ZOOPLANKTON OF CENTRAL PACIFIC 



375 



2° 0° Z° 



S — LATITUDE — N 



Fku'rk 7. — Vertical tcmporature section (adapted from C'roniwell and Austin, 1054, figure 28) based on bathythermograph 

 observations along 172° W. longitude, made February 27-March 12, li)51, on Hugh M. Smith cruise 8, showing the 

 boundaries of the 6 areas u.sed in this study in relating variations in zooplankton abundance to particular features of 

 the equatorial current systems. 



tlio iiortlipni limit of our sampling to the northern 

 boiiiularv of the Countercurrent, a region of 

 relatively shallow thermocliiie; (2) the Counter- 

 eurrent with its boundaries being determined at 

 the time of each crossing from vertical temperature 

 sections, a region with shallow thermodine to the 

 north, deepening to the south; (3) a zone of 

 convergence in the South Equatorial Current 

 extending (according to our definition) from the 

 soutiiern boundary of the Countercurrent to 1^° 

 X. latitude, a region of deep thermodine; (4) a 

 zone of divergence and upwelling in the South 

 Equatorial Current along the Equator from U$° 

 X. to 1)2° S. latitude, evidenced by a doming of the 

 isotherms, a reduction in surface temperatiu-e, and 

 an increase in surface inorganic phosphate; (5) 

 the South Equatorial Current from 1K° S. to 5° S. 

 latitude, a region of deep thermodine; and (6) 

 the South E{|uatorial Current from 5° S. latitude to 

 the southern limit of our sampling (about 14° S.), 

 a region of shoaling thermodine to the south. 

 Figure 7 shows the boimdaries of these six areas 

 superimposed on a vertical temperature section 

 based on bathythermograph observations along 

 172° W. longitude. 



When the zooplankton volumes, adjusted for the 

 day-niglit variation but disregarding differences 

 related to longitude and season, are combined 

 according to these natural divisions of tlie current 

 system, we obtain the distril)ution siiown in 



figure' 8, with the greatest concentration of 

 zooplankton occurring at the Equator (1K° X". to 

 1K° S.) in the region of divergence. Average 

 volumes for the areas just north of the Ecjuator, 

 i. e., the convergent zone and the Countercurrent 

 were considerably higher than those for the cor- 

 responding areas south of the Equator. The 

 X'orth Equatorial Current and the South Equa- 

 torial Current at the southern extent of our 

 sampling were equally poor in zooplankton. 

 From an analvsis of variance we conclude that the 



2 

 o 



< 20 



SEC 



(331 



ISECIDIV|C0NV| CO ' 



"I — r 



(661 I (59) I (87) 



NEC 



(65) 



14° 12° 10° 8° 



I I 



_Li_ 



go go iQO igo 140 150 igo 



S — LATITUDE — N 



FiGiRE 8. — Variations with the current system in 

 zooplankton volumes (adjusted) for longitudes 120° W. 

 to 180°, with the limits of the 0.!».5 fiducial interval 

 shown for each mean. (The number of samples for 

 each area is indicated in parentheses]. 



