Categories of marine pollution

Before examining in detail the major types of marine pollution and the specific problems which they cause, or to which they contribute, let us see how they are grouped according to their basic properties.

Biodegradable and non-biodegradable materials

Most of the materials which reach the sea disintegrate either through simple chemical reactions or because of the activities of bacteria and some larger organisms. There are some substances nevertheless which are either extremely stable or else they have a very slow rate of degradation. The organic compounds in domestic sewage, wastes from the food industry and agricultural fertilizers, all belong to the first category while plastics, heavy metals and nuclear wastes belong to the second.
Figure 24 (essential - non essential) illustrates the main difference between the above categories: in the first one a substance is necessary for the ecosystem in small or moderate quantities, i.e. both absence and excessive concentration inhibit vital biological functions, while in the second case the substance is anything but necessary for the environment and every increase in its concentration leads to deterioration of the environmental conditions.



Bioaccumulated and non-bioaccumulated substances

Many of these long-lasting materials demonstrate the phenomenon of bioaccumulation, i.e. they accumulate in the tissues of living organisms in concentrations much higher than those in the sediment or in the water. Such concentrations may not be lethal for these organisms either because they have developed a certain resistance or perhaps because the toxic material is gradually deposited in their fat tissues. Severe effects stemming from such bioaccumulation appear in those predators (including human consumers) who are exposed without previous warning to high concentrations of the toxic substances in question.

Solid wastes - Plastics

Pollution caused by solid wastes, in particular plastic wastes, displays some of the most conspicuous effects from human activities, very often in the form of used bottles, cups, plastic bags and other objects scattered on the seashore. Though the end result is undoubtedly aesthetically unpleasant, there are other more objectionable features. Plastics decompose very slowly, and therefore they are very difficult to eliminate. Figure 26.

   

It has to be said however, that the biological consequences of this type of pollution are negligible or extremely limited in comparison with other types of pollution. There have been cases reported of fish having swallowed plastic objects which caused a blockage in the digestive tube; there are other cases of fish having been trapped inside plastic bags or other plastic objects and dying of suffocation; still other cases have been reported of fish getting entangled in plastic fishing nets which have been torn off fishing boats and drift in the water column, causing a phenomenon known as "ghost fishing".

The discharge of plastic sheets, coming for instance from greenhouses, and being subsequently deposited on the sea bottom does cause local problems in respect of the oxygenation of the sediments, giving rise to anoxic conditions underneath the plastic sheet. However, mortality related to plastic solid waste is not very important and does not affect the faunal composition of marine areas to any notable extent.
Of course the discharge of large quantities of solid material into the sea and especially on areas with sandy or silty bottoms will result in the alteration of the texture of the substrate and hence will be the agent of changes in the composition of the fauna. This process though is not always undesirable (i.e. if the solid material does not contain toxic compounds), and in some cases the development of artificial reefs has been considered as a very good way to increase the environmental heterogeneity in areas which function as nursery or feeding grounds for important fishing stocks.

Organic enrichment

Organic pollution or organic enrichment is the most common and the oldest type of pollution on the planet. Urbanisation, i.e. the high concentration of human beings in a limited space, created a need for the construction of networks of sewage outfalls which usually end up in the sea. The major characteristic of this type of pollution is the presence of high quantities of organic compounds of biological origin, i.e. produced by plants or animals. An equivalent quality of pollution is caused by certain industrial discharges such as the food industry, paper mills etc., the raw materials of which are also of biological origin.

The organic load concentration is expressed by means of a quantity called Biochemical Oxygen Demand (BOD), i.e., the amount of oxygen required by the micro-organisms for the decomposition of the organic compounds dissolved in a given volume of sewage. Consequently high BOD values indicate high concentrations of organic material and therefore low quantities of dissolved oxygen (DO).

The oxygen consumption for the decomposition of the organic material which results in anoxic conditions near the discharge point is the major consequence of organic enrichment. Since most marine organisms cannot survive under these conditions, the faunal composition changes dramatically within a short distance of the sewage discharge point. See figure 25.



Four sub-areas are distinguished, with sediments being more and more organically loaded and thus less oxygenated:

1. a heavily polluted area completely unsuitable for all macrofaunal organisms
2. a polluted area occupied by so-called opportunistic species, i.e. small species with a high reproduction potential, easily able to colonise azoic areas, whose major representative is the polychaete species Capitella capitata
3. a transitional area with species from both polluted and unpolluted areas
4. an unpolluted area where the oxygenation conditions are natural and the fauna is diverse, and includes species from various taxonomic groups as well as a wide range of sizes.

Macrofauna

Macrofauna (and especially the infaunal component) reflects environmental changes caused by organic enrichment with remarkable sensitivity. Because of this sensitivity, macrofauna is often used in pollution studies. On the other hand, nekton species (and epifauna to a certain extent) can easily emigrate from a polluted area but can also return from time to time in order to take advantage of the large quantities of food which can be found there.

Bacteria

Besides the effects on concentrations of oxygen, an important hazard caused by domestic sewage is the existence of pathogenic bacteria, viruses and all sorts of parasites. These organisms do not die when they come into contact with sea water (as was previously held to be the case) but they do suffer a considerable drop in population size due to the ultraviolet radiation effect. A high proportion, however, do change into tolerant forms not easily detected by the usual microbiological methods. Any swallowing of contaminated sea water during a swim, or any consumption of shellfish caught in polluted areas may give rise to severe public health problems.

Purification plants

Difficulties caused by organic enrichment can be overcome up to a point by making use of biological purification plant technology. Such treatment plants fall into three categories in respect of the proportion of sewage undergoing treatment.

1. primary treatment consists of the elimination of suspended solids using mechanical systems (racks);
2. secondary treatment: consists of the reduction of BOD using aerobic decomposition by bacteria. This process usually needs energy for the continuous aeration of the fermentation tanks.
3. tertiary treatment consists of the elimination of microbes, heavy metals and other chemical compounds by employing special techniques e.g. filtration, ultra-violet radiation, chemical sedimentation etc.

Sewage treatment technology has made a good deal of progress over the last decades so that by the end phase the water is considered to be pure enough to be used for agricultural irrigation instead of being discharged into the sea. Of course this technology is not cheap (both in terms of construction expenses and function costs) and therefore in most cases it is primary and secondary treatments which are used.

Pesticides

The so called "green revolution" led to the establishment of intensive agriculture, or, to put it another way, to an increase of the areas taken up by cultivated organisms. This concentration of many plants in a small space had certain advantages but also certain side-effects, namely,an increased susceptibility of the plants to parasites for those organisms which took advantage of these high concentrations or merely took advantage of the general change in the conditions of cultivation. These parasites belong to different taxonomic groups (plants, insects, fungi, acarea etc.) and nowadays they are kept under control by the use (sometimes excessive) of chemical compounds. These chemical compounds belong to four main categories.

1. Organochlorine compounds, such as DDT): most of these do not decompose or else decompose very slowly (half-life period from 9 to 116 years), and they usually accumulate in the fat tissues of certain organisms, in concentrations much higher than those encountered in the environment. It is noteworthy that these compounds have been detected in the fat tissues of Antarctic penguins who live in areas where such compounds have never been used.
2. Organophosphate esters, such as Parathion : these are very toxic compounds, much more so than the organochlorines
3. Carbamid compounds, such as Baygon: they decompose rapidly through hydrolysis and have a half-life period of approximately one week..
4. Chlorophenyl-acids such as 2,4,5-T used during the Vietnam war for defoliation: these are hazardous compounds whose use has been banned.

These chemical compounds usually end up in the sea, either through runoff into rivers or having been transported by the wind (dust - aerial spray). Once in the sea they enter the biological cycle of certain organisms and, through the bioaccumulation mechanism, some amounts are stored in the fat tissues of particular marine organisms. This storage has no visible effects on these organisms except when a shortage of food occurs, and the fat deposits are used up. It is then that large quantities of the chemical compounds enter the metabolic cycle with possible lethal consequences. This time-lag in the physiological activity of these pesticides facilitates the biomagnification phenomenon, that is to say, the multiplication of the concentrations of pesticides in the higher levels of the food web with its concomitant dramatic effects on marine birds and mammals.

Fertilisers

Fertilisers are industrial products rich in nutrient salts, especially nitrates and phosphates, and are used in order to increase agricultural yield. These salts are soluble in water and are often carried through the runoff of rainwater to the sea via streams and rivers. Another major pathway of nutrient input to the sea (and the land) is through wet and dry atmospheric deposition. The atmospheric nitrogen input in northern Europe may reach 10-20 kg Nitrogen per hectare and year. The origin is not only the car traffic but also intense culture of cattle where cows are fed like pigs. It may be noteworthy that one cow fed like this releases the same amount of nitrogen as 1-2 cars without catalysators. Nutrients, as has been mentioned are necessary for primary production both in the terrestrial and the marine environment. In this sense they cannot be considered as pollution factors except when occurring in very large quantities, i.e.when they stimulate excessive vegetal production, at levels which disturb the equilibrium of the ecosystem capacity.

Oil Pollution

Oil is a mixture of organic compounds, many of which are toxic to marine organisms. Generally, these organic compounds are considered under three headings:

1. low molecular weight These are volatile, that is to say they evaporate easily and thus do not make a major impact on the marine environment;
2. medium molecular weight These remain in the marine environment for considerably longer and are mainly responsible for oil toxicity;
3. high molecular weight These are tars which are inert from the chemical point of view but have mechanical side effects on the ecosystem.

Every year 3 billion tonnes of crude oil are produced worldwide. Out of this total, half is transported by sea and it seems that approximately 3 million tonnes of crude oil end up in the sea itself. These figures then break down as follows:

7%	comes from non-anthropogenic inputs
5%	comes from leaks from permanent installations (refineries, offshore platforms etc.)
45%	comes from the transport process itself (tanker operations, bilge and fuel oils,docking, accidents etc.).
43%	comes from domestic (as seen opposite) and industrial wastes

From the above only 15% results from tanker accidents and these are the best known to the public, almost certainly because of their dramatic consequences.

Effects of oil on marine organisms

The effects of oil pollution differ according to the mode of life of the marine organisms and to the general characteristics of each component of the ecosystem, each of which is described below.


Rocky shores

Because of the high energy which characterises these ecosystems (wave action, tides etc.), oil is easily flushed away. Nevertheless green and red algae can be affected directly by the toxicity, and considerable effects can be incurred on the herbivores' populations, and these will cause significant alterations in the structure of the communities.

Sandy and silty coasts

The consequences here can be much more substantial because oil covers the sea bed, then becomes incorporated into the sediments and results in anoxic conditions which has catastrophic consequences for the infauna.Epifauna, although generally more mobile, suffers from direct oil toxicity as well as from the indirect effects of oil on its food resources which are of course the infauna. In addition, the low concentrations of oxygen in these coasts will result in slower degradation of the oil and hence the toxic activity will be prolonged.

Plankton

Plankton and especially neuston, suffer more intensively from oil toxicity in the initial phase, i.e. when the oil is floating, because at that stage it still contains significant quantities of low and medium molecular weight compounds which are the more toxic. In addition it has a relatively low specific weight and hence it will remain on the top of the water column. Apart from its chemically toxic activity there is also the mechanical effect: oil has a tendency to expand into as large an area as possible, creating a thin layer or film, which not only prevents the exchange of gases but also the penetration of solar radiation that is required for photosynthesis. It seems however that these consequences are transitory and the planktonic system recovers relatively fast.

Marine birds and mammals

Marine birds display the most important (and visible) effect of oil pollution which comes about principally because of its mechanical activity; when oil makes contact with the plumage of a bird, the water-repellent properties of the feathers are destroyed and hence the birds either sink (because they can no longer float on the water) or die of hypothermia.
On the other hand, the chemical-toxic oil effects do not seem to be important because these can only take place after oil is swallowed, a relatively unlikely event. The reported losses in the populations of marine birds are high and because of the relatively small number of offspring produced in relation to other marine organisms, they cannot easily be replaced.
Marine mammals do not seem to be very vulnerable to this type of pollution; there have been only a few cases reported of young seals having been contaminated by oil.

Other effects of oil pollution

Apart from the effects on the ecosystems, there is another major issue of concern, especially where there is chronic oil pollution, and that is its economic consequences.

Fisheries.

Even a tiny amount of hydrocarbons will give a characteristic and unpleasant taste to fish and shellfish and this makes it impossible to sell them in the open market. This is major threat to aquaculture of finfish (salmon) and shellfish (mussels and oysters)

Tourism

Coasts (and therefore beaches) affected by oil spills. are completely unacceptable to the tourist industry. One particular irritant is the presence of tars. Although they do not have toxic effects, they make a very unpleasant impression and they remain where they are deposited for a very long time.

The cleaning process

The consequences of oil pollution and especially those associated with major leakages, bring to the forefront the need for the development of an effective anti-pollution technology.

In large modern harbours there are special installations for bilge water; modern tankers are built to specific standards (load-on-top system, double walls), and materials for the disposal and elimination of oil-spills, namely floating booms, are being marketed. See figure 26.

Absorbents and dispersants.

Dispersants, chemical compounds which dissolve oil in sea water and emulsify it, are even more toxic than oil, despite the improvements made over the past decade. It is recommended that dispersants be used in the open sea only. This is so as to prevent oil from reaching the coastline where it is more difficult and more expensive to clean up.

Heavy Metals

Heavy metals are not as visible as the types of pollution we have considered so far, but from their position hidden within the sediments, must be considered as a very serious marine pollution hazard. The term "heavy metals" is used to denote elements with specific weight higher than those of Iron (Fe) and mainly Lead (Pb), Mercury (Hg), Copper (Cu), Cadmium (Cd)and Chromium (Cr).

Sources/ origins

Heavy metals are associated with several different kinds of human activities. Their use in the chemical industry is widespread: factories producing paint, photographic materials, electronic materials, pesticides, batteries, explosives, metallurgy etc. all use Several industries use copper tubes in their cooling systems and significant amounts of copper end up in the sea after the cooling water is discharged , because of corrosion.

Lead (Pb) from automobile fuel is released into the atmosphere where it enters the water cycle and results in the deposition of considerable quantities of lead compounds on land, running off into rivers and streams which flow directly into the sea.

Domestic sewage also contains a certain amount of heavy metals deriving from domestic industrial products making use of heavy metal compounds, as well as from certain types of detergents etc.

The burning of solid wastes is yet another factor which has led to an increased concentration of particles containing heavy metal compounds in the atmosphere, where the water cycle phenomenon again ensures that these particles once more end up in the sea.

Nevertheless, some of the heavy metal concentrations in the sea is due to natural inputs, in particular to the erosion of minerals which have a high level of heavy metal compounds.