the Chaetognatha, Hydromedusae and Ctenophora. Though they do not make a 

 significant fraction of the diet of the animals at higher trophic levels 

 they, nevertheless, may influence their distribution, by consuming the 

 planktonic crustaceans to a significant extent and greatly reducing their 

 biomass. Of the chaetognaths , the most numerous are Sagitta elegans and 

 Eukrohnia hamata , regularly encountered in the plankton throughout the 

 year. Their greatest numbers are observed in the Norwegian Sea in late 

 spring and early summer (Gruzov, 1963), when they may represent up to 20% 

 of the plankton biomass. In the Labrador Basin, they are also present in 

 the plankton throughout the year: S^. elegans is encountered primarily 

 in the surface zone near the continental slope, while E. hamata is more 

 oceanically distributed and is found over a greater range of depths 

 (Kielhorn, 1952). The chaetognaths are not as important in the boreal 

 communities of the Atlantic as they are in certain tropical communities of 

 the world ocean, either in terms of number or in terms of biomass. 



The hydromedusa Aglantha digitale , widespread in the Arctic and 

 temperate regions, may form great concentrations in the temperate waters 

 of the iJorth Atlantic. In the central regions of the Norwegian Sea, it 

 develops in large numbers, regardless of the hydrologic conditions of the 

 year, approximately one month after C. finmarchicus s.l. reaches its 

 maximum population, usually in July. At this point there is a sudden drop 

 in the population and biomass of Calanus (Timokhina, 1968). A negative 

 correlation has been found between the population of A. digitale and C^. 

 finmarchicus s.l . (Gruzov, 1963; Pavshtiks, 1964). 



Seasonal changes in the plankton communities of the boreal Atlantic 

 have been studied in some detail for the Norwegian Sea, less for the 

 Labrador Basin, and still less for the open waters of the boreal Atlantic. 

 The season of biologic spring, characterized by the maximum development 

 of phytoplankton and the beginning of breeding of C^. finmarchicus s.l., 

 the Euphausiacea and certain others, begins earliest in the southern regions 

 of the boreal area, then gradually extends northward, being delayed in the 

 northern boreal waters by 2-3 months in comparison to the southern boreal 

 waters. Figure 10, based on the data of Pavshtiks (1966, 1969), Timokhina 

 (1962), Semenova (1964), Colebrook and Robinson (1965), shows the times 

 when biologic spring begins in the various regions of the boreal Atlantic. 

 Spring starts in the coastal waters of the southern boreal zone (February- 

 March), then encompasses the regions of the North Sea and the oceanic area 

 of the Atlantic south of 59°N (March-April). In May and June, biologic 

 spring extends to the central regions of the Labrador Basin and the 

 Norwegian Sea and, finally, in June-July, biologic spring extends into the 

 northern boreal regions of the Davis Strait and the northeastern area of 

 the Norwegian Sea. The maximum development of phytoplankton is followed by 

 a gradual increase in the biomass of phytophages. In the southern boreal 

 regions, the times of the maxima are closer together (Fig. 11). In the 

 Central areas of the Norwegian and Labrador Seas, the maximum biomass of 

 zooplankton is delayed by approximately one month relative to the peak of 

 production of phytoplankton: In the central waters of the Norwegian Sea, 

 the maximum biomass of Calanus usually occurs in June. 



72 



