If fish farming in enclosed bodies of seawater presents difficulties, why not simply raise the productivity of the open sea by enriching the surface waters over wide areas by the addition of plant nutrients? We spread fertilizers on the land to promote the growth of crops; why not spread them on the sea?
To produce any appreciable increase in concentration of plant nutrients in the open sea would require enormous quantities of fertilizer, and the costs would be tremendous. It would be an extremely wasteful process because so small a proportion of the nutrients absorbed by phytoplankton eventually becomes incorporated in fish flesh. Any additional plant growth obtained as a result of fertilization would contribute very largely to the production of unwanted organisms. Calculations of the increase of yield from sea fisheries that might be obtained by large-scale fertilization of seawater do not stand up to comparison with those known to be obtained from the use of equal quantities of fertilizer applied to the land.
None the less, there are unintentional processes of artificial enrichment of seawater going on in some areas, though not with the deliberate aim of benefiting fisheries. Close to large centres of population, great quantities of sewage are discharged into the sea, which decompose to provide nutrients for marine plant growth. For example, the fertility of the southern North Sea is augmented by the outflow of London's sewage via the Thames. It may be doubted, however, whether this form of sewage disposal is really in our long-term interests. Present-day problems with coastal pollution from sewage are currently in the limelight and are described in Chapter 10. In 1948, Dr L.H.N. Cooper, chemist at the Plymouth Laboratory, wrote:
As matters now stand, very large amounts of nutrients are being poured into the sea, the great sink, as sewage from coastal towns and by way of the rivers from inland towns and farms fertilized and unfertilized. Phosphorus is a very precious commodity which in not so many years will become very scarce. The scale on which phosphorus even now is being dissipated to the sea is more than the world can afford. In years to come the cry will be for more methods for recovering phosphorus from the sea, not for putting it in.
Other measures for raising the productivity of seawater have been proposed which do not involve the addition of valuable fertilizers. The seas already contain vast reserves of plant nutrients in deep water, and it might be possible to devise means of bringing these to the surface. Any measures which increase the mixing of surface and deep water would be likely to lead to increased production. For example, it has been suggested that upwelling could be brought about artificially by sinking atomic reactors in deep water to generate heat and cause convection currents to carry the nutrient-rich deep water to the surface. The Japanese have already created some artificial reefs and breakwaters to redirect coastal currents and create artificial upwelling on a smale scale.
In low latitudes where warm, high-salinity layers overlie water which is much colder and of lower salinity, the temperature and density differences above and below the thermocline can power the raising of deep water to the surface. Water drawn up a pipe from a depth of 1 km or more becomes warmed by conduction as it rises, and the density is correspondingly reduced, so that the system once started continues to provide its own energy for raising the water as long as the temperature and salinity differences between the two ends of the pipe persist.
In some areas it might be possible to alter the natural circulation in ways that could lead to a better supply of nutrients at the surface, or a higher water temperature. Digging out the Strait of Gibraltar to a greater depth has been advocated as a means of raising the productivity of the Mediterranean by allowing the entry of deeper levels of water from the Atlantic. It has also been predicted that a barrage across the Strait of Dover which allowed only a one-way flow of water from the Channel to the North Sea would reduce the entry of colder water into the northern part of the North Sea, and gradually raise the North Sea temperature, thereby promoting higher productivity.
None of these ideas is ever likely to become a practical proposition. The costs would be prohibitive and the long-term consequences would be difficult to predict. There would also most likely be strong opposition from conservationists. However, there are some methods which may work and which are being experimented with today. One of the most promising is briefly described here.
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