Consequently, the quantity of individual species of plankton also serves as an indicator of 

 currents . 



7. Temperature is an important factor influencing the vertical distribution of plankton. 



A thermocline arises with the warming of the upjjer layers of water in spring. The mass 

 development of plankton normally begins at this time of year; its volume increases noticeably, and 

 the plankton collects above the thermocline, which serves as a "floor". 



In the summer, in the areas of the polar front, there is a large plankton biomass above and 

 in the thermocline, cold-loving organisms - especially Calanus hyperboreus - collecting in this 

 layer . 



8 . Waters of different origins differ considerably in their qualitative plankton composition. 

 In view of this fact, charts of plankton distribution can be used for drawing the boundaries of a cur- 

 rent more accurately when making current charts of the areas under investigation. 



9 . A certain deviation in the distribution and predominance of populations of different 

 species is observed in some years during the period of mass development of plankton. 



Thus, Aglantha digitale was the predominant species of plankton in the central areas of the 

 Norwegian Sea during the spring and summer period of 1951. In the south, Aglantha was observed 

 in association with Oikopleura labradoriensis . Further north, for instance in latitude 65°N, Aglan- 

 tha is observed more and more in association with_P. elongatus in the western branch of the Nor- 

 wegian current, as far as latitude 73 N. 



At the same season in 1952, patches of Euphausiacea and Siphonophora were observed in the 

 plankton in the southern part of the Norwegian Sea . An outburst of the pelagic mollusc Limacina 

 retroversa was observed in the central areas . There is a population of Amphipoda in the central 

 plateau and in the area of Jan Mayen . 



In the 1953 season, the predominant species of plankton was Pseudocalanus elongatus - one 

 of the Copepoda. In the area of Jan Mayen, P. elongatus was observed in the 0-50 m. layer in 

 association with Oithona similis and Oncea borealis . 



100 



