500 Dynamics of the Ecosystem 



in our information, and the large range in values draws attention to 

 the variability in the abundance of the organisms, in the yield, and in 

 the production rates. It will be observed that an average of 3 million 

 g-cal of radiant energy fall on each square meter of sea surface over 

 the Bank per day and that the energy content of the fish landed per 

 day averages about 5 g-cal per sq m, or an overall efficiency of 0.00015 

 per cent. This very low value is due to the small fraction of the sun's 

 radiation actually absorbed by the plant producers and the losses at 

 each step in the long food chains involved ( Clarke, 1946 ) . 



PRODUCTIVITY OF LAND AND WATER 



Let us conclude our consideration of the ecosystem with a general 

 comparison of the circumstances and efficiencies of its operation on 

 land and in the water. The terrestrial environment clearly has certain 

 advantages. More light is usually available, and large plants are 

 ordinarily much more abundant. For this reason food chains can be 

 shorter with correspondingly greater efficiency of production. Be- 

 cause the soil does not have the mobility of the water medium, 

 nutrients are typically more concentrated, and products of decomposi- 

 tion tend to be retained as a humus near the surface of the ground 

 where they can be immediately used again. On the other hand, 

 the lack of water and the extreme changes in temperature are promi- 

 nent among the disadvantages of the land as a place to live. 



In the littoral zone of the ocean and of lakes, light is sufficiently 

 intense for the growth of both planktonic and benthic plants. Since 

 the water in this zone is continually mixed, nutrients are rapidly re- 

 distributed and food particles are carried in abundance to sessile ani- 

 mals. The disadvantages of the littoral zone include excessive molar 

 action, considerable temperature change, and other special dangers 

 near the water's edge. Beyond the littoral zone the replenishment of 

 nutrients to the surface and of oxygen to the depths must await sea- 

 sonal stirring. In the open sea and in the deepest lakes the euphotic 

 zone represents only a tiny fraction of the entire depth, and effective 

 wind stirring never extends below the permanent thermocline. The 

 result is that nutritive materials sinking into deep water are regen- 

 erated only very slowly. Although organisms inhabiting large water 

 bodies enjoy such advantages as relatively stable temperatures, they 

 are handicapped by a restricted nutrient supply at the surface and by 

 a lack of light at greater depths. 



In relation to the distribution and availability of the critical ma- 

 terials in the major environments of the world we realize that the run- 



