Figure b. --Logarithms of zooplankton volumes of 



Hugh M. Smith cruise 16 plotted against 

 the sine value corresponding to the hour 

 of hauling, and showing the calculated 

 regression line for the relationship. 



the ZOO-meter oblique hauls provided a 

 means for approximately removing the 

 diurnal variation in this group of surface 

 samples. This nnethod of adjustment 

 was not considered applicable to the in- 

 termediate samples where there was al- 

 most no day-night difference nor to the 

 deep samples where there may be a 

 difference but in a direction opposite to 

 that at the surface. 



The numerical data show 

 essentially the same diurnal variation in 

 zooplankton abundance as the volume data, 

 although statistically the day-night dif- 

 ference must be considered nonsignifi- 

 cant (since P > 0.05) at all three levels. 

 Table 7 gives the night-day ratios based 

 on numbers of organisms for the total 

 samples and for Copepoda, the major 

 constituent. Although the differences 

 are not significant, the ratios suggest 

 that nnore zooplankton was captured at 

 the surface at night than in the day time, 

 with just the reverse being true for the 

 intermediate and deep levels. 



VARIATIONS WITH DEPTH AND LATITUDE 



The average volumes for the three sample depths, surface (adjusted for hour of haul- 

 ing), intermediate, and deep, were 62.7, 29.2, and 16.6 cc/lOOOm-* respectively (table 6). The 

 corresponding variances were 581.4, 129.9 and 46.2. Since most of the variation in the surface 

 volumes related to hour of sampling had already been removed, the chief sources of variation re- 

 maining are those associated with latitude, depth, and sampling error. 



When the zooplankton volumes (transformed by means of logarithms) are subjected to 

 an analysis of variance with 2-way classification, we find significant differences (P < 0. 05) among 

 stations (latitudes) and highly significant differences (P < 0. 01) related to the depth of sampling. 

 In respect to latitude, the area of best catch, particularly for the surface samples, extended from 

 approximately 2°S. to 8°N. (fig. 7A). About the same degree of variation with latitude is evident 

 at all three depths. Although there was no marked indication of increased abundance immediately 

 at the Equator, the largest single volume was taken at the surface at about 1 S. latitude. 



Table 7. --Average numbers and night-day ratios of total zooplankton and 

 of Copepoda as obtained in the Clarke -Bunnpus hauls of cruise 

 16 for 3 depths of sampling (twilight hauls omitted) 



