248 THE SEAS 



As a result of the formation of this discontinuity layer 

 we find the clue to the poor plant -life during the summer 

 giving rise to a fresh outburst in the autumn. For while all 

 the nutrient salts have become used up in the surface 

 layers a fresh supply is being formed in the deeper layers 

 from the sinking down of dead and dying animals and from 

 other organic remains. But owing to the inability of the 

 upper and lower layers to mix, this supply of food substance 

 is completely cut off from those plants living in the surface 

 waters above the discontinuity layer where the light 

 conditions are best for growth. In the autumn, when the 

 surface layers begin to cool down, the waters become once 

 more of the same temperature from top to bottom. All the 

 water masses can now be mixed and this is soon brought 

 about by the autumnal gales. As a result fresh supplies 

 are brought up into the surface layers where there is still 

 sufficient light for the plants to grow actively and a renewed 

 flowering of the diatoms takes place. Soon, however, 

 with the onset of winter, the sun's light becomes too weak 

 and the plants die down until they are wakened once more 

 into life in the following spring. 



From the changes in the amount of phosphates in the 

 waters of the English Channel, off Plymouth, a minimum 

 estimate has been made of the actual extent of this diatom 

 crop during the year. Assuming a depth of thirty-eight 

 fathoms the minimum annual yield of diatoms would be as 

 much as five and a half tons (wet weight) in the water 

 layers lying beneath one acre of the surface. It is interest- 

 ing to compare this with the quantity of crops obtained on 

 land. For instance, between the years 1895 to 1904 the 

 average annual yield of potatoes was 4*84 tons per acre, 

 the highest value for any one year being 5-81 tons. Over 

 the same period that for turnips and swedes was 13*21 tons 

 per acre. 



