180 W 



150'W 



a) 



Boundary of 

 Tradewind 

 Current 



180 W 170' 



150' 



5' 



x|x convergence 



o 



b) 



180 W 170 ' 



Fig. I. Character of currents in the region studied: (a) circulation of waters 

 at the surface; (h) circulation of waters at a depth of 100 m; and (c) 

 circulation of waters at a depth of 2(H) m. 



largest animals ( over 2 mm long ) were washed out and classified 

 into systematic groups using a biolam R-7 microscope. The 

 residue of the sample was diluted 1 0-20-fold, depending on the 

 concentration, and 2-3 portions of 10-20 ml each were taken 

 with a 5-ml plunger pipet for counting. On the basis of this data, 

 the mesozooplankton numbers in 1 m 1 and under 1 m : were 

 calculated. A micrometer eyepiece was used to determine the 

 body length of representatives of each species. On the basis of 

 these values and Chislenko's nomograms (Chislenko, 1968; 

 Vinogradov & Shushkina, 1987), the biomass of each species 

 was determined, and the total biomass of mesozooplankton in 

 I in' and under 1 m was calculated. Twenty samples were 

 analyzed. 



Results and Discussion 



li is well known (Korshenko, 1988) that the characteristics 

 of the distribution of zooplankton depend, to a considerable 

 degree, on the hydrological structure of the water masses. 

 Figure 1 shows a diagram of the currents in the studied region 

 on the surface and at depths of 1 00 m and 200 m. The dynamics 

 of the water masses at the easternmost station near Caroline 



Atoll and at adjacent Station 1 14 are low; no major currents 

 pass through this area. The depths of 100 m and 200 m at 

 stations in the central portion of the region are characterized by 

 a water mass transport to the west. The w ater masses of the 

 western portion of the section (Stations 119 and 120) are 

 affected by the surface South Tradew ind Current. In addition, 

 at the westernmost point of the region (Station 120). an 

 anticyclonic equatorial current is observed (Gorshkov, 1974). 



The heterogeneity of the regional water structures suggests 

 a nonuniform horizontal and vertical distribution of the 

 mesozooplankton as well as its qualitative diversity. As is 

 evident from Table 1 . the composition of the mesozooplankton 

 differed qualitatively at different points of the section. A total 

 of about 1 80 forms of mesozooplankton were determined as a 

 result of the analysis of the samples. The average number of 

 species for a station ranged from 75 to 139 and. for an 

 individual level, from 39 to 84 (i.e., it differed by a factor of 

 two) (Table 1; Fig. 2). The smallest number of species was 

 found at the station near Caroline Atoll: at the 0-50-m and 

 50-100-m levels, there were 39 and 56 species, respectively. 

 The number of species increased from south to north toward the 

 equator, reaching a maximum of 1 39 at Station 17. The vertical 

 structure of species composition also varied. In the majority of 

 the cases, the number of species increased appreciably with 

 depth, and it remained practically unchanged only at the 

 equator (Fig. 2a). 



The species observed in the waters of the region studied 

 were encountered at different frequencies. Four gradations of 

 species occurrence were distinguished: rare, under 1 sp/m'; 



Number of species 



Number of species 

 140 



120 119 118 117 116 115 114 Sun.) 



I ig 2. Number ol meso/ooplankton species: (a) at an individual level; and 

 Ibl average for the station. 



