2.5 Global Eutrophication?
Apart from polar and upwelling regions, oceans on the whole are deficient in nutrients,
oligotrophic, if one only observes the productive surface layer, which is permeated by
light (Fig. 75). The water of the surface layer is warm and thus lighter than the cold
deep water. Across broad expanses of the warm oceans in the tropics, only little
exchange of water takes place between the surface and the deep water layers. Plankton
which are in the process of dying and fecal pellets from plankton animals sink slowly
and conduct phosphorus and nitrogen away from the surface. Therefore surface primary production is poor. If one could compensate for the loss of nutrients through
fertilization, it would then probably be possible to increase the amount of commercial
fish in tropical seas. Technically and economically, however, this idea is an illusion.
Looked at from the perspective of marine pollution, eutrophication is not a problem
which is likely to assume ocean-wide proportions.The outlook for the supply of raw
materials is more important. The chemical industry can produce nitrous salts in
unlimited amounts as long as sufficient energy is available for the synthesis of nitrogen compounds from the atmosphere. The deposits of phosphate ore on the other
hand cannot be replenished. At the present rate of consumption, the known deposits
of ore would be sufficient for another 500 years. But if one takes a global view,
thinks of the future, and keeps the well-being of future generations in mind, then
the overuse of irreplaceable phosphates in fertilizers and detergents cannot be justified. The amounts of phosphorus that are present in the world's oceans contribute
little to the fertility of the world; they are mainly in the light-less depths of the
world's oceans and cannot be utilized.
2.6 Sewage Treatment Plants in Coastal Regions?
Should coastal city sewage processing plants be built which also remove phosphorus
and nitrogen from waste water so that the seas are protected from the dangers of
eutrophication? For freshwater, the answer is in many cases obvious: eutrophication
must be avoided if rivers and lakes are to maintain their capacity for purifying themselves and supplying drinking water. For the Baltic, the answer depends on the evaluation scientists will provide regarding natural versus man-made causes of oxygen depletion in the deep water (see Chap. 2.4). For coastal areas of other seas, like the North
Sea and Kattegat, there are, at present, not many arguments in favor of a chemical
elimination of nutrients from sewage. One can, by the way, go one step further and
question whether waste water treatment is at all necessary in coastal situations.
A biological sewage treatment plant reduces the nutrient load in the effluent by
about one third and the amount of biodegradable organic substances, expressed as
BOD, by about 90%. To protect narrow fjords, bays, and estuaries, such treatment
plants are necessary, otherwise anaerobic conditions could spread. To a certain degree
a reduction of pathogenic bacteria and viruses is achieved, too, by the treatment of
waste water. The question, however, has to be discussed whether a waste water treatment plant, which is a rather complicated, costly kind of a factory, is the cheapest and
most efficient technical means if no other effect is wanted but to kill pathogens.
27
Apart from polar and upwelling regions, oceans on the whole are deficient in nutrients,
oligotrophic, if one only observes the productive surface layer, which is permeated by
light (Fig. 75). The water of the surface layer is warm and thus lighter than the cold
deep water. Across broad expanses of the warm oceans in the tropics, only little
exchange of water takes place between the surface and the deep water layers. Plankton
which are in the process of dying and fecal pellets from plankton animals sink slowly
and conduct phosphorus and nitrogen away from the surface. Therefore surface primary production is poor. If one could compensate for the loss of nutrients through
fertilization, it would then probably be possible to increase the amount of commercial
fish in tropical seas. Technically and economically, however, this idea is an illusion.
Looked at from the perspective of marine pollution, eutrophication is not a problem
which is likely to assume ocean-wide proportions.The outlook for the supply of raw
materials is more important. The chemical industry can produce nitrous salts in
unlimited amounts as long as sufficient energy is available for the synthesis of nitrogen compounds from the atmosphere. The deposits of phosphate ore on the other
hand cannot be replenished. At the present rate of consumption, the known deposits
of ore would be sufficient for another 500 years. But if one takes a global view,
thinks of the future, and keeps the well-being of future generations in mind, then
the overuse of irreplaceable phosphates in fertilizers and detergents cannot be justified. The amounts of phosphorus that are present in the world's oceans contribute
little to the fertility of the world; they are mainly in the light-less depths of the
world's oceans and cannot be utilized.
2.6 Sewage Treatment Plants in Coastal Regions?
Should coastal city sewage processing plants be built which also remove phosphorus
and nitrogen from waste water so that the seas are protected from the dangers of
eutrophication? For freshwater, the answer is in many cases obvious: eutrophication
must be avoided if rivers and lakes are to maintain their capacity for purifying themselves and supplying drinking water. For the Baltic, the answer depends on the evaluation scientists will provide regarding natural versus man-made causes of oxygen depletion in the deep water (see Chap. 2.4). For coastal areas of other seas, like the North
Sea and Kattegat, there are, at present, not many arguments in favor of a chemical
elimination of nutrients from sewage. One can, by the way, go one step further and
question whether waste water treatment is at all necessary in coastal situations.
A biological sewage treatment plant reduces the nutrient load in the effluent by
about one third and the amount of biodegradable organic substances, expressed as
BOD, by about 90%. To protect narrow fjords, bays, and estuaries, such treatment
plants are necessary, otherwise anaerobic conditions could spread. To a certain degree
a reduction of pathogenic bacteria and viruses is achieved, too, by the treatment of
waste water. The question, however, has to be discussed whether a waste water treatment plant, which is a rather complicated, costly kind of a factory, is the cheapest and
most efficient technical means if no other effect is wanted but to kill pathogens.
27