O'CONNELL: VARIABILITY OF NEAR-SL'RFACE PLANKTON 



in 



z 



CHftETOGNftTHS 



SMALL COPEPODS/IOO 



LARGE COPEPODS 



EUPHAUSMOS 



06 10 14 18 22 02 06 



HOUR 



Figure 6. — The relation of median density to time of day 

 for four species groups. The points are medians for 

 block densities in 4-hr intervals. The small copepod 

 medians were divided by 100 to put them on the same 

 scale as the others. 



Table 6. — Rank difference correlation coefficients for 

 median block densities of four species groups for the 

 night periods of five cruises. 



* p = 0.05. 

 •• p = 0.01. 



the parallel patterns of diurnal change in the 

 crustacean groups. 



The coefficients for each of the six species 

 group combinations varied widel.y among the 

 five cruises, with only four of the entire 30 co- 

 efficients indicating significant correlations. It 

 appears that, while occasional correlations can 

 be expected to occur over the area, consistent 

 trends of association in density do not occur 

 among these four species groups near the surface 

 at night. 



DRY WEIGHT VARIATION 



Dry weight concentrations are summarized 

 by cruise period in Table 7 and by weight class 

 for all cruises in Figure 7. Low and high values 

 occur both night and day, but there is clearly 

 a shift to higher values at night. 



The sample concentrations may underestimate 

 the true concentrations by as much as 15 or 20%. 

 Ahlstrom and Thrailkill (1963) showed that for 

 copepods dry weight decreased about 15% after 

 Formalin preservation. Lasker (1966) showed 

 that dry weight of euphausiids was about 35% 



15 25 35 45 55 



DRY WEIGHT (mg/m') 



Figure 7. — Dry weight frequency distribution of all 

 sampling blocks. The wide bars .show day frequencies 

 and the narrow bars night frequencies. 



Table 7. — Summary of sample dry weight concentration (mg/m^) by cruise and day (D) or night (N) period. 



691 



