Productivity of Land and Water 501 



off from the land, including river and sewage discharge, represents a 

 transfer of nutrients from the terrestrial environment to inland waters 

 and thence to the ocean. Much of the nutrient material is utilized 

 by the marine communit)', but, as we have seen, a portion of it is being 

 permanently removed from the system as inert dissolved compounds, 

 as submarine deposits, and as additions to the reservoir in the ocean 

 abyss. The land is therefore continuously losing nutrient materials 

 to fresh water and finally to the sea, and very little return movement 

 of these materials to the terrestrial environment takes place. 



Relatively few channels exist by which the crucial nutritive sub- 

 stances are being restored to the land. The use of fish, shellfish, and 

 algae as food by land animals and the harvesting of marine products 

 by man for fertilizers and for industrial uses, as well as for food, 

 represent a return movement of materials to the land. A large frac- 

 tion of the world's photosynthesis takes place in the sea. Estimates 

 differ as to whether the average annual growth of the phytoplankton 

 per unit area is as great as that of the land vegetation ( Riley, 1944 ) or 

 somewhat lower (Steeman Nielsen, 1952). Nevertheless, since the 

 oceans cover about 70 per cent of the earth and since phytoplankton 

 exists in all surface waters, perhaps half or more of the primary syn- 

 thesis of organic matter for our globe is carried out by marine plant 

 producers. Yet less than 1 per cent of man's food comes from the 

 marine environment! From the point of view of conservation what- 

 ever increase can be made in man's use of marine resources will help 

 to restore the balance and will represent a gain from the point of view 

 of the inhabitants of the land ( Clarke, 1950 ) . 



In view of the widely varying ecological conditions found in the 

 major environments it is of interest to compare all aspects of their 

 productivities in so far as this is possible. Characteristic differences 

 in the standing crop of various areas have already been discussed. 

 Far more difficult is a comparison of the production rates in view of 

 the complexity of the relations between the various trophic levels in 

 each ecosystem. Very few data exist for comparative purposes in 

 natural environments. In general we may say that the efficiency of 

 plant growth is usually greater than that of animal growth, and the 

 efficiency of animal production is greater in the terrestrial than in the 

 aquatic environment because of the shorter food chains. 



Of particular interest to man is the yield which may be obtained 

 from different types of environment. As we have seen, the magnitude 

 of a sustained yield depends upon the supply of energy and of material 

 and upon the production rates and efficiencies of the various links in 

 the food chain. Table 22 gives some sample values for man's annual 



