Rate of Re ({generation 305 



habitats the return portions of the same currents that carry oxygen 

 down to the deeper levels in lakes and in the ocean bring nutrients 

 up from the richer reservoirs in the lower strata. This upward trans- 

 port of nutrients by wind stirring, eddy conduction, or mass circula- 

 tion which also affect other properties of the water is another mani- 

 festation of the principle of common transport. 



Rate of Regeneration. The rate at which regeneration takes place 

 varies very widely and depends not only upon the speed of decom- 

 position but also upon the rapidity of restoration of nutrients to the 

 growtli zone. The rate of regeneration is rapid in some instances 

 and slow in others. The rapidity with which the processes of decom- 

 position and transformation go forward depends upon temperature, 

 supply of oxygen, and other conditions that influence the industrious- 

 ness of the various kinds of bacteria required. In very cold regions 

 and in poorly aerated soils and muds the chemical steps in regenera- 

 tion take place at a sluggish rate and sometimes come to a standstill. 

 Under these circumstances undecomposed, or partially decomposed, 

 organic matter accumulates and, if time and circumstances are right, 

 deposits of peat, coal, or oil may be formed. 



When decomposition and transformation to inorganic compounds 

 have taken place, the rate at which nutrients are restored to surface 

 layers depends on the physical conditions in the soil and in the water. 

 In oceanic areas of active upwelling of water from deeper layers the 

 regeneration of nutrients may take place as fast as the plants can use 

 them. The extensive regions of permanent upwelling off the west 

 coast of Africa and the west coast of North and South America are 

 characterized by water rich in nutrients, and consequently they sup- 

 port a vigorous plant growth at all seasons. In most temperate waters 

 vertical circulation is retarded or stopped completely during the sum- 

 mer. The deep stirring in temperate seas during the winter period 

 brings a new charge of nutrient-rich water to the surface and gives rise 

 to the expression: "Once a year the sea is plowed." In deep lakes of 

 the temperate region phosphate and nitrate are restored to the euphotic 

 zone during the spring and fall overturns. 



During periods when virtually no vertical circulation takes place 

 plant growth in natural waters must depend upon the nutrients fur- 

 nished by decomposition within the surface layers and by the rela- 

 tively slow eddy conduction from deeper strata— often further retarded 

 by the presence of a thermocline. An adequate rate of replacement 

 is just as necessary as an adequate concentration of nutrients. A 

 single measurement of the phosphate and nitrate in the water might 

 indicate the presence of adequate amounts of these substances, but 



