66 



REDFIELD, KETCHUM AND RICHARDS 



[CHAP. 2 



it moves toward the site of upwelling, as the result of the sinking of organized 

 matter from the surface layers as they move offshore. In support of this, the 

 oxygen is diminished in about the proportions required for the decomposition 

 of organic matter. 



The classic examples of upwelling are found along the eastern boundaries of 

 the oceans in the trade-wind belt. The distribution of total phosphorus in a 

 section crossing the Benguela current off the western coast of Africa, where up- 

 welling occurs, is shown in Fig. 15. At all depths the concentrations of phos- 



200 



400 



600 



800 — 



1000 



Fig. 15. Distribution of total phosphorus and salinity in a section along 24th parallel off 

 west coast of Africa. Solid contours, total phosphorus, mg atoms/m 3 ; broken con- 

 tours, salinity, %„. Depths in meters. (Based on Crawford Stations 450-458, October, 

 1958.) 



phorus increase markedly as the coast is approached. If this enrichment were 

 due entirely to the movement of deep water into the upper layers, the isohalines 

 should parallel the phosphorus isopleths, which is not the case. The fact that 

 along any isohaline the phosphorus increases markedly toward the coast 

 indicates that some process other than the physical motions of the water 

 leads to an accumulation of this element in the upwelling area. 



Upwelling can only be maintained by a flow of deep water toward the site 

 of upwelling to replace the surface water moved offshore by the wind. Off the 

 coasts of California (Sverdrup and Fleming, 1941) and Peru (Gunther, 1936) 

 counter currents are present in the sub -surface waters during the periods of 

 upwelling. It seems probable that nutrients accumulate in areas of upwelling 

 because of the differential advection of water at different depths and in ac- 

 cordance with the principles which apply in estuaries. This accumulation must 

 contribute substantially to the fertility of the water in such areas. 



D. Dynamic Equilibria in Three Dimensions 



In the cases discussed above special considerations have justified a simplifica- 

 tion of the problem by the assumption that exchanges in one or two dimensions 

 could be neglected. Such assumptions are not justified when the distribution 

 of properties in large bodies of water are under consideration. The treatment in 



