Laymen tend to think of oceans as limitless or bottomless but this concept is erroneous for the oceans are quite limited in respect of their capacity to act as huge sinks and receptacles for wastes. The production capacity of oceans for food and other resources has also certain limits.
Oceanic pollution can be caused by ship-generated discharges of oil and petroleum products, noxious liquids, packaged dangerous goods, sewage and garbage, etc. The fate of a pollutant discharged into the sea is determined by many routes and processes.
Thus, the pollutant may become dispersed by turbulence and ocean currents, or transported by ocean currents and migratory birds, or concentrated by means of various biological or physicochemical processes. Such biological processes may include uptake by seaweeds, fish, birds and phytoplankton (and from the latter it may pass on through a food- web involving zooplankton, benthic invertebrates, fish and mammals). Physicochemical processes of concentration can involve adsorption, precipitation, accumulation in bottom sediments and ion exchange phenomena.
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Any serious pollution of seawater can result in a loss of diversity in the marine organisms and unstabilize and simplify the otherwise complex and stable ecosystem. This has been clearly demonstrated as a result of recent studies of the extent of pollution in the Baltic Sea (see Bagge, 1973) along the coast of Finland. The pollution here mainly comes from sewage and effluents from wood-processing industries.
The bottom fauna of polluted coast shows a zonation in keeping with the extent of pollution. In highly polluted areas, macroscopic benthic animals are absent but chironomid larvae occur at the sea bottom. The commonest pollution-tolerant benthic animal is Macoma baltica.
Most crustaceans avoid polluted areas. The greatest diversity of bottom fauna was found in clear or only slightly polluted water and it was found to decrease progressively with increasing load of pollution. Similar studies of fishes in polluted and non-polluted areas near Helsinki revealed that though the production and biomass of fish in polluted areas was much greater than that in relatively clean waters, the species diversity was much lesser in polluted areas than non-polluted, and the catches of edible fish were also smaller.
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Terrestrially produced or eroded substances and pollutants are mostly carried into an ocean via (a) the atmosphere, e.g., rain: (b) rivers; and (c) deliberate disposal of substances by man through pipelines, etc. This last source is the most dangerous since, unlike terrestrial habitats, marine sediments and seawater are relatively free of decomposers and hence are unable to degrade the inputs.
Oceans can cope up with reasonable amounts of particur late minerals, common soluble chemicals, mud and salt, but they are unable to rot or decompose organic matter. Organic materials can, however, be eaten, digested and excreted by marine animals, or may enter into simple chemical reactions (Maclntyre and Holmes, 1971).
However, certain microbiological processes such as sulphate reduction and release of ammonia and phosphate from organic compounds in anoxic marine sediments have been studied by Berner (1973). He proposed the following generalizations: (a) dissolved ammonia and phosphate seem to originate from the same general organic decomposition process which in some marine sediments is believed to be brought about by microbes in addition to those concerned with sulphate reduction; (b) heavily polluted sediments seem to be subject to faster decomposition of their organic matter than less polluted ones; (c) in sediments containing high concentrations of fine particles of CaCO3, precipitation of dissolved phosphate occurs on the surfaces of CaCO3 grains. Consequently, phosphate concentrations in the interstitial water of such sediments are much lower than those in anoxic siliceous sediments.
During early diagenesis, the major change in organic matter seems to involve completing of simple monomers to form humic acid. These are oxygen rich compounds, probably composed or reaction products of carbohydrates and amino acids. Virtually nothing is known about the nature of the carbon compounds utilized by sulphate-reducing bacteria (Kaplan, 1973).
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The capacity of oceans to cope up with the various kinds of substances being dumped by man is limited and mere dilution of the discharged pollutants in the vast oceans is no solution to the ever-increasing problem.
Paper wastes and cellulose are non-biodegradable in the oceans and have been reported even to intercept light transmission to marine phytoplankton (Mainnheim et al., 1970).
In recent years oil pollution has attracted considerable attention. Recent increases in shipping and drilling operations have caused increased contamination of our oceans with chemical pollutants, including petroleum hydrocarbons. Marine fish, crustaceans, molluscans, and zooplankton accumulate hydrocarbons from polluted water.
Aquatic species may be more sensitive than mammals to the acute lethal effects of a variety of chemicals, and they may also exhibit the biological effects of nonlethal exposure to chemicals more quickly, or at lower doses, than mammals. Marine species and fish form an important constituent of the diet of many people and greater knowledge of the interactions between marine species and pollutants is needed in view of the increasing chemical content of our oceans.
Most of our knowledge on the effects of pollutants in marine systems comes from experiments to determine concentrations which are lethal or developmentally or physiologically effective for species grown in laboratory aquaria, or from observations of stressed conditions in grossly polluted areas. Under these conditions in natural ecosystems only a small fraction of the numerous species present can be examined, and it is possible that critical or sensitive species are missed.
Much of the oil and petroleum products are shipped by sea and any spill of this oil can cause serious pollution problems. ‘Spills’ are defined as occasional discharges or dumping that happen as a consequence of accidents, human error, malfunctioning of vehicles and equipment, deliberate discharges of ballast water, and convenience dumping into sewers, streams, estuaries, coastal waters, etc.
Many of these spills involve such things as oil, gasoline, grease and waste lubricating oil. Some of these spills cause obvious, even pronounced, damage to marine fish, birds, invertebrates, algae, and mammals whereas the damage caused by other spills may largely remain unnoticed for some period and hence is liable to be ignored.
Sometime the spills themselves may not be very harmful to marine life, but the detergents employed to clean up the spill prove extremely harmful. Thus those areas of British coasts around which large amounts of toxic detergents were used to clean up the oil slick caused by the notorious Torrey Canyon accident a few years ago, witnessed a high mortality of marine life whereas other areas where the detergents were not used, remained more or less unaffected. Oil and oil products seem to persist in food chains for long periods.
Some other kinds of pollutants that can upset marine ecosystems include metals (e.g., mercury, lead), DDT, polychlorinated biphenyls, other agricultural chemicals, fungicides, pesticides (which are leached off from land into rivers, streams and then into the sea), 2, 4, 5-trichlorophenoxyacetic acid, and various radioactive substances.”