Clearly the opportunities to study the effects of oil presented by continuous natural seepage, refinery effluent, oil rig discharges, etc., on the one hand, and the more massive short-term injections of oil arising from oil rig blowouts and tanker accidents on the other, are different, and differences exist even within these two broad classes.
It may therefore by though that results obtained in one case may not be applicable in the others. It is thus worthwhile to consider types differences and to assess the extent to which results obtained at the various sites and incidents may be of universal application.
The first point to note in this endeavour is that oil does not dissolve in water. It floats on the sea surface as a thin layer when spilled or otherwise discharged because it is less dense than water and insoluble in it.
This thin layer tends to break into droplets by wave action and these droplets become dispersed in the volume of sea water beneath the slick. Photochemical and biological oxidation of the slicks and of the dispersed oil droplets take place producing oxygenated derivatives of the original oil components and these may form true solutions in water.
Thus, the area of sea covered by the surface slick is proportional to the magnitude of the spillage, but the concentrations in the sea arise from the slow dispersion of droplets of oxygenated derivatives. Sea water concentrations are therefore a function of mass transfer rates from slick to water and not a function of the quantity spilled.
Sea water concentrations are in fact independent of the quantity spilled, provided a surface layer has formed above the sea water volume in question.
If more or less oil is spilled or otherwise introduced to the sea the result is more or less sea affected rather than that a given volume is affected to a greater or lesser extent.
Oil concentrations are therefore controlled by the insolubility of the oil and mass transfer rates from surface slicks to the underlying sea.
In the absence of detailed knowledge of mass transfer rates it is not possible to calculate the resulting concentrations, but upper limits can easily be established. Thus, if the slick thickness to which the oil spreads and from which it subsequently disperses to the water is 0.1 mm, then the maximum concentration which can be achieved if the oil were instantaneously dispersed uniformly in the top metre of sea would be 100 ppm. Concentrations over other depth ranges and for other thicknesses are of course pro data.
It must be concluded therefore that is we leave aside for the time being the possible environmental effects of continuous oil slicks on the sea surface which occur when the capacity of the aqueous phase for oil is exceeded, we can discuss the other oil concentration-related aspects in the same manner whether a surface slick is present or not and whether the oil in question arose from a natural seep, a blowout, cargo tank rupture or operational discharge from ship, oil rig or refinery.
Generally speaking, we have limited our discussions to those materials introduced into the marine environment that are the direct result of man’s activities, and with a few minor exceptions we shall continue to limit our discussions of pollution to those activities of man and to consider the deleterious effects of natural occurrences.
Some idea as to what pollution does and why we consider it to be so undesirable. There are three types of pollutants: the pathogenic, the aesthetic, and the ecomorphic. Pathogenic pollutants are those which cause disease. This disease may be fatal if the pollutant is a lethal poison.
Aesthetic pollutants are those pollutants causing a change in the environment displeasing to the eye, ear, or nose of a man. Ecomorphic pollutants, on the other hand, are those pollutants which produce a change in the physical characteristics of the environment in such a way that there may be drastic changes in the structure or composition of the biosphere.
Obviously, these three types of pollutants are not of equal import. Pathogenic pollutants are certainly much more serious than the other two; however, ecomorphic pollutants are often an indicator of more serious types of pollution to follow.
Furthermore, since pollution is man- produced and the effects are suffered by man, aesthetic problems are certainly of interest.
Nevertheless in any management situation when trade-offs are required, it will certainly be necessary to rank different types of pollutants so that society can determine how it would like to spend is limited funds of time, effort, and money.
Let us look first at the pathogenic pollutants since these are obviously the most important. The effects of pathogenic pollutants are either acute or chronic. Acute effects are those which occur in a relatively short period of time while chronic effects may take time to be noticed.
As in the case of carcinogenic pollutants or the effect may be noticed immediately but continued for a very long period of time.
The acute effects are the easiest to determine because of the short time between administration and affection. The easiest way of measuring acute effect is by feeding the pollutant in question to test organisms, such as since or selected fish, and observing the dosage required to will 50% of the organisms involved.
This is usually done by feeding a pollution of test organisms increasing amounts of the pollutant, and for each dosage the number of individuals that succumb is noted. When these data are plotted, an S-shaped curve results, such as shown in Graph-3.1 wherein dosage versus number of mortalities is plotted.
From this curve it may be seen that there are a small number of individuals that require an extremely large dose before they are killed while some are killed with a relatively small dose. By plotting this type of curve, it is easy to locate the dose for which half the individuals die. This dosage is called the LD 50, and it is this LD 50 which is usually used to compare the relative toxicity of one pollutant with another.
Another way of examining acute effect is to observe the effect of a given pollutant concentration on organisms over a somewhat longer period of time, nothing how long it takes for given dosage to produce mortalities.
This may be done by giving a predetermined concentration of the pollutant to the test animals and observing how long it takes to kill half of them. When data from an experiment such as this are plotted, a curve similar to that shown in graph 3.2 results.
As may be seen, the curve is a hyperbola with the horizontal asymptotic suggesting a threshold concentration. In other words, below this asymptotic value there is no apparent effect, since at this concentration a finite amount of time is required for mortality.
The vertical asymptote, on the other hand, appears to be a saturation level. At this level any increase in the concentration will not produce any further decrease in the amount of time required for fatality. Evidently there is a finite time required for the pathogen to do its work, so that no matter how high the concentration, the time required to kill will not go below this amount.
With the asympototic values, suggests another concept: the threshold value. There appears to be a dose size for each pathogen below which the pollutant has no effect. The concern with threshold values for various pollutants is one aspect of the study of chronic effects.
Chronic effects are the most difficult to measure because of the time involved; in some cases cancers have developed as long as 25 years after initial exposure to the carcinogenic agent.
Nevertheless, measurements have been made and for many materials threshold value seems to describe the data reasonably well.
Most experiments need some measure of the effect on the organisms other than death, so that many different measures are used.
These are: deformity in the growth of organisms, damage to particular organs, and change in heartbeat or breathing rate, genetic damage to individuals, change in the rate of increase of a population, life expectancy of individuals within a particular population, and fecundity of females with the population.
Unfortunately, and index may be very descriptive of the effect of a pollutant on a particular species, but it may be completely ineffective in describing what happens when this pollutant comes in contact with other species.
A typical experiment using one of these measures would involve the variation in pathogen concentration while noting the variation in the heartbeat or some other measure. When these data are plotted the result is very often a straight line as shown in graph 3.3.
This linear relationship between effect and concentration may be extrapolated back to determine the concentration for zero effect. When this is done the line will cross the axis at a concentration from zero. This is called the threshold value.
A great deal of caution must be used in interpreting these data. Some pathogens apparently have no threshold value associated with them, and it is always risky business to extrapolate a straight line. The actual manner in which the organisms react to the stimulus might very well be nonlinear at lower dosages.
In addition some chronic effects may be so suitable that two or three generations of the organisms might be required to observe any change.
Another complication in the examination and quantification of pathogenic pollutants is the phenomenon of synergism. A synergistic effect occurs when the combined effect of two or more materials acting together is greater than would be expected from the simple sum of the individual effects.
Temperature, for example, is a very strong synergistic parameter, generally enhancing most pollutant effects as temperature is increased.
The same is true of salinity and dissolved oxygen. Some pollutants, on the other hand, tend to decrease the effects of others, as might be in the case of a strong acid and a strong base present at the same time. Other pollutants may cause strong pathogens to be precipitated out of the water column or to be collected on sediments, so that in either case they end up on the bottom.
Thus, it is extremely important in the analysis of any pollutant to be aware of the other materials present and their synergistic. Within the marine biosphere many organisms affected by various pollutants.
Whether or not an organism is affected by any given pollutants will depend on any variable. For example, many pollutants are found only in the water column while others, such as some of the pesticides and heavy metals, are adsorbed onto suspended sediments and usually find their way to the bottom within a relatively short period of time.
Consequent, these latter types would tend to affect those organisms living on the bottom or feeding from bottom organisms more than they would affect the organisms living within the water column, such as free swimming fish.
Also of import in determining the effectiveness of a particular pollutant on living organisms is its relative solubility in water and oil. Pollutants more soluble in oil will tend to find their way to organisms that have a larger content of oil in their body issues.
This is particularly true of some of the pesticides which are more soluble in oil than water. The individual habits of marine organisms also affect their susceptibility to a particular pollutant. Feeding habits with respect to time of day, portion of water column for aged, type of material ingested, and the method of digestion utilized all determine to a great extent the types of pollutant the organism will be exposed to.
Similarly spawning habits determine the type of area and time of year in which a species is particularly sensitive to subtle changes in the environment. These changes affect not only the parents but also the offspring, and may be even their progeny.
Other habits not having to do with spawning also help to determine, whether an organism will be exposed to particular pollutants.
Whether the organism is sensitive to light or sound, for example, will often determine whether it is attracted to a particular out-fall. Thus, we find that a pure and simple cause and effect relationship between a pollutant and an organism is not enough to determine completely the actual response of the organism in the real world.
Laboratory results might be completely refused in the field simply because the organism does not respond to extraneous characteristics of the environment under natural conditions in the same manner in which it did in the laboratory.
Pathways to Man:
Once some of the pollutant has been ingested by marine organisms, the major concern of man is the possibility of this pollutant appearing on his dinner table and causing a pathogenic response. Obviously if a fish ingests a pollutant and a human eats that fish, then that person will have ingested a dose of that pollutant and there will probably be some danger.
The question is, “How much?” In order to arrive at an approximation of the actual amount of undesired material finding its way into the reader’s stomach, one must follows the pollutant through a number of steps.
Most organisms living in the sea tend to concentrate materials existing in the sea to much greater values that they are in the ocean itself. For example, a diatom, a small form of marine algae having silicate frustules, has a body concentration of silicon about 40 thousand times as great as the oceanic waters from which the organism derives all of its material.
Thus, the diatom concentrates silicon very effectively so that if we eat diatoms, a large portion of our diet would be glassy. Similarly, other organisms, including fish, will concentrate other materials even if these do not naturally occur in the environment. But this amplification or concentration does not stop with one step.
We must continually keep in mind the fact that an organism does not exist by itself in the marine environment. Any organism is dependent upon many other organisms for its existence. An edible fish, for example, might very well subsist on smaller fish which, in turn, might subsist on small zooplankton, which, in turn, may subsist on phytoplankton.
There may be 5 to 10 individual links in a food chain leading to man, and there is the distinct possibility that for each one of these links a concentration of the pollutant occurs. Thus, the total pathway to man for the pollutant might be a rather tortuous one, but it might result in a very high concentration of the pollutant material in the fish.
This in itself may be of no concern to the individual unless he ingests the fish and if so, how often and how much he consumes. It is not enough simply to know that there is some pollutant present in marine organisms: one also should know how much is present, and in addition two other facts are required. This first is the accepted maximum level of ingestion of the pollutant before harm results, and the second is the amount of fish that can be eaten before this level is reached.
In this way it might very well be possible to have a particular kind of fish one or two times a year with perfect safety, while if it is eaten once a day, it might very well have a toxic effect.
There are many different types of pollutants that may be classified as ecomorphic, while some pollutants fall into all three of the classes stipulated above and many fall into at least two. Sediment is a pollutant whose effect is primarily that of changing the environment since it is not pathogenic and very often is not aesthetically displeasing, although it can be.
As it settles to the bottom, it will tend to bury the organisms that live on the bottom, such as cysters and clams, and eventually smother them. Sediment also tends to fill in marsh areas, killing the marsh grass, and completely changing the habitat.
Since marsh areas are breeding grounds for small fish and an important ecological link in most of the oceanic life, any destructive process such as this is bound to markedly change the marine biological environment.
On the other hand, in some cases where man’s activities have produced large amounts of erosion and sediment has been brought down by rivers into estuaries, some estuarine areas have been filled enough to support marsh grasses.
In some cases excess erosion can produce marsh lands rather than destroy them. Whenever there is a depth change in an estuarine or coastal area, the effects are many.
They range from changing the biological environment so that larger fish will no longer live in the shoal areas to affecting the total circulation pattern of the area. But suffice it to say that changing depth in many cases does change the manner in which salt and fresh water mix in the coastal area and also the amount of flushing present in a particular region.
When the sediment is in suspension it also has an effect on the eco-system, the most obvious aspect being a change in transparency. By making the water more opaque, light will not penetrate as deeply as before so that the thickness of the layer of water in which plants grow will be decreased.
If the water is fairly shallow to begin with, this decrease in transparency may very well result in the inability of rooted plants to grow and some marsh lands may be destroyed. The effect of sediment then is two-fold: changing both bottom and water properties. Both of these effects tend to change the physical characteristics of the ecosystem.
Another ecomorphic pollutant is oil, although oil sometimes produces toxic effects. The result of oil coating bird feathers is a well known physical effect.
The oil does not poison birds; it simply makes it impossible for them to fly and consequently many of them die. A film of oil on the surface will also decrease the amount of sunlight entering the water and, similarly, will limit the exchange of oxygen from the atmosphere.
In addition to the aesthetic and pathogenic effects of oil, there are thus also ecomorphic effects. The toxic effects of various nitrogen compounds are well known but fertilizers and other nutrients washed into the sea have a nontoxic effect on marine waters.
These materials will do just what they are designed to do: make plants grow, whether these plants are terrestrial or aquatic plants. Those plants on the bottom of the blanket will tie, due to lack of light, and those rotting plants will utilize oxygen in their decay processes.
Even though there are more plants than before, producing more oxygen in the process of photosynthesis, so many of them are dying and decaying the result is to decrease the dissolved oxygen. This phenomenon is called “Eutrophication.”
Another ecomorphic pollutant that seems to be receiving a lot of attention lately is heat. Usually supplied to the marine environment form electrical power plants or other industrial sources, this heat will increase the water temperature and may result in different kinds of damage to marine organisms.
Since most mutations are caused to thermal activity under normal conditions, the possibility of genetic damage is real. It is estimated that about 90% of the mutations occurring normally are thermally caused, while only about 10% are caused by radiation.
The last type of pollutant mentioned above was aesthetic pollutants. Aesthetic pollutants are those which offend the human senses and as such are considered by many to be unimportant.
In many environmental problem areas, though, the aesthetic considerations often receive more attention by environmentally oriented citizens than some of the pathogenic pollutants.
This is simply because the aesthetic pollutants are visible and what can be seen, smelled, or heard appears much more bothersome than that which is hidden, even though the hidden pollutant might be much more dangerous to life.
Consequently, any scheme for controlling marine pollution must include some consideration of the aesthetic pollutants if it is to succeed. An excellent strategy seems to be to focus on the aesthetic pollutants and use them as a mechanism to gather momentum to clean up the rest of the pollution problems.
Power transmission cable has caused more than one power company grief because of the environmental consideration given the local citizens who are concerned about the appearance of power cables running across the countryside.
Oil wells, especially the offshore variety visible from resort areas are of great concern to most environmentalists. The same thing is true of equipment required for dredging operations and the spoil areas resulting from these operations.
To some people, especially those living in relatively pristine environments near the water, boats can also be an aesthetic pollutant, especially those that cause waves and noise.
Effluents that cause the colour of the sea to change to increase the amount of suspended sediments also may be considered aesthetic pollutants. It simply is not as pleasant to go swimming in water that is cloudy and contains coloring material different from what one would expect in a normal, natural body of water.
In essence, any change from the pristine environment appears too many to be aesthetic pollution. Obviously we are living in an unreal world if we think the marine environment can be maintained in an unspoiled form.
It should be borne in mind that many complaints about aesthetic pollution come from individuals who would farther live in the primeval environment not realizing that it would not be available to them unless the commercial world about them supported it.
There appears to be a dichotomy wherein try to maintain the unspoiled environment in certain areas while letting others become over utilized. Nevertheless, there is a growing movement to include in the cost of every project a relatively small sum to improve its aesthetic qualities.
This is, of course, exactly the same philosophy that is applied to the control of other forms of pollution. Additional money is spent in a chemical process, for example, to control the pollution- producing properties of the effluent since society believes that this additional sum of money is desirable expenditure.
It may similarly be maintained that the expenditure for aesthetic pollution control is desirable as it also will tend to increase the quality of life.
These paragraphs briefly state those portions of these studies that are relevant to determining the effects of oil on marine wetlands. Also, the literature and out personal experiences with oiled marine wetlands are synthesized to allow an evaluation of methods or protection, clean-up, and restoration attempts that have been carried out in marine wetlands.
This section accomplishes there two objectives by:
I. Providing a brief review of the effects of oil spills and related clean-up activities on salt marshes and mangrove ecosystems
II. Reviewing methods of protecting marine wetlands from being oiled.
III. Reviewing successful means of cleaning marine wetlands following oil spills.
IV. Reviewing and presenting techniques that have proven successful in restoring marine wetlands damaged by oil spill and/or clean-up operations. Establishing a set of criteria and discussing guidelines for decisions on means of protecting susceptible areas, and for cleaning and restoring oiled marine wetlands.
V. The differences in growth patterns and recycles are essential elements in consideration of methods of clean-up and restoration of marine wetlands.
VI. It should be noted that salt marsh and mangrove vegetation occur together in some parts of the world, e.g., southern Brazil, southeast Australia. For the purpose of this discussion, we need to identify two terms.
VII. Recovery return of a site to dominance by native organisms that are within the natural range of limits for structure and function in unprotected examples of the ecosystem within the local geographic area.
VIII. Restoration man’s efforts to initiate and/or enhance the recovery process.