human activities such as salmon (or fish) farming. Changes in N:P:Si ratios may also cause a shift in species composition. Consequently, alterations in pelagic and benthic communities are to be expected. Which species is stimulated by eutrophication is highly dependent on the local environmental conditions.
Industry
Since most contaminants enter the sea by flows from the surrounding land, in particular via rivers, the highest concentrations are often found in estuaries and coastal areas and thus maximal effects of contaminants on the ecosystem could be expected to occur here. This general picture can be influenced by additional inputs from sources at sea - ships, off-shore platforms - and by inputs via the atmosphere. After entering the sea, contaminants are usually diluted and widely dispersed. However, the adsorption of contaminants to suspended solid material in the sea leads to the occurrence of elevated concentrations in the seabed in areas where this material settles. Areas, which are also close to direct sources of input, are doubly at risk for example estuaries and lagoons. Water quality is affected by toxic substances, which are persistent in the marine environment. These substances are of varying origin and composition, but they are together classed as stable or persistent, and have decisive properties in common. They are not readily degradable, or not at all degradable, toxic to living organisms, and bio-available (living organisms can take them up and accumulate them). The persistence of certain groups of contaminants, recognised as “toxic” in the marine environment, varies. Some of the factors which have an influence include their chemical reactivity, which describes the probability for reactions with other substances, e.g. photochemical reactivity which deals with the probability of reactions initiated by light. Their biological reactivity is dependant upon the probability for different biological systems to modify or metabolise a chemical compound. In turn, the distribution and availability of chemicals in the environment depend on a number of parameters, such as:
- The emission - whether the contaminant is emitted as a gas, a solution, adsorbed on particles or included in a solid matrix, and
- The volatility - the influence the distribution of the chemical;
- The lipophilicity - depends on the solubility of the chemical in lipids and is one of the factors affecting accumulation in organisms;
- The structure of the molecule - influences the stability and the biological availability of the chemical.
- The geographical distribution of pollutants - is dependant on partition between different compartments. The following factors can have a significant influence:
- The source pattern - whether it is a point or a diffuse source,
- The wind - is essential for a long range transport,
- The precipitations - are an important mode of transport from the air to seawater and to sediments,
- The contribution from the watershed and riverine inputs - are dependant upon factors such as the geology, the geomorphology, the relief…
- The currents in the sea are dependant on tides, location, exposure...
- The movements of organisms - are another transport mode of contaminants.
The bio-concentration or bio-accumulation of contaminants in the tissues of organisms is essential to understand. Bio-accumulation describes the ratio of a compound in the organism and the concentration in the surrounding medium. This factor is a function of the stability of the chemical but also how it will accumulate in the fat of the body, i.e. its lipophilicity. Accumulation presents a risk to consumer organisms, including the human species.
Bio-magnification describes a higher concentration in an organism than in its diet. It is related to the biological availability of contaminants to organisms and to their metabolism and excretion rate. Therefore, identical levels of specific contaminant can have different effects due to the fact that they can be present in different forms with different availability for uptake.
Organic synthetic substances
More than 7,000,000 compounds are known as Persistent Organic Compounds (POCs) and there are almost infinite possibilities to combine new substances. Serious environmental damage is caused by some of these POCs in the sea. Effects take place at metabolic and physiological levels, both in marine vertebrates and invertebrates. The great number of organic substances are due to the numerous possible substituents and substitution patterns. In fact, there are only few dangerous chemicals, and in the marine environment, the acute toxicity of compounds is only relevant after accidental spills. The only important exception to this rule is the bird casualties due to operational spills from ships of lipophilic floating substances and surfactants. The more important effects to focus on from a scientific point of view are caused by chronic exposure to relatively low concentrations and affect reproduction, immunology and carcinogenicity.
In common, POCs all have the following characteristics: they are stable and toxic, and share a similar structure. It has emerged that some highly stable organic compounds - chiefly halogenated hydrocarbons - can have serious environmental effects in the sea. Such substances have in common the presence of a halogen in their molecule (chlorine, iodine, fluorine, astatine), have a low polarity and low water solubility. Aromatic compounds are more reactive and susceptible to chemical and biochemical transformation and include pesticides (chlorinated such as DDT – DDE, Polycyclic Aromatic Hydrocarbons, Hexa Cyclo Hexan, and organometallics such as tributyltin). There are 209 congeners of Poly Chloro Biphenyls, all with different properties. This variety makes both analysis and effect studies complicated. The metabolic pathways of polychlorinated biphenyls (PCBs) congeners are complex. Though the use of PCBs has been prohibited for a long time, emissions from unidentified sites still occur. For example, re-circulated waste paper used as raw material for new pulp was discovered downstream from a paper mill. It is unclear how organic synthetic organic chemicals affect marine organisms but PCBs, for instance, are frequently found in fish liver, seal blubber, bird eggs, and human fat. Organochlorines have been associated with impaired reproductive ability in seals and whales. For instance, octachlorostyrene (OCSs) have been found in benthic organisms. OCS concentrations can be taken as an indication of incomplete combustion resulting in the accumulation of chlorinated hydrocarbons in marine organisms.
Organometallic compounds such as tributyltin have been used extensively as antifouling agents and are now banned in many countries because of its effect known as imposex. Imposex refers to a change of sexual characteristics in invertebrates, female gastropods growing a penis, for instance. Compounds in alternative antifouling products form a special field of interest, since these compounds are especially applied to display their toxic effects in the marine environment. Their use is expected to increase in the near future due to the total ban on tributyltin-based antifouling chemicals by the International Maritime Organisation IMO in 2003.
Effect on wildlife
It is now well established that there are anthropogenic chemicals released to the environment that can disrupt the endocrine systems of a wide range of wildlife species. The reproductive hormone-receptor systems appear to be especially vulnerable. Indeed, changes in sperm counts, genital tract malformations, infertility, an increased frequency of mammary, prostate and testicular tumours, feminisation of male individuals of diverse vertebrate species and altered reproductive behaviours, have all been reported (Sharpe & Skakkebaek, 1993; Colborn et al., 1996). With regard to environmental management, the problem of endocrine disrupting chemicals is extremely difficult to address. Basic research is required to strengthen the scientific foundation for risk assessment: e.g., baseline studies on endocrine dysfunction across classes of animals to reduce the uncertainty associated with species extrapolations (NSTC, 1996).
An extensive list of chemicals which are thought to be capable of disrupting the reproductive endocrine systems of animals has been assembled. They fall into the following categories:
- Environmental oestrogens (oestrogen receptor mediated) (e.g., methoxychlor, bis¬phenolic compounds);
- Environmental antioestrogens (e.g., Dioxin, Endosulphan);
- Environmental antiandrogens (e.g., Vinclozolin, DDE, Kraft mill effluent);
- Toxicants that reduce steroid hormone levels (e.g., Fenarimol and other fungicides; endosulphan);
- Toxicants that affect reproduction primarily through effects on the CNS (e.g., dithiocarbamate pesticides, methanol); and
- Other toxicants that affect hormonal status (e.g., cadmium, benzidine-based dyes).
Metals
Heavy metals are naturally occurring and do not degrade. They are not particularly toxic as the condensed free elements (except Hg vapour) but they are dangerous to living organisms in the form of cations and when bonded to short chains of atom carbon. In particular cations have a strong affinity for sulphur. For example, sulfhydril groups in enzymes attach themselves to cations or molecules and so the enzyme is blocked. They are a problem in the marine environment because they bio-accumulate in marine organisms and despite measures taken to combat pollution, they are still concentrating year after year. Pollution above background levels in the environment can cause serious effects. For instance, copper is a useful oligoelement bit in excess it affects trophic levels. As a free element, mercury has hundreds of applications, for example in electrical switches. Despite emissions of vapour from the industry have been curtailed, there are still releases from unregulated burning of fuel or wastes. This human source of pollutant is added and its atmospheric inputs rival volcanoes. The ultimate sink for metals and many organic compounds is the sediment. Processes involved include deposition and burial as heavy metals are retained by:
- adsorption onto the surface of mineral particles
- complexation by molecules in organic particles
- precipitation reactions
Radioactive substances
Present day levels of radioactive substances found in coastal waters are the result of natural radioactivity (cosmic rays, earth crust), and possibly released radioactivity due to human activities such as oil exploration and combustion, phosphate production and use, land-based mining, managed discharges from nuclear power and reprocessing facilities, fallout from atmospheric nuclear weapons testing and accidents, medical diagnosis and therapy, and food conservation. The world's oceans have been a sink for radioactive waste from the production of nuclear weapons and electric power since 1944. Radioactive waste enters the ocean from nuclear weapon testing and the resulting atmospheric fallout, the releasing or dumping of wastes from nuclear fuel cycle systems, and nuclear accidents (for example Chernobyl in 1985). Dumping of high-level radioactive waste is no longer permitted in the ocean, but dumping of low-level wastes is still permitted. Low-level waste contains less radioactivity per gram than high-level waste. High-level wastes usually have longer half-lives. For example, one common high-level waste that is produced by spent nuclear fuel has a half-life of 24,100 years.
Oil and gas and offshore installations
Oil is at the heart of the modern economy in providing a cheap source of energy and as a raw material for making plastics, etc. It is a mixture of hydrocarbons and up to 25% non-hydrocarbons such as sulphur, vanadium, and metals. Environmental impacts occur at all stages of oil and gas production and use. They result from prospecting activities (including seismic techniques), physical impact due to the installation of rigs, operational discharges when production starts, accidental and routine spills, and finally combustion. Nihoul & Ducrotoy (1994)[3] have estimated the input of oil to the North Sea, due to the offshore industry, at 29% of the total input of oil. Offshore installations may disturb the environment through the placement of structures on the seabed, which disturb benthic organisms, acoustic disturbances and light emission. An increasing number of installations is currently reaching the end of their productive life and will need to be dismantled or removed throughout the world seas.
Overall, coastal ecosystems remain largely affected by direct discharges of oil from offshore activities and illegal discharges from ships. Operational discharges consist of production water and drilling cuttings. Although the amount of oil discharged via production water is increasing as platforms are getting older, cuttings still account for 75% of the oil entering the sea as a result of normal operations. The effects on the marine environment have been extensively studied by national authorities as well as by the industry.
Despite uncertainty about possible long-term effects, problems with oiled cuttings (drilling muds) are acknowledged. Effects in the vicinity of platforms are well known , in particular those on the macrobenthic invertebrates, which range from lethal to insignificant, depending on proximity. Spills of oil and the release of chemicals (i.e. lubricants) used during exploitation constitute an important source of chronic pollution to coastal seas.
Even if accidental spills represent a relatively small source of oil, they directly affect birds and mammals and have devastative effects on local vulnerable economies. Shipping is a main source of oil slicks (chronic and accidental) showing no downward trend. Combustion of oil is the ultimate stage in the chain of production-use. Polycyclic aromatic hydrocarbons (PAHs) originate from various sources such flaring or engines, including on land. Nevertheless, their major sources are not the rivers but sources in the sea itself from platforms and ships (Laane et al., 1998). PAHs are volatile material which travel well airborne, showing the importance of the atmospheric pathway in the distribution of contaminants. Airborne pollutants are dissolved by rain and some are carried to the coast as dust particles or in solution from the atmosphere. The current knowledge about the dependence of the deposition velocity upon the particle size and about the processes controlling wet deposition fluxes, and the quality and completeness of the emission data are still inadequate for describing the environmental cycle and impact of such pollutants.
Conclusion
The effects of contaminants on coastal ecosystems are very difficult to assess. In the estimation of possible effects, the actual concentrations are compared with the levels that can cause effects. Results of laboratory experiments give only limited information in relation to the field situation, due to the complexity of natural systems and (in general) the co-occurrence of a multitude of contaminants in the field. Actual changes in the sea are often very difficult to discern from the large variability, which occurs naturally.
Measurement of marine pollution using biological indicators provides information on the bio-availability of contaminants and the integration of the effects of multiple exposures and exposure over time. Multiple exposure is the combined action of all chemical contaminants. Even if the contaminants are present at concentrations too low to cause gross harmful effects, they can cause a suite of bio-chemical reactions in marine organisms generally called stress. Amongst the result of prolonged stress is the suppression of the immune system, thus increasing sensitivity towards the impact of infectious agents and parasites.
Natural factors such as temperature extremes and fluctuations of salinity or anthropogenic activities, such as fisheries, can aggravate these reactions. A suite of bio-chemical reactions in marine organisms may occur as a response increasing sensitivity towards the impact of infectious agents and parasites.
References
See also
Please note that others may also have edited the contents of this article.
|
