306 



Nutrients 



in view of the rapidity with which the nutrients are used by a grow- 

 ing plant population, the supply of nitrate and /or of phosphate will 

 soon be exhausted unless regeneration keeps pace with utilization. 

 Quantitative data on the rates of decomposition, regeneration, and 

 assimilation of phosphate by phytoplankton are available from con- 

 trolled experiments in outdoor tanks (Pratt, 1950) in addition to lab- 

 oratory tests (Rice, 1949). Analysis of the seasonal cycle of phos- 

 phate in the Gulf of Maine by Redfield, Smith, and Ketchum (1937) 

 gives the following approximate estimates for the fractions of this 

 nutrient assimilated by the phytoplankton from the various sources 

 of supply: 2 per cent from the original inorganic phosphate in the 

 surface water layer, 73 per cent from vertical transport, and 25 per 

 cent from decomposition and animal excretion within the surface 

 layer (Ketchum, 1947). Similar experiments in the terrestrial en- 

 vironment have been carried out principally in relation to cultivation 

 of crops, and relatively little is known about the rate of regeneration 

 in wild areas. For further information on this subject for cultivated 

 areas the reader may consult the Yearbooks of the United States De- 

 partment of Agriculture. 



Ratio of Regenerated Materials. The proportions of the materials 

 provided by the regeneration process are limited by the relative abun- 



600 



500 



'400 



^300 



200 



100 



Atlantic 



9m- 



50 100 



200 



50 100 



Phosphate (Pa O5) 



Fig. 8.9. Correlation between concentrations of nitrate and phosphate in waters 



of the Atlantic, Indian, and Pacific Oceans expressed in mimgrams per cubic meter 



( Redfield, 1934, Copyright, Univ. Press of Liverpool. ) 



