During the year, the maxima of population of the individual species 

 follow one another in sequence. This phenomenon has been analyzed in 

 detail by L. N. Gruzov (1963) for various regions of the Norwegian Sea. 

 The relative significance of individual species in the community also 

 changes significantly from season to season. The differences from year 

 to year in the development of plankton have been most thoroughly studied 

 for the Norwegian Sea. Although in the open regions of the North Atlantic, 

 regular multiannual collections of plankton have also been made using an 

 automatic continuous plankton recorder, these data are difficult to compare 

 with the materials obtained using plankton nets. The differences from 

 year to year in the development of the plankton of the Labrador Basin have 

 not been studied, but it has been demonstrated for the Norwegian Sea 

 (Pavshtiks, Timokhina, 1972) that the beginning of biologic spring and 

 the duration of the seasons in the plankton depend on many abiotic factors, 

 particularly the number of sunny, storm-free days in March and April, 

 the intensity of the influx of warm water with the Norwegian Current, the 

 degree of summer heating of the water, etc. Storms and overall cloud cover 

 during the spring months delay the development of phytoplankton and the 

 subsequent mass breeding of the crustaceans. During relatively cold years 

 (1958, 1962, 1965, 1966), a delay and an extension in the time of biologic 

 spring has been observed in comparison to moderately warm or warm (1960) 

 years. Depending on abiotic factors, the interrelationships of species 

 within a community change from year to year. For example, the largest number 

 of the hydromedusa A. digitale occurs during cold years (1962-1963), the 

 maximum biomass--during warm years (1960-1961) (Timokhina, 1968). Apparently, 

 this is a result of the more rapid growth of medusae during warm years. 

 Since the number of medusae influences the number of planktonic crustaceans, 

 during different years the production of different links in the plankton 

 community may differ significantly (Table 4). 



Table 4. Production of main forms of zooplankton over entire Norwegian Sea 

 area, occupied by Atlantic waters, in 1959-1963 (in millions of tons) 

 (Timokhina, 1968). 



74 



