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



The upper temperature limit for Sagitta is 10° or 11°, for Pleurobrachia — 

 12° or 13°, for Calanus and Pseudocalanus — 13°, and for Oithona — 14°. 



Finally, a third group develops only in summer, keeping to the upper, warm 

 layer of water. During the summer warming-up these forms occupy a greater 

 and greater depth of water. When cooling begins they become gradually 

 scarcer, disappearing completely from the plankton in the winter. This group 

 includes Centropages kroeyeri, Evadne nordmanni, Evadne spinifera and Podon 

 polyphaemoides. 



The lower temperature limit for these species frequently coincides with the 

 upper temperature limit of the previous groups of forms. 



Hence in different inhabited zones of the Black Sea both constant and tem- 

 porary plankton species can be observed, the temporary ones appearing 

 either as a result of migration from the deeper layers, or developing in the 

 upper, warm layer in summer only. This is shown by V. Nikitin (1929) in a 

 clear diagram reproduced by us in an abbreviated form {Table 162). 



Vertical migrations. Thus some plankton species have seasonal vertical 

 migrations. V. Nikitin thinks (1929) that under Black Sea conditions the 

 main factor causing these migrations is temperature, which masks the effect 

 of light. 



The plankton forms inhabiting layers below 50 m must have the capacity 

 to exist, under Black Sea conditions, with little oxygen. In the deepest in- 

 habited layers, where the amount of oxygen is no more than 4 per cent and 

 may be less, five or six species are still found, among them Calanus helgo- 

 landicus and Pseudocalanus elongatus. Their high eurybiotic form was proved 

 experimentally by V. Nikitin and E. Malm (1927). 



Apart from the seasonal migrations, daily migrations have been observed 

 for a number of species, conditioned primarily by variations in light. The most 

 pronounced daily migrations are those of Calanus helgolandicus and Sagitta 

 euxina. 



The lower limit of distribution. Owing to the hydrological and hydrochemical 

 conditions of the Black Sea, both plankton and benthos exist only in the upper 

 layer of the Sea. In the central parts the plankton is concentrated in the upper 

 layer at 100 to 150 m, and in the littoral areas and in those of the middle of 

 the Sea between the shores of the Crimea and Anatolia, in the 150 to 175 m 

 layer. In the littoral areas of the western part of the Sea the lower boundary 

 of the inhabited zone lies a little higher (125 to 150 m) and in the eastern area 

 a little lower ( 1 75 to 200 m) than the average position. Thus the lower boundary 

 of the Black Sea inhabited zone is not horizontal, but slopes from west to 

 east with about 50 m difference in level. This sloping of the lower boundary 

 of the inhabited zone is conditioned by the greater decrease in salinity in the 

 western part of the Sea, which hinders vertical circulation. We shall see below 

 that the same phenomenon is found for the lower boundary of benthos dis- 

 tribution. 



As is shown by a closer examination of the distribution limit of the Black 

 Sea pelagic plankton (Fig. 193), this is mostly in accordance with the general 



