ZOOPLANKTON OF CENTRAL PACITIC 



377 



differences among subdivisions of the current 

 system are highly significant (F = 4.08 P<0.01). 

 The degree of overlap in the 0.95 fiducial intervals 

 of the means is shown in figure 8. 



The asymmetrical distribution of the zooplank- 

 ton in respect to the Equator results, we believe, 

 from the prevalence of the southeast trades during 

 most of the year. The occurrence of the zooplank- 

 ton peak at the site of the divergence in an area 

 of newly upwelled water, rather than in "older" 

 water to the north or south of the Equator, is 

 somewhat surprising and may be evidence that, 

 on the average, the northward and southward 

 components in the westerly surface current at the 

 Equator are slight compared with the rate of 

 development of zooplankton. 



The distribution of zooplankton around well- 

 marked fronts suggests a causal relation. Three 

 well defined fronts have been observed on POFI 

 cruises in the convergent or transition zone to the 

 north of the Equator. On all three occasions 

 strong southeast winds were experienced between 

 the Equator and the region of the front. The 

 latitudinal variation in zooplankton abundance as 

 related to these fronts is illustrated in figure 9 

 for the three series of stations along 120° W., 

 158° W., and 172° W. longitude. On each of the 

 three meridians the zooplankton abundance peaks 

 south of the front and drops off sharply to the 

 north. 



VARIATION WITH LONGITUDE 



To examine the east-west variation in zooplank- 

 ton abundance in respect to divisions of the cur- 

 rent system as previously defined, the adjusted 

 volumes were first combined by 10-degree inter- 

 vals of longitude disregarding season. Because of 

 the shortage of data for some subdivisions, 

 longitudes 170°\V. and 180°; were then combined 

 as were 150° W. and 160° W.; 120° W. was 

 grouped with 130° W. and 140° W. The lati- 

 tudinal zooplankton distributions in the two 

 western regions, 150° \V.-160° W. and 170° W.- 

 180°, are essentially alike (fig. 10) with peak 

 abundance occurring at the equatorial divergence, 

 and with the convergent zone next in importance. 

 In the eastern region (120° W.-140° W.), we find 

 the highest average volume in the Countercurrent 

 with the area of divergence second in rank. 

 Only in the Countercurrent are there significant 

 differences among longitudes, as indicated by the 



418106 O — 57 3 



60 



50 - 



o 

 o 



10 - 



14° 12° 



10° 8° 6° 4° 2° 0° 2° 4° 6° 

 S — LATITUDE-N 



8° 10° 12° 14° 16° 18° 



' Only 1 sample. 



Figure 10. — Longitudinal and latitudinal variations in the 

 distribution of zooplankton volumes (adjusted) with the 

 data segregated into three longitudinal groups and in 

 accordance with natural features of the current system. 

 The limits of the 0.95 fiducial interval are indicated for 

 each mean. 



lack of overlap in the 0.95 fiducial intervals of the 

 means. And it is only in the eastern Pacific that 

 production in the Countercurrent equals that of 

 the divergent zone. WhUe these apparent rela- 

 tions may change with further sampling and more 

 complete seasonal coverage, we believe the results 

 are logical in view of longitudinal variations in 

 thermocline depth and winds. 



As previously mentioned, toward the northern 

 boundary of the Countercurrent in the eastern 

 and central Pacific, there is a doming in the 

 isotherms (figs. 7 and 17) and the thermocline is 

 relatively shallow; consequently high-phosphate 

 water is within the photosynthetic zone and within 

 the reach of wind-induced turbulence. To the 

 westward the thermocline deepens (Sverdrup and 

 others, 1942: 708), reducing the likelihood of such 

 enrichment. Figure 11 shows the relation of 

 the average zooplankton volumes for the range 

 of latitude 8° N. to 11° N., and the depth of 

 the 70° isotherm for four meridians (140° W., 

 150° W., 160° W., and 170° W. long.). The 

 chosen range of latitude (8° N.-ll° N.) includes 

 the doming in the isotherms at the northern 

 boundary of the Countercurrent and represents the 

 zotie of most shallow thermocline in the tropical 

 Pacific. The results indicate a highly significant 



