ZONES OF THE SEA-WHERE THEY LIVE 



13 



less dense, rises to the surface, and flows northward. At the polar regions, it 

 cools, sinks and flows very slowly back to the equator at the rate of a mile a 

 day. In warm seas, water evaporates rapidly and becomes denser due to in- 

 creased salinity. In ihe Sargasso Sea, for instance, water sinks due to increased 

 salinity. Tropical rains have an eflfect in diluting water and making it less dense. 

 These interplaying factors vary so as to produce quite complex results, but 

 temperature has generally the greatest effect in the oceans. Thus, circulation 

 is mostly toward the poles on the surface and toward the equator in deep water. 

 UpwelHng is a special and very important vertical movement caused mosdy by 

 winds. The principal areas of upwelling in North America are off the coast 

 of Newfoundland and widely along the southwest coasts of the United States 

 and northern Mexico. On the lee coasts of prevailing winds, water is blown 

 away from the shore and must be replaced by upwelling water, floating up 



Fig. 3. Currents of North American seas. The names of the curreyits are indicated 

 on the map. 



from the depths. Conversely, windward coasts accumulate surface water which 

 sinks. This is reflected biologically by the abundant coral forests in the Caribbean 

 where warm water accumulates, and the comparatively depauperate coral fauna 

 of western Mexico where cool water wells up from below. However, these 

 upwelling seas compensate for lack of coral by possessing a great abundance 

 of plant and animal plankton, and very rich oceanic fisheries, largely com- 

 posed of schooling fishes, become established as a result. In fact, areas of 

 upwelling show the greatest abundance of life to be found anywhere on earth. 

 The champion of all upwelling areas seems to be off the coast of Peru and 

 northern Chile. 



There are other causes of upwelling besides winds. These are storms, tides, 

 and the passing of two currents of unequal density (when this occurs, one sinks 



