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



forms keep mostly to the surface layer, the adults to the depths. They like- 

 wise prefer the deep layers in the summer and the surface ones in the winter ; 

 this is an example of a peculiar type of migration conditioned by photo- 

 tropism but adapted not to the daily change of light, but to the yearly alter- 

 nation of the polar day and night. 



A certain change of the qualitative composition of plankton is observed 

 during the year and is particularly marked in inlets. 



In the spring and early summer, the oceanic forms of plankton are domi- 

 nant, while in the second half of the summer and in the autumn there is a 

 considerable admixture of neritic forms. A considerable amount of the larval 

 stages of bottom fauna appears also in the plankton during the second half of 

 the summer. 



B. Manteufel (1937) distinguishes for the Motovsky Gulf four groups of 

 forms, producing their greatest development at different seasons of the year 

 (Fig. 33). 



In early spring (April to May) Copepoda are almost absent, while plankton, 

 consisting mainly of the larval forms which have risen from the bottom for 

 breeding, is concentrated in the uppermost layers of water. In this group the 

 larvae of Balanus, Fritillaria borealis and those of the polychaetes, decapods 

 and some medusa such as Sarsia and Cyanea are prominent. In June the notable 

 preponderance of three forms — Calamis finmarchicus, Thysanoessa inermis and 

 Sagitta elegans — has been observed. Calanus finmarchicus is found mainly in 

 the third stage which is vigorously fed upon by all its numerous predators. 

 It we take the amount of third stage Calanus finmarchicus as the unit, then only 

 9 per cent of it develops to the fifth stage and only 0-1 per cent to the sixth. 

 Thysanoessa and Sagitta as they grow depart into the depths and partly, 

 perhaps, move away from the shores. 



In their place there appear in summer (August to September) different 

 Copepoda (Acartia, Centropages, Temora, Paracalanus and others), Clado- 

 cera (Evadne, Podon), sometimes some Pteropoda (Limacina) and the mollusc 

 larvae. These are mostly thermophilic forms. The warmest water forms (some 

 Copepoda) are, in the early stages of their development, brought from the 

 west with warm water; they disappear with the coming of cold weather. 

 Others (Cladocera) live through the winter in the stage of resting eggs. 



Finally, in the late autumn and in the winter the fourth group develops 

 significantly. 



This group contains a whole series of Copepoda (Metridia, Calanus hyper- 

 horeus, Euchaeta, Oithona and others), Euphausiacea (Thysanoessa, Meganyc- 

 tiphanes), Oikopleura labradoriensis and Aglantha digitale. In the spring their 

 number is greatly reduced and in summer they are met only in the deepest 

 parts of the inlet, and then only in small numbers. 



In the open sea the seasonal changes of plankton are not so sharply defined, 

 Calanus finmarchicus and Euphausiacea are, however, sharply predominant 

 even in June, and different Copepoda and other Euphausiacea in December. 



Quantitative distribution of plankton. It is possible to obtain an idea of plankton 

 productivity in the southwesterly half of the Barents Sea, southwest of a line 



