As noted previously, the distribution of mesozooplankton 

 within the region is mainly due to the heterogeneity of 

 hydrological conditions found there (Korshenko, 1988). Asa 

 result, two bands of increased zooplankton biomass are formed 

 in the surface layer along the equator (Vinogradov & Voronina, 

 1963). In the Southern Hemisphere west of 140°W, the zone 

 of abundant plankton narrows to 5°S^°S. At the same time, 

 zooplankton productivity in the equatorial region increases 

 from east to west. We found that zooplankton biomass ranged 

 from 5 1 to 200 mg/m 3 . The western portion of the region 

 (Stations 1 19 and 120) was located in the zone of abundant 

 plankton (Fig. 1 ). The total biomass of mesozooplankton 

 ranged from 89 to 340 mg/m 3 and averaged 201 mg/m 3 

 (Table 2). The total biomass of zooplankton in the eastern 

 portion of the section in the upper 200 m layer did not exceed 

 163 mg/m 1 (Fig. 3). At the westernmost point, and at Station 

 117. the biomass was maximum and amounted to 321 and 

 340 mg/m 3 , respectively. The total numbers of mesozooplankton 

 at different stations ranged from 547 to 2,170 sp/m 3 and 

 averaged 1,044 sp/m 3 (Fig. 3; Table 2), which doubled the 

 known quantity of plankton organisms (493 sp/m 3 ) known in 

 the northern portion of the equatorial region (Korshenko, 

 1 988). In the eastern section, the numbers of mesozooplankton 

 ranged from 547 to 765 sp/m 3 . As one moved toward the 

 equator, starting at 6°S, the numbers of mesozooplankton 

 increased to a maximum of 2,170 sp/m 1 at Station 120 

 (Table 2). It was shown that the qualitative composition and 

 structure of the community at Stations 1 17 and 120 differed 

 appreciably from those recorded at the other stations in the 

 region. This is evidently related to the characteristics of the 

 structure and dynamics of the water masses. 



The vertical distribution of the mesozooplankton was also 

 different at different stations. At six of seven stations (Fig. 4). 

 most of the mesozooplankton was concentrated in the upper 

 100-m layer, and the maximum was in the 0-50-m layer 

 (Table 3; Fig. 4), corresponding to findings cited by Korshenko 

 (1988). A significant increase in zooplankton numbers and 

 biomass with depth was tracked only at the westernmost point 

 (Station 120). Mesozooplankton biomass as a whole increased 

 with depth owing to an increase in the fraction of large-sized 

 animals. 



In addition to analyzing the vertical distribution of total 

 numbers and biomass, we also examined the vertical distribution 

 of individual taxonomic groups. We found that the numbers of 

 Calanoida and Euphausiasea decreased with depth, yet the 

 biomass remained practically unchanged because of an increase 

 in the fraction of large-sized forms (Fig. 5; Table 3). The 

 numbers and biomass of Siphonophora, Polychaeta, Ostracoda, 

 Hapracticoida, Amphipoda, Chaetognatha, and Salpidae 

 increased with depth. The numbers and biomass of Cyclopoida, 

 Mysidae, Decapoda, and Echinodermata, on the other hand, 

 decreased with depth. The numbers of Doliolidae were constant 

 at all the levels, and the biomass increased with the relative 

 content of large-sized animals. 



In order to analyze the structure of the mesozooplankton 

 community, we divided the determined forms into 17 major 

 systematic groups. The frequency of occurrence (by numbers 

 and biomass) of 14 of these basic taxonomic groups is shown 

 in Table 4. The maximum fraction of numbers, as an average 

 for the cross section, belonged to Calanoida — 53.8%. In 60% 

 of the cases, Calanoida comprised over half of the total numbers. 

 According to Korshenko (1988), the numbers of Calanoida in 

 the 0-200-m layer amounted to 45% of the total value. The 

 highest numerical percentage of Calanoida (66.5%) was found 

 on Station 1 14. In the western portion of the section, Calanoida 

 numbers decreased to 40%. The biomass amounted to over half 

 of the total value only in the region of the Caroline Atoll, owing 

 to the small number of other taxonomic groups composing the 

 community. To the west, the relative content of Doliolidae was 

 comparatively high, and the fraction of groups other than 

 Calanoida was insignificant. We noted a tendency for the 

 number of taxonomic groups to increase toward the equator. 

 On one hand, the fraction of numbers and biomass of the 

 dominant group (Calanoida) decreased, while the relative 

 content of Cyclopoida, Chaetognatha, and Siphonophora 

 increased. For example, the fraction of numbers and biomass 

 of Cyclopoida at the equator was 48 and 29%, respectively. In 

 addition to the general tendency of the Calanoida fraction to 

 decrease, in many cases the typical (Fig. 6) structure of the 

 community broke down as a result of concentrations of animals 

 of a particular species. For example, the biomass fraction of 

 Euphausiacea was maximum (35.5%) at Station 1 17 as a result 



TABLE 2 



Total numbers and biomass of mesozooplankton in the 200 m surface layer. 



Remark: For Station 1 14. the data and results were obtained for the 0-100 m layer. 



