made at the 13 sample levels at each station of cruises 12 and 17 o Dis- 

 solved oxygen was measured at all sample levels throughout cruise 10 „ 

 Salinity and reversing thermometer data were available for all three 

 cruises o The hydrographic data resulting from these three cruises have 

 not as yet received thorough oceanographic study » The conclusions ex- 

 pressed here are based on information available at the time of this 

 writing and are, therefore, of a preliminary nature o 



Temperature 



For Smith cruises 4 and 6^, in Liay and August 1950 respec- 

 tively j, the deviations from the mean zooplankton volume at the 50- 

 meter level and the ratios of 50-meter/surface zooplankton volumes 

 were examined in respect to the depth to the top of the thermoclinej, 

 the hypothesis being that a large positive deviation or a large ratio 

 should more likely occur at stations 7n.th a shallow thermocline., A 

 shallow thermocline would result in nutrient-rich water's being nearer 

 the surface and more available to plant life than in the case of a 

 deep thermocline with the discontinuity occurring belov; the photo- 

 synthetic zoneo The results did not confirm this hypothesise Graphs 

 of the data (not presented here) showed a random distribution in re- 

 lation to thermocline depth for both deviation from the mean and the 

 50-meter/surface ratios » 



Since surface temperature may fluctuate as a result of diurnal 

 heatings, the authors chose the 10-meter depth as providing a tempera- 

 ture more truly indicative of temperatui'e conditions in the surface 

 layero Figiare 14 shovv-s the isotherms for the 10-meter depth as found 

 on three cruises of the Smith o Although conditions were remarkably 

 uniform throughout the area„ waters to the southwest held the highest 

 temperatures while the northeastern and southeastern areas showed the 

 lovrest temperatures « Correlations between adjusted zooplankton volojaies 

 and temperatures at 10 meters vrere calculated for cruises 10, 12., and 

 17 of the Smith and cruise 8 of the ^fanning (table 11) „ For only one 

 of the four cruises (Smith cruise 12) was a" significant correlation 

 obtained . 



As stated previously, we believe that thermocline depth 

 may have a significant influence on biological productivity o Occasion- 

 ally, however, the thermal structure is such that no distinct thermo- 

 cline is present or more than one gradient may be shovm on the BT 

 trace. Thus it is frequently difficult to follov; set rules in desig- 

 nating thermocline depth and as a result the m.easurement tends to be 

 rather subjective « The depth to the 70 isotl'ierm may be more objec- 

 tively read from the BT traces and, in this area of the central Pacific, 

 falls within the thermocline and varies generally with thermocline 

 deptho Adjusted zooplankton vol'omes and depth to the 70° isotherm were 

 compared for cruises 10^ 12 .s and 17 of the Smith and cruise 8 of the 

 Manning (table 11) „ No significant correlation was found in any of the 

 four analyses <> 



39 



