SECT. 4] 



GEOGRAPHIC VARIATIONS IN PRODUCTIVITY 



353 



and light penetration for an incident radiation of 400 g cals/cm 2 /day from the 

 equations of Ryther and Yentsch (1957). 



According to Table I, by the time a phytoplankton crop of 2 g/m 3 has 

 developed, the euphotic zone is limited to 3.5 m and the nitrogen in this water 

 is exhausted. The same situation is depicted in Fig. 3, showing the maximum 

 standing crop which can develop at each depth before the light becomes 

 extinguished or the nitrogen consumed. It is obvious that only at the very 

 surface can dense populations develop. Even if all the organisms produced 

 were able to persist in the water column after the light had been shut out from 



0- 



Fig. 



1000 2000 



g dry wt./m 3 



3. The population of phytoplankton, expressed as g dry wt./m 3 , which can develop 

 at different depths within a water column under conditions stipulated in the text. 



above, the maximum amount of organic matter which could accrue beneath 

 a square meter of ocean surface (obtained by integrating the curve in Fig. 3) 

 is no more than about 25 g — less than one-thousandth the potential standing 

 crop on land. 



In terms of the population of plants which may exist at any time, then, the 

 sea is a desert compared to moderately productive land. Furthermore, although 

 the rate of phytoplankton production in the sea may momentarily approach or 

 equal the highest observed rates on land (Ryther, 1959), these high rates may 

 be maintained in a given body of water for no more than a matter of days in 

 the sea in contrast to years on land. High levels of marine production may be 

 maintained only by the constant replacement of the water in which the or- 

 ganisms have grown with a new supply of nutrient-rich water. In other words, 

 high production may persist only in terms of a geographical location through 

 which there flows a continuous supply of new, nutrient-rich water. 



Upwelling of rich water from intermediate depths to the surface does occur 

 more or less continuously in certain restricted areas. Over most of the ocean, 



