Table 4. Variation in biomass of plankton in dependence on temperature variation. 



Owing to the sharp increase in the temperature of the upper layers of water, the thermo- 

 cline sinks somewhat and almost ceases to act as a barrier containing the plankton. At this period, 

 the biomass of plankton rises to 1200 mg/^S (graph HI) in the thermocline. This phenomenon is 

 particularly pronounced in the areas near the polar front (in the vicinity of the Faroes and Jan 

 Mayen), where warm and cold waters meet. Here, in the layer of abrupt temperature change, 

 cold-loving organisms - especially Calanus hyperboreus - also collect in large quantities (graphs 

 III and IV) . 



In the polar-front areas, the development of the biological processes is later than in other 

 areas, and the biological spring occurs during the summer months. The cold-water copepods 

 multiply at low temperatures, and as the upper layers of water are well warmed by this time, the 

 bulk of C. hyperboreus becomes concentrated in the thermocline, where the temperature is low. 



SPECIAL FEATURES OF PLANKTON DISTRIBUTION IN 1951-53 



It is well known that the qualitative composition of plankton varies in the course of the year 

 and that there is a specific composition of organisms, or complex, for each biological season. 

 This is due to variations in the physicochemical conditions of the environment (12). 



However, in some years certain deviations are found during the period of mass develop- 

 ment of the plankton; variations occur in the ratio of individual species to one another and in the 

 area which they occupy. 



Thus, in 1951, there was a very extensive spread of Aglantha digitale (see Figure 5) during 

 the spring and the summer season in the central areas of the Norwegian Sea to a depth of 200 m. 

 This medusa, being an oceanic deep-water species, remains in the currents. In different areas, 

 however, it is found in association with other species. At the end of March and during April, 

 Aglantha has been found in the south of the Norwegian Sea, on the Viking Bank, in association with 

 Oikopleura labradoriensis . 



O. labradoriensis occurs in an isolated patch in the channel between the Faroes and Shet- 



lands , 



Further north, for instance in latitude 65°N, large quantities of Aglantha (in May and June) 

 in association with Pseudocalanus elongatus have been observed in the western branch of the 

 Norwegian current as far north as latitude 72°N. 



In 1952, at this same season, a patch of Euphausiacea ( Meganyctiphanes norvegica) and 

 Siphonophora (Physophora hydrostatica ) was observed in the plankton in the southern part of the 

 Norwegian Sea (see Figure 6). An "outburst" of the pelagic mollusc Limacina retroversa in assoc- 

 iation with Physophora hydrostatica was found on the Norwegian Plateau in the continental stream 

 of the Norwegian current. 



96 



