In 1953, 0_. simtlis was found In large quantities with On. borealis , the latter being a wide- 

 spread species in the northern seas . The largest quantities of these two species were found in the 

 0-25 m. layer in the area of the polar front, near Jan Mayen and between latitudes 71°30' and 73°N 

 in longitude 3°E . 



Conclusions 



The physi CO -geographical characteristics of the Greenland and Norwegian Seas and the 

 existing system of warm and cold currents in the individual areas create peculiar hydrological con- 

 ditions - the phenomenon known as the polar front - where warm-water and cold-water organisms 

 are found simultaneously, their presence enabling us to ascertain the nature of the water masses 

 in a given area of the sea . 



2 . The most numerous forms in the plankton found in the North Atlantic are Copepoda - 

 Calanus finmarcliicus and Pseudocalanus elongatus . 



The quantities of these are particularly great in the currents (continental and western) . 



The second place is occupied by the group Amphipoda ( Themisto abyssorum and Th. 

 libellula ) in the north central part of the Norwegian Sea. Themisto libellula is the predominant 

 species in the Greenland Sea. 



The marine Cladocera and individual species of Copepoda (Cyclopidae and Centropagidae) 

 which inhabit the surfece layer of water (0-25 m.) constitute a numerous group during the summer 

 in the central areas of the Norwegian Sea . 



3. Warm-water plankton organisms are widespread in the Norwegian Sea. This is due to 

 the presence of the constant Norwegian current in this sea . 



Individual warm- water plankton organisms ( CoUozoum gro^nlandicum , Limacina retroversa) , 

 the main bulk of which is concentrated in the 0-100 m. layer, travel far northwards into the Green- 

 land Sea (reaching latitude 77°N) . 



Deep-water organisms do not enter the Greenland Sea, as they are checked by Mohn's 

 Threshold . 



4. Some plankton organisms which inhabit higjh latitudes, where they constitute the basis of 

 the zooplankton, normally inhabit the bottom waters in the central areas of the Norwegian Sea . 

 Among these are the species Dimophyes arctica, Calanus hyperboreus, Pareuchaeta norvegica , 



P . glacialis, Metrldia longa, Scolocithrix minor, Clione limacina, Sagitta elegans, Eukrohnia hamata, 

 Oikopleura labradoriensis . 



5. Variations in the strength of the warm- or cold-water currents have a considerable in- 

 fluence on the development of the biological processes and on the times of the biological seasons. 



Any strengthening of a cold- or warm-water current can be assessed from an increase in 

 the quantity and from the distribution of warm-water or cold-water plankton organisms, such as 

 took place in 1953 (strengthening of the western branch of the Norwegian current). 



6. Plankton is considerably more abundant in currents than it is beyond their boundaries . 

 During the summer the biomass in the 0-50 m. layer is 4, 000 mg/m3, whereas in the space between 

 currents it scarcely reaches 300 mg/jn3. 



99 



