302 



Nutrients 



decompose. Under diese circumstances, when the phosphate and 

 nitrate are released in form suitable for absorption by green plants, 

 they will be at levels where light is too weak for photosynthesis. 

 These nutrients cannot be used again for plant growth until they have 

 been returned to the euphotic zone by vertical currents and mixing, 

 as will be discussed in the next section. A cycle thus exists involving 

 this movement of nutrient materials to the deeper parts of water 

 bodies and the restoration of the decomposition products to the upper 



1000 



- 2000 - 



3000- 



4000 



1.0 2.0 3.0 



PO^P, Mg-atoms/L 

 Fig. 8.8. Vertical distribution of phosphate in the Atlantic, Pacific, and Indian 

 Oceans. (From The Oceans by Sverdrup et al., 1942, Copyright, Prentice-Hall, 



N. Y.) 



layers by vertical transport. In deep lakes and in the ocean the con- 

 centration of nutrient salts is much greater in the lower strata than it 

 is near the surface, as may be illustrated by the condition in the 

 Pacific (Fig. 8.8), The rates of upward and downward transport for 

 the year as a whole are presumably in approximate balance, and the 

 cycle tends toward a steady state. However, mineral salts are con- 



