COMPARISONS OF PLANKTON 



examples will illustrate the evidences of such stratifica- 

 tion more clearly. 



The list for station :;5 shows lo species confined to the 

 surface tow, i^ to the 50-meter tow, and 24 to the 100- 

 meter tow. Forty-seven species, therefore, out of a total 

 of 96 showed definite selection of a single depth to the 

 exclusion of the other two. Thirty other species were 

 found in two of the tows but not in the third one, thus 

 manifesting a similar but less exclusive selection. Fur- 

 thermore, in only 2 instances do we find the abundance 

 records of any species in the two tows exactly the same. 

 In 7 other instances, however, both records are expressed 

 in numerals, with not enough difference between them to 

 warrant any assumption of real preference. Eliminating 

 these 9 records, the other 16 species do manifest a more 

 or less decided preference for one of the two depths over 

 the other. Finally, there were 19 species which were found 

 at all three depths. In 5 instances the abundance record 

 was exactly the same at the three depths. For 9 other 

 species it was the same at two depths and differed in the 

 third. In the remaining 5 species the records show a de- 

 cided preference for one depth over the other two. Of 

 the 96 species taken at this station, therefore, 61S (71 per 

 cent) showed good evidence of selection in the depth at 

 which they were found. 



In the list for station 56, 49 species were restricted to a 

 single depth, 44 were taken at two depths but not at the 

 third, and 15 were present at all three depths. Of the 44 

 two-depth species, 19 showed a definite preference for 

 one depth over the other, and of the 15 three-depth 

 species, 11 distinctly favored one of the depths in prefer- 

 ence to the other two. Here, then, 79 species (7^ per cent) 

 manifested a choice of depth. 



At station 51, 39 of 50 species were confined to a single 

 depth; 10 others were present at two depths, with 6 of 

 them preferring one of the depths to the other. A single 

 species was found at all three depths, with the same 

 record for two of them and a much smaller one for the 

 third. Here, then, 45 species (90 per cent) showed selec- 

 tive stratification. 



Every one of the lists gives evidence of similar stratifi- 

 cation of the copepods captured at the station. The num- 

 ber of species manifesting an exclusive preference for one 

 depth only and those that were present at all three depths 

 arc recorded in the remarks accompanying each station 

 list. The former constitute from 50 to 90 per cent of the 

 species total in three-fourths of the station lists, and in 

 the other fourth they fall below 40 per cent only twice. 

 The species present at all depths vary from o to 30 per 

 cent of the species total, with an average of about 15 per 

 cent. Among these species the number of instances in 

 which the abundance record is the same for all three 

 depths is so small as to be practically negligible. Sum- 



ming up the evidence here presented, we find that almost 

 every species in each of the station lists shows some 

 degree of preference as to depth. 



Causes of Daytime Stratification 



We naturally look for the cause of this universal day- 

 time stratification within the upper 100 meters of the 

 ocean. Is it the salinity, the temperature, the density, the 

 hydrogen-ion concentration, any combination of two or 

 more of these factors, or the united effect of all of them? 

 Or may it be something outside, more powerful than any 

 of them, or even than all of them combined? Since we 

 have the necessary data in connection with the station 

 lists, some answer to these questions ought to be possible. 

 Let us consider the data separately, beginning with tem- 

 perature. 



Temperature 



There is no doubt that temperature does exert con- 

 siderable influence on copepod distribution. Is it strong 

 enough to produce such a universal stratification? And 

 if so, is there sufficient difference in temperature between 

 the three depths at which the tows were taken to warrant 

 its selection as the principal cause? At first it would seem 

 as if the answer might be in the affirmative. 



The greatest variations in temperature of the ocean 

 water are nearest the surface. The upper 100 meters, 

 within which all but one of the Carnegie tows were 

 taken, show the maximum differences between the 

 various regions of an ocean — polar, temperate, and trop- 

 ical. Here also are found the maximum differences in 

 temperature between the vertical depths o, 50, and 100 

 meters. Below 100 meters there is a gradual approach to 

 uniformity of temperature over the entire area of all the 

 oceans. 



Variations in temperature are undoubtedly one of the 

 causes of the marked differences in the copepod species 

 that make up the regional plankton of the upper 100 

 meters. The Carnegie stations in the Pacific ranged from 

 52° north latitude through the tropics to 40° south lati- 

 tude. The low temperatures of the upper 100 meters 

 north of parallel 40° north entirely exclude many of the 

 copepods that are common in the tropics. The number 

 of species taken at each of the 12 stations in that part of 

 the Pacific was thus reduced to an average of only 16. A 

 comparison of this average of 16 with the average of 83 

 for stations 35 to 44 in the tropics suggests the powerful 

 influence of temperature on regional distribution. Is its 

 influence on vertical distribution equally great? These 

 few species at the northern stations showed the same 

 evidence of stratification as the more numerous species in 

 the tropics. That this stratification is not due to the ver- 



