OCONNELL: VARIABILITY OF NEAR-SURFACE PLANKTON 



RELATION OF RANGE TO MEDIAN 

 IN SAMPLING BLOCKS 



Within block variation is indicated for eacli 

 species group and for all species groups together 

 by regression log R = a + b log M (Table 4), 

 where R = block range and M = block median. 

 The slopes, b. for the five equations do not differ 

 significantly from each other (p = 0.05). All 

 are significantly greater than (p = 0.01), and 

 all but that for small copepods are significantly 

 less than 1.0 (/) = 0.05). The intercepts are 

 all significantly greater than 0, again except for 

 the small copepods, but they do not diflfer sig- 

 nificantly from each other (p = 0.05). In view 



Table 4. — Estimated coefficients for regressions of log 

 block range on log block median for each species group 

 and all species groups combined." 



Species group 



^ N = number of sampling blocks; 

 = stondord deviation about the line; 

 " p = 0.01. 



= intercept; b = slope; 

 correlation coefficient. 



of the similarities, the regression for all groups 

 combined (Figure 5) is a satisfactory descrip- 

 tion of the average relation of block range to 

 block median for each of the species groups. 



The regression for all groups combined indi- 

 cates that average block range increases with 

 block median but not proportionately. The ex- 

 pected ranges for difl["erent medians are: 



1,000 10,000 

 562 3,548 



Median 1 10 50 100 

 Range 2 14 51 89 



Thus range will tend to be greater than the me- 

 dian when the latter is below 50, but appreciably 

 less than the median when the median is above 

 100. Small copepods are the only group with 

 consistently high medians, and they also have 

 the greatest standard deviation about the line. 

 This suggests that, while the ratio of range to 

 median is lower, on the average, for this group, 

 it also tends to be more variable than for the 

 other groups. 



The relation of block range to the total varia- 

 tion within the area is suggested by the mag- 

 nitude of block ranges relative to the class in- 



tervals of the frequency distributions shown 

 earlier (Table 5) . In the case of small copepods, 

 block ranges only slightly exceed the size of class 

 intervals for the two highest classes in the distri- 

 bution. For the other three species groups, block 

 ranges exceed the class intervals for all but the 

 lowest class. Ranges tend to be spread over 

 three or more class intervals for the upper halves 

 of the euphausiid and chaetognath distributions. 

 Even though the highest classes tend to be rare, 

 it can be seen that ranges extending over two 

 or more class intervals would not be uncommon 

 for large copepods, euphausiids, and chaeto- 

 gnaths. 



Table 5. — The number of class intervals encompassed 

 by the range for the midpoint of each class interval in 

 the frequency distribution for each species group. The 

 0-1 class is excluded and other classes are numbered con- 

 secutively. 



Frequency 

 class 



Small 

 copepods 



Large 

 copepods 



Euphousiids 



Chaetognaths 



0.1 

 0.3 

 0.9 



1.2 

 1.5 



0.6 

 1.4 



2 1 

 28 



3.4 



0.7 

 1.6 

 2,4 

 3.2 

 3.9 

 4.6 

 5.2 



0,6 

 1.4 

 2 1 

 2.8 

 3.4 

 4.0 

 4.6 



DIURNAL VARIATION 



It is evident from the differences in day and 

 night sets of data that density level near the 

 surface is influenced by diurnal vertical move- 

 ments, pai-ticularly for the larger crustacean 

 groups. The pattern of change is indicated for 

 each of the species groups by medians for 4-hr 

 time intervals (Figure 6). A sequence of change 

 is most apparent for the large copepods and 

 euphausiids, the lowest medians occurring be- 

 tween 1000 and 1400 hr and the highest between 

 1800 and 0200 hr. The increase in the evening 

 appears to be more rapid than the decrease in 

 the morning for both groups. 



The small copepods show a pattern similar to 

 that for large copepods, but much weaker. The 

 highest time interval median is almost 10 times 

 the lowest for large copepods but less than 2 

 times the lowest for small copepods. 



Chaetognaths show only slight evidence of 

 diurnal change. The peak between 0600 and 



689 



