OCONNELL: VARIABILITY OF NEAR-SURFACE PLANKTON 



the species is 1.0 to 15.5 mm in lenj>-th ( Alverino, 

 1961), and the samples contained individuals 

 from 2.0 to 14.5 mm long. 



VARIATION BETWEEN CRUISES 



Table 2 shows characteristics of the frequency 

 distributions of all 1.6-km samples by cruise and 

 period of the day for the four species groups. 

 Median densities are lower than mean densities 

 in every instance but one, the nighttime period 

 for small copepods on cruise 5, indicating that 

 distributions for virtually all arrays show some 

 degree of positive skewness. Differences be- 

 tween cruises are described in terms of the me- 

 dians to avoid undue effects of extreme values 

 that can arise in moment measures on nonnormal 

 populations. 



Nighttime median densities are higher than 

 daytime medians on all cruises for small cope- 

 pods and euphausiids, and on all but the fifth 

 cruise for large copepods. However, variation 

 was such that day and night medians differed 

 significantly (p<0.05, Tate and Clelland, 1957) 

 in only one instance for small copepods, three 

 instances for large copepods, and two instances 

 for euphausiids. Though none of these groups 

 can be considered to show consistently higher 

 densities at night for the area as a whole, real 

 differences occurred more often for the large 

 copepods and euphausiids than for the small 

 copepods. Day and night median densities for 

 chaetognaths do not differ significantly for any 

 of the cruises. 



Daytime median densities do not differ sig- 

 nificantly between cruises for chaetognaths or 

 euphausiids, but those for small copepods and 

 large copepods differ significantly (?j<0.05) for 

 about half the comparisons. Nighttime medians 

 (Figure 3) show a pattern similar to the day- 

 time sets for the two copepod groups but show, 

 in addition, a number of significant differences 

 between cruises for the euphausiids and one dif- 

 ference for chaetognaths. Median densities of 

 the small copepods were significantly higher for 



Figure 3. — The nighttime cruise medians for four species 

 groups shown by cruise date. Vertical bars indicate the 

 95% confidence intervals. 



60 



30 



1 NOV. 1 DEC. 



1 DEC. 



1 OCT. 1 NOV. 



CHAETOGNATHS 

 1962 



1 DEC. 



1 OCT 



1 NOV. 

 MONTH 



1 DEC. 



685 



