28.0 



d 26 



22 

 35 30 



35.20 



J 1 \ I I \ 1 1 



J \ I I I I I 



T r I I — I — I — I — I — I — I — I — I — r 



J L 



J I \ L 



J J A S N D J FMAMJ JASONO 



r 1 ' 



1957 1958 



Figure 6. — Monthly variation In temperature and salinity 

 at the IGY station from June 1957 to December 1958. 

 A. Mean to 60 m. temperatures. B. Mean to 60 m. 

 salinities (solid line) and mean 200 to 300 m. salinities 

 (dashed line). 



to sijniificance at the 5 percent probability (for 

 P = 0.05 and d.f. = 16, r = 0.468). Negative cor- 

 relations were significant between volumes of zoo- 

 plankton for both night and day haids and depths 

 from the surface to the top of the tliennocline. 

 Neither temperature, oxygen, nor inorganic i:)hos- 

 phate was correlated with volumes of zooplankton. 

 We believe the significant relations between zoo- 

 plankton and salinity and between zooplankton 

 and the thickness of the isothermal layer resulted 

 from tlie advection of dift'erent types of water into 

 the Hawaiian area. The biomass of zooplankton 

 was higher in waters of high salinity. 



ZOOPLANKTON AND TEMPERATURE- 

 SALINITY CURVES 



Seckel (1962) indicated that the Hawaiian Is- 

 lands are located near the northern boundary of 

 the transition zone between two major types of 



Table 3. — Monthly values of surface salinity, inorganic 

 phosphate, and oxygen during low and high tides at the 

 IGY station 



Month 



Salinity °/o, 



Low High 

 tide tide 



Oxygen ml.A 



Phosphate 

 (ig.at./l 



Low High Low High 

 tide tide tide tide 



1957 



June - 34.97 



July 34.87 



August. 34.96 



September 35. 07 



October 35.19 



Novem'^er 35.12 



December 34.72 



19S8 



January 34.81 



February 35.03 



March 34.94 



April 35.10 



May 34.80 



June 34.90 



July 34.81 



August 34.91 



September 34.86 



October 34.94 



November 34.96 



December 34.94 



34.97 0.22 0.20 4.29 4.32 



34.88 .29 .24 4.47 4.46 

 34.96 .20 .22 4.79 4.78 



35.12 .13 .16 4.46 



35.16 .10 .16 4.63 4.61 



35.12 .11 .16 4.78 4.64 



34.81 3.65 4.58 4.66 



34.85 .12 .13 4.79 4.69 



35.05 .22 .20 4.85 4.84 



34.96 .64 1.60 4.60 4.66 



35.07 



34.77 .06 .62 4.65 4.62 



34.82 4.08 4.18 



34.80 .49 .70 4.77 4.80 



34.89 .39 .22 4.43 4.65 



34.87 4.65 4.64 



34.96 .21 .23 4.40 



34.94 .11 .10 



35.05 36 4.64 



surface water: (1) the high-salinity (surface 

 values >35.0%o) North Pacific Central "Water, and 

 (2) the low-salinity (surface values <34.2%o) 

 North Pacific Equatorial Water. These water 

 types corresponded approximately with the sub- 

 surface water masses as defined by Sverdrup, 

 Johnson, and Fleming (1942). Se«kel (1962) clas- 

 sified the Califoniia Current Extension, which 

 forms the core of the transition zone, as a third 

 type. He showed that the boundaries of these water 

 types shifted seasonally and affected the type of 

 water to be found near Oahu. 



Our data were analyzed from the standpoint of 

 advection of different types of water as indicated 

 by changes in the temperature and salinity during 

 the 19 months of observation. The data were di- 

 vided into the following nine chronological periods 

 (fig. 7) on the basis of the similarity in the tem- 

 perature-salinity (T-S) curves for successive 

 months : June to July 1957, August 1957, Septem- 

 ber to November 1957, December 1957 to March 

 1958, April 1958, May 1958, June to July 1958, 

 August to November 1958, and December 1958. 

 Each period had temperature and salinity char- 

 acteristics that were different from the preceding 

 and succeeding periods. The average monthly 

 (0-60 m.) temperatures, the average monthly 

 salinities (0-60 m. and 200-300 m.), and the 

 average depth from the surface to the top of the 

 thermocline for the nine periods are illustrated 

 in figure 8. 



VARIATIONS IN MARINE ZOOPLANKTON IN HAWAIIAN WATERS 



93 



