NO. 4 PLANKTONIC FORAM IXIF ERA CIFELLI 3 



waters of the North Atlantic. Separating these water bodies is the 

 Gulf Stream, a well-defined band of currents originating in the straits 

 of Florida that moves clockwise and can be traced as far north and 

 east as the Grand Banks. Significant short- and long-period variations 

 occur in the velocity and position of the Gulf Stream, but throughout 

 the year it forms a sharp pressure gradient between the slope waters 

 and the Sargasso Sea (Iselin, 1936). Because of this pressure 

 gradient the slope waters and the Sargasso Sea maintain their more or 

 less unique hydrographic properties. 



The waters in the Sargasso Sea are warm, and surface temperatures 

 remain relatively uniform throughout the year, ranging from about 

 19° C. to 27° C. In contrast, the surface temperatures of the shelf and 

 slope waters range from about 12° C. in March, the coldest month, 

 to about 26° C. in September, the warmest month (Stommel, 1958, 

 fig. 5). During the warmer months there is a pronounced stratified 

 surficial layer in the Sargasso Sea with a summer thermocline de- 

 veloping between 40 and 70 meters (Cifelli, 1962, figs. 3, 6). In the 

 slope and shelf waters the summer thermocline develops between 10 

 and 40 meters, but there is relatively little surficial stratification. The 

 summer warming afifects mostly the upper 100 meters ; below that 

 depth there is little change, and at 200 meters the temperature remains 

 relatively constant throughout the year, which is generally between 

 9° and 12° C. in the slope waters and between 19° and 21° C. in the 

 Sargasso Sea. Lateral variations in temperature are more common 

 in the slope waters than in the Sargasso Sea and, moreover, subsur- 

 face temperature inversions occur in the former (Grice and Hart, 

 1962, p. 289). 



The Sargasso Sea is more saline than the shelf and slope waters. 

 In the Sargasso Sea salinities are between 36-37 o/oo, while those in 

 the slope waters normally range between 34-36 o/oo. The shelf waters 

 tend to be even less saline, with salinities as low as 25-31 o/oo having 

 been recorded at the location of the inshore shelf stations (Grice and 

 Hart, 1962, p. 289). 



The hydrographic differences between the shelf slope waters and 

 the Sargasso Sea are reflected in the composition of their planktonic 

 foramini feral faunas. In the vicinity of the Gulf Stream is located 

 the boundary between the temperate and the subtropical faunas. 



DISTRIBUTION 



Tables 1-4 show the percentage of total population at the stations 

 sampled expressed in number of specimens per half-hour tow. 



The principal features of the distribution and the faunal changes 

 observed in the traverses studied as ascertained from these tables 



