ummer iillll Autumn 



Figure 1 . The duration of the biological seasons in the northern seas. 



However, regardless of the fact that the time or length of the biological seasons in different 

 years can change or vary somewhat, it is, nevertheless, possible to establish some regularity in 

 the development and sequence of seasonal plankton jrfienomena. 



north. 



Figure 1 shows how the beginning of the biological seasons change gradually from south to 

 This is related, to a considerable extent, to climatic conditions . 



The spring- summer period lasts for a half year in the coast waters of Norway and in the 

 southern part of the Norwegian Sea. In the central deep-water regions, it does not exceed four 

 months . But in the Greenland Sea it does not always last three months . The shortest winter occurs 

 in the southern part of the Norwegian Sea, whereas the Greenland Sea has the longest winter. 



The plankton of the entire area of the Norwegian and Greenland Seas is poor in the area dur- 

 ing the winter time. Its quantity increases somewhat with depth, but even at more than 300 m. it 

 does not exceed 10 mg/ni3. In the upper 100- meter layer during the winter months (December- 

 March), the weight of plankton organisms is barely several milligrams per cubic meter. Copepoda 

 and Eui^iausiacea occur only as individual specimens in plankton samples. For example, the com- 

 position of a sample of plankton obtained from the 0-25 m. level on February 4, 1954, in the south- 

 em part of the Norwegian Sea, reads as follows: 



108 



