time of occurrence of the bloom are determined primarily by the develop- 

 ment of the summer density stratification of the water it is this factor 

 which is most important in determining the trophic conditions during the 

 spawning period. 



However, various species do not reach the surface and breed simul- 

 taneously, but rather in a strictly defined succession. For example, 

 among the copepods in the Antarctic, the earliest species is C^. acutus , 

 followed by C^. propinquus , then R^. g igas (Voronina, 1956a). Therefore, 

 in various places of the area either one or another species is in optimal 

 conditions and yields the greatest brood. These factors are responsible 

 for the changes in abundance of each of the species within the limits of 

 its area of distribution, and the variation in the quantitative relation- 

 ships between different species. Illustrations of this can be found in 

 materials from the expeditions of the DISCOVERY (Baker, 1959) and OB' 

 (Voronina, 1966b). 



In some cases, the influence of the hydrologic structure of the 

 waters is manifested differently. During the spring migration, great 

 hydrologic gradients may prevent the spawning pool from rising into the 

 productive layer, and then breeding occurs in the subsurface waters poor 

 in phytoplankton. This has a sharp influence on the abundance of the 

 brood (Voronina, 1970a). 



These are the main factors determining the regional distribution of 

 the abundance of new generations of plankters. The distribution is 

 subsequently transformed under the influence of the currents. The 

 primary significance is the meridional transfer. The transverse studied 

 (Deacon, 1937; B. G. Neyman, 1968; Arsen'yev et al . , 1975). The Antarctic 

 surface water with its population moves northward at a mean speed of 

 5-20 cm/s (in the Subantarctic, somewhat more slowly). In the zones of 

 the Antarctic and Subantarctic convergences, the surface waters move 

 downward, but the organisms remain at their natural depths. As a result, 

 in these narrow zones there is a continuous increase in the abundance 

 of zooplankton due to the influx of allochthonous material. The rate of 

 this increase in one transect through the Antarctic convergence has 

 been found to amount to 3% per day (Voronina, 1968). 



The influence of currents on the distribution of various species 

 depends on the time spent by their populations in the corresponding 

 waters. For example, for Euphausia triacantha , which has a two-year 

 life cycle, inhabiting the surface water and, apparently, performing 

 only slight seasonal migrations, the peak at the Antarctic convergence 

 is quite clearly expressed (Baker, 1959), while for the one-year inter- 

 zonal copepods it is less clearly expressed, although it is still quite 

 clear. For inhabitants of the middle depths, e.g., Sagitta gazellae , on 

 the other hand, a great decrease in the population along the convergence 

 is characteristic, apparently related to the moving apart of their biotope 

 by the descending surface water. The descending layer has so thoroughly 

 disjoined these animals that it has facilitated the formation of isolated 

 populations on either side of the zone of convergence, which have developed 

 into independent races. 



99 



