SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 



65 



above the critical level at which it can flow across the sill, and its escape is 

 unimpeded by stability. 



d. Upwelling 



Where persistent winds, such as the Trades, blow the surface water off- 

 shore sub-surface water upwells to replace it. Upwelling takes place off the 

 western coasts of the continents in the trade-wind zones. It is recognized by the 

 anomalous coolness of the surface waters, their high nutrient content and 

 the abundance of life. 



A small-scale example of upwelling, and its influence on the distribution of 

 the components of sea-water, is found in the outer part of the Gulf of Venezuela. 

 During the winter season, when the northeast trade winds blow persistently, 

 upwelling occurs in the lee of the peninsula of Paraguana, which forms the 

 eastern boundary of the Gulf. Fig. 14 shows that along a section extending 



Fig. 14. Distribution of salinity (S°/ 00 ), total phosphorus (P( mg atoms/m 3 ), and oxygen 

 (O2 ml/1.) in a section across Gulf of Venezuela. Ordinate, depths in meters. (After 

 Redfield, 1955.) 



westward from the peninsula the isohalines slope upward as the coast is ap- 

 proached. This indicates that the more saline water at depth is being drawn 

 landward and upward by the upwelling process to replace the surface water 

 being blown offshore. The phosphorus and oxygen isolines slope in much the 

 same way but more markedly. The concentrations of phosphorus are greater 

 and those of oxygen less in deep water close to the coast than anywhere off- 

 shore. Apparently there is an accumulation of phosphorus in the deep water as 



