O'CONNELL: VARIABILITY OF NEAR-SURFACE PLANKTON 



-^Z^ 



••• r. 





CALIFORNIA 



Q^ 



MEDIAN DENSITY/m' 

 • 1-25 

 # 26-75 



A 76-150 



151 + 



DIEGO 



34° 



-33° 



119° 118° 



Figure 9. — Geographical distribution of chaetognath medians for all blocks on all cruises. 



117° 



with a median above 9000 m'', when the general 

 area median was about 3000 m''. Large co]ie- 

 pods showed a few occurrences of blocks with 

 medians well above 200 when the general area 

 median was 50/'m^ or less. Euphausiids showed 

 occurrences of blocks with medians above 75 m'' 

 when the general area median was between 25 

 and 40/m^. It seems probable, in other words, 

 that high crustacean densities are always present 

 somewhere in the area. At the lower general 

 levels they would be scarce but perhaps as much 

 as three times higher than densities over most 

 of the area. 



The data indicate that higher densities of 

 chaetognaths are most likely to occur near the 

 mainland, but they failed to demonstrate such 

 trends for the three crustacean groups. They 



indicate only that the crustaceans, and dry 

 weight, may sometimes be at higher levels where 

 and when temperatures are relatively low. Re- 

 gressions with temperature show, for example, 

 that the density of small copepods was 6400/m'^ 

 in 15° C water, on the average, as compared with 

 2400, m'' in 19° C water, and that dry weight 

 concentration was 29 mg/m-' in 15° C water, but 

 only 15 mg/m'' in 19° C water. Since variation 

 associated with these trends is wide, it can only 

 be concluded that water of low temperature may 

 sometimes, though not always, contain much 

 higher standing crops of zooplankton than are 

 likely to be found in warmer water in the survey 

 area. 



Dry weight factors were determined for the 

 different species groups because it is impossible 



695 



