SECT. 4] 



GEOGRAPHIC VARIATIONS IN PRODUCTIVITY 



355 



Woods Hole Oceanographic Institution. As nearly as possible the data are 

 taken from one-degree squares of latitude and longitude, but records from the 

 closest position available have been interpolated liberally. Data for the three 

 southern latitudes represent mid-oceanic positions with corresponding west 

 longitudes of approximately 35°, 38° and 31° from south to north respectively. 

 The data for 60°N originated from the Norwegian Fisheries Investigations and 

 represents temperatures off the west coast of Norway. 



Such data as these do not, of course, provide a reliable index of vertical 

 mixing, surface enrichment and other related phenomena. They do give some 

 indication of the relative stability of the surface waters on a seasonal basis at 

 the four different latitudes. It is clear, for example, that the euphotic zone is 

 thermally stratified throughout the year at both 0° and 20°. At the equator, 



200 



100- 



D J 



S N D 



Fig. 4. The thermal structure of the upper 300 m throughout the year at the following 

 approximate locations: (A) 0°N, 35°W; (B) 20°N, 38°W; (C) 40°N, 31°W; and 

 (D) 61°N, 03°W. 



particularly, the thermocline is strongly developed just at the compensation 

 depth. At the temperate latitude of 40° the upper 100 m shows pronounced 

 stratification only during the summer though a thermal gradient exists for 

 over half the year. From November to April, however, the surface waters are 

 isothermal and potentially wind-mixed to depths below 100 m. Off the 

 Norwegian coast, at 60°, vernal warming is far less pronounced ; surface tem- 

 peratures never exceed 12°C and the surface layers are only weakly stratified 

 in mid-summer. In winter, these waters are isothermal to depths greater 

 than 200 m. 



Although the degree to which the euphotic layer is enriched from below by 



