COMPARISONS OF PLANKTON 



also some of the usiuil instances of a northward trans- 

 portation of species whose normal habitat was farther to 

 the south. The course of the Carnegie in the Athmtic did 

 not come in contact with other surface currents, and con- 

 sequently the station lists of copepods cannot give us 

 more than this minimum of evidence with reference to 

 the influence of the currents on the plankton. A similar 

 relation between the Gulf Stream and the volume of the 

 plankton was shown by Jespersen (1926) in his discus- 

 sion of the quantity of macroplankton in the North 

 Atlantic, but species were not mentioned. 



In the Pacific Ocean, however, conditions are very 

 dilTerent. In one of the papers already published by the 

 Carnegie Institution of Washington, Sverdrup has made 

 use of the map of the surface currents of the Pacific 

 which is here reproduced (map 2). This map shows an 

 approximately symmetrical surface circulation north and 

 south of the thermal equator, which is a few degrees 

 north of the geographic equator. In the South Pacific the 

 west-wind drift generates a current flowing east and 

 striking the western coast of South America, where it is 

 deflected to the north along the coast as the Peruvian 

 Current. At the equator this current is turned back west- 

 ward across the central Pacific as the South Equatorial 

 Current. A part of it flows directly west to the East 

 Indies, another part flows southwest to Australia. 



In the North Pacific the west-wind drift forms the 

 Japan Current, which flows eastward and, striking the 

 western coast of North America, is deflected to the south 

 as the California Current. This follows the coast to the 

 southern boundary of Mexico, where it is turned west- 

 ward as the North Et]uatorial Current; this flows directly 

 west to the Philippine Islands, where it joins the Japan 



Current. Between the two equatorial currents, both of 

 which flow west, is a countercurrent flowing east along 

 the thermal equator and striking the western coast of 

 Central America. Here it divides, one half turning sf)Uth 

 and then west and joining the South Equatorial Current, 

 the other half turning north and west and joining the 

 North Equatorial Current. 



The course of the Carnegie followed part of each of 

 these currents. Stations 35 to 40 were in the half of the 

 eastern end of the Counter Equatorial Current that turns 

 south and west. Stations 40 to (So were within or close to 

 the Peruvian Current. Stations 93 to 98 were in the 

 South Equatorial Current. Stations 102 to no were in the 

 North Equatorial Current. Stations in to 115 were in 

 the Japan Current. Stations 12S to 135 were in the Cali- 

 fornia Current. Stations 150 to 160 crossed the North and 

 South Equatorial and the Counter Equatorial currents. 



The above stations in every instance were the ones that 

 yielded the greatest variety of copepod species and the 

 largest number of individuals. There must be something, 

 therefore, in connection with these currents which is con- 

 ducive to copepod life and development. 



Sverdrup has plainly shown a combination of low 

 salinity and low temperature near the surface in the 

 Peruvian Current, and this combination appears to a 

 greater or lesser degree in each of the other localities 

 mentioned above. We cannot escape the conclusion that 

 such a combination is favorable to the copepods, or at 

 least not antagonistic. There is also the probability of 

 better aeration of the water in these currents than outside 

 of them, and the possibility that they contain a richer and 

 more uniform supply of food than can be found in water 

 that does not flow. 



COMPARISONS OF PLANKTON 



Since all the regular tows were made at the same time 

 of day, by the same persons, using the same nets, and 

 employing the same methods, the species lists will be as 

 free from diversity as they could possibly be. If we com- 

 bine with these lists of species the supplementary data 

 mentioned above, we find ourselves in possession of an 

 accurate and really scientific basis for a comparison of 

 the copepod plankton of the two oceans and of the dif- 

 ferent parts- of each of them. 



Comparison' of the Two Oceans 



The Carnegie towings show that the Pacific plankton 

 is much richer in copepod species than that of the 

 Atlantic over those parts of the two oceans covered by 

 the cruise. For instance, station ^4, the last one in the 



Atlantic, may be compared with station 35, the first one 

 in the Pacific. These two stations are about equally distant 

 from the two ends of the Panama Canal; the bottom 

 depths are each over 3500 meters, with a difference of 

 only 47 meters. The Atlantic station yielded 48 species; 

 the Pacific station yielded 96 species. On comparing sta- 

 tions 15 to 25, down through the center of the North 

 Atlantic, with stations 1^0 to i :;5 and 14S to 152 at cor- 

 responding latitudes in the Pacific, we get a total of 394 

 species for the Atlantic stations with an average of 56 per 

 station, and 661 species for the Pacific stations with an 

 average of 66 per station. Unfortunately the total volume 

 of the plankton is not recorded for the Atlantic stations, 

 but in view of the great inferiority in number of species 

 it is reasonable to assume that it was much less than that 

 at the Pacific stations. 



