THE BLACK SEA 421 



С helgolandicus breeds throughout the year, apparently producing five or 

 six generations (N. Klucharev, 1948; L. Chayanova, 1950). Its average 

 amount, in all its stages, under 1 m 2 of surface is about 1 ,000 specimens. 



During the cold season of the year C. helgolandicus keeps mostly to the upper 

 layers of the Sea and in the summer to the lower ones, but in summer also 

 Calanus travels vertically to the depth each day. During daylight its mass is 

 concentrated at a depth of 75 to 100 m, and during darkness in the to 10 m 

 layer {Table 167). Pseudocalanus elongatus behaves in a similar manner. 



Table 167. Vertical distribution of edible zooplankton biomass in April 1949, percentage 

 of total biomass in open part of Sea. {A. Kusmorskaya) 



In contrast to C. helgolandicus, the development of Acartia clausi proceeds 

 throughout the year in the upper layer (0 to 50 m), and in the warm period of 

 the year the upper maximum of plankton development depends on the growth 

 of A. clausi and Penilla avirostris. As regards numbers Penilla occupies the 

 first place, Acartia the second and Calanus the third {Table 168). 



Table 168. The numbers of Calanus, Acartia and Penilla, April to August, in the 

 to 150 m layer of the open Sea under 1 m 2 {A. Kusmorskaya) 



Acartia clausi (L. Chayanova, 1950) produces nine generations in one year. 

 The numbers of Noctiluca miliaris reach 2,000 to 6,000, sometimes even 

 9,000, specimens per 1 m 3 (in one case 80,000 specimens/m 3 were recorded) 

 and Pleurobrachia pileus gives in the 50 to 100 m layer 2-5-6 and up to 15,000 

 specimens per 1 m 3 . At times they form a fairly considerable supplement to 

 food-plankton {Table 169). 



An approximate distribution of food-zooplankton for August 1950 is 

 given in Fig. 203 (except the Medusa, Pleurobrachia and Noctiluca). The total 



