plastic dish) which had 300 subdivisions ruled 

 into its bottom. The organisms in 40 randomly 

 selected squares were identified and counted 

 under a dissecting microscope. The estimated 

 number of a group of organisms in 1,000 m.' 

 of water was calculated by the following formula 

 (modified from King and Demond, 1953) : 



PA 



where E = number of organisms per 1,000 m.' of 

 water 

 C = counted number of organisms 

 A=area of counting cell (300 cm.^) 

 f=fraction of total sample in the counting 



cell 

 a=area of square (1 cm.^) 

 n=number of squares counted (40) 

 W= cubic meters of water strained by net 



Usually, on each visit to the IGY station, two 

 hydrographic casts were made, one during the 

 highest tide and the other during the lowest tide. 

 Ordinarily, each cast was from to 500 m. depth 

 and consisted of 12 Nansen bottles, but on some 

 occasions the casts were deeper. Temperature 

 and salinity data were obtained with each cast, 

 and with a few exceptions, oxygen and inorganic 

 phosphate were also measured. Additional temper- 

 ature data were obtained with bucket thermom- 

 eters and bathythermographs. 



Salinity determinations were made by a modi- 

 fication of the Knudsen method ("Van Landing- 

 ham, 1957) ; inorganic phosphate determinations 

 were made with a Beckman ^ spectrophotometer. 



Temperature, salinity, phosphate, and oxygen 

 values used in the study were averaged as follows : 



Surface value: average of measurements ob- 

 tained at water surface during high tide and low 

 tide. 



to 60 m. value: average of measurements at 

 depths of 0, 10, 20, 30, 40, 50, and 60 m. during 

 high tide and low tide. 



200 to 300 m. value: average of values at 15- 

 unit increments between the 400 and 300 thenno- 

 Bteric anomaly surfaces on a temperature-salinity 

 plot. These two anomaly surfaces represented ap- 

 proximately 200 and 300 m., respectively. 



' Trade names referred to in this publication do not imply 

 endorsement of commercial products. 



Vertical temperature distribution: isotherms 

 were contoured on the basis of the average tem- 

 peratures at the following depths : 0, 9.2, 15.2, 30.5, 

 45.8, 61.0, 76.2, 91.5, 100.6, 122.0, 149.4, 152.5, 183.0, 

 199.8, 213.5, 244.0, and 274.5 m. Readings were 

 limited to a maximum of five bathythermograms 

 per month. 



DIEL VARIATIONS 



In central Pacific waters, diel variation in vol- 

 umes of zooplankton and in abundance of certain 

 zooplankton has been shown to vary as the curve 

 of the sine function (King and Hida, 1954; 

 Legand, 1958; Nakamura, 1967) ; the peak equated 

 to midnight. The volumes and abundance of zoo- 

 plankters determined from samples obtained at 3- 

 hour intervals from June 21 to 23, 1957, were 

 examined for similar variations. Temperatures and 

 depths fo the top of the thennocline for this 

 period were also examined for diel variations. 



VOLUMES OF ZOOPLANKTON 



The volumes of zooplankton obtained for the 48- 

 hour series ranged from a low of 13 cc./l,000 m.^ 

 of water strained to a high of 40 cc./l,000 m.'', with 

 a mean of 26 cc./l,000 m.^ (fig. 1). The volumes of 

 the samples obtained during the night were greater 

 than those of the samples collected by daylight; 

 the night to day ratio was 1.3 : 1. The greater abun- 

 dance of zooplankton in darkness has been attrib- 

 uted by investigators to some combination of an 

 upward migration by the zooplankters and an 

 increased avoidance of the net by some organisms 

 in daylight (King and Hida, 1954, 1957a; Flem- 

 inger and Clutter, 1965 : Brinton, 1967). 



The volumes from the 48-hour series appeared 

 to conform approximately to a sinusoid for the first 

 24-hour period but not for the second. The volumes 

 remained high through the second morning. An 

 unusually large amount of an undetermined species 

 of diatom was present in samples 11, 12, 15, and 

 16. The greater resistance of these samples to mois- 

 ture drainage than of those without diatoms re- 

 sulted in a greater determination of wet volume. 

 This fact may have explained the unusually high 

 volumes in samples 12, 15, and 16. On the other 

 hand, the volume of sample 11 did not appear to 

 be unusual even though it too contained diatoms. 



88 



U.S. FISH AND WILDLIFE SERVICE 



