96 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



of Phaeocystis and only partly by the diatoms ; these latter are preponderant 

 in July and the peridineans in August and September. The May maximum of 

 zooplankton is connected with the larval forms of the bottom animals, 

 mainly with Balanus, Calanus finmarchicus and Thysanoessa inermis ; it is 

 rather low and persists through June only. Starting from June the neritic 

 Copepoda and medusa become the preponderant groups. Moreover, an inter- 

 esting fact is noted by B. Manteufel — the population of the deeper layers of 

 water of these inlets, which is not affected by the surface loss of salinity, 

 approaches much nearer to the plankton composition of the open seas than 

 does the population of the upper, always somewhat saline, layer. 



Vertical migrations and seasonal variations. As was shown by V. Bogorov 

 (1932) and V. Jashnov (1939) for a number of the highest mass forms of Cope- 

 poda during their young stages they keep to the surface layer of water (in the 

 south of the Sea mainly at a depth of 10 to 25 m and in the north at 25 to 

 75 m) ; the adult forms, however, descend into deeper layers (75 to 300 m). 



It is most curious that under the conditions of a polar day the Copepoda 

 remain on the same level at different hours of the day and night (V. Bogorov, 

 (1938) ; they do not migrate vertically every 24 hours, as they do in other lati- 

 tudes with the change of day to night. However, as we have noted in our general 

 section, owing to the presence of the deep waves, the layers of water are sub- 

 ject to 24-hourly vertical oscillations, sometimes of several tens of metres. 

 Evidently, Copepoda, in order to keep their position within the same inten- 

 sity of light, are forced to travel in the opposite direction to the wave motion. 

 Hence the purpose of the known vertical semidiurnal migrations of Copepoda 

 is to remain at a constant level. V. Bogorov has observed the same pheno- 

 menon in other plankton. In the autumn with the alternation of day and night 

 Copepoda begin their daily vertical migration of the usual type. On the other 

 hand, some organisms change their position of greatest density twice daily, 

 i.e. they do not actually migrate but go upwards and downwards with the 

 wave motion of the water layers. 



V. Jashnov's (1939) detailed analysis of the succession of generations and the 

 seasonal changes in the distribution of the stages of growth of Calanus 

 finmarchicus led him to conclude that its nature was monocyclic. According 

 to his data from the Barents Sea only one generation of Copepoda succeeds 

 in developing within a year (Fig. 35). This fact is especially interesting when 

 compared with data from other parts of the northern Atlantic. Thus, accord- 

 ing to V. Bogorov's data (1934), in the Plymouth region three generations of 

 Calanus finmarchicus manage to develop within one year — the spring, summer 

 and winter ones. As has been shown by a number of foreign biologists, 

 Calanus finmarchicus gives two generations in the northern Atlantic, bred in 

 the spring and by the end of the summer (Scotland, the western shores of 

 Norway, etc.). However, a very circumstantial survey by M. Kamshilov (1955), 

 carried out almost fifteen years later, has led him to the conclusion that in the 

 eastern part of the Barents Sea Calanus finmarchicus breeds twice a year. The 

 first breeding period begins in April, and at the end of June or the beginning 

 of July there appears the second brood, with considerably bigger females. 



