boundary amount to only 4-7°. In the central portion of the Pacific Ocean 

 sector (120-150°W), where the seasonal movements of the ice boundary amount 

 to about 10°, high stability is observed over the entire Antarctic zone 

 (Hasle, 1969). In general features, the boundaries of the maximum extent 

 of the icepack in winter agree well with the areas of stability of water in 

 the surface layer. Individual, local deviations are explained by vertical 

 movement of the water, related to the topography of the bottom (Ishino, 

 1963) or frontal zones. In the more northern regions, where the summer 

 stratification is formed only due to heating of the surface layer, its 

 stability is significantly lower. 



The formation of the summer type of density structure of the water 

 begins in the northern Antarctic zone in mid-October (Fig. 13); in the 

 Southern Antarctic later. The duration of its existence varies from 6 months 

 in the region of the convergence to 1-2 months in the high latitudes 

 (Makerov, 1956). 



Seasonal changes in the quantity of phytoplankton . The beginning of 

 the development of phytoplankton is determined by the increase of light 

 intensity and the formation of density stratification. What are its 

 subsequent changes? Long-term observations in the open waters of the 

 Southern Ocean have never been performed, and our concept of the seasonal 

 development of the plankton is therefore based on the results of summarization 

 of materials from cruises. Hart (1942) divided the Antarctic water area into 

 three latitudinal zones and averaged for each of them by months all the 

 quantitative data on phytoplankton, based on the concentration of pigments in 

 Harvey units. These results are presented in slightly altered form in Fig. 20. 

 In the northern zone, where light stops limiting photosynthesis as early as 

 September, the increase in the quantity of algae begins in October; the 

 maximum is reached in December, during the period of sharply increasing 

 stability of the surface layer; in March, the late-summer minimum occurs; 

 in April, a second maximum, significantly less than the December maximum, is 

 observed, followed by a sharp decrease, accompanying the autumnal mixing. 

 From north to south, the length of the period of vegetation decreases, the 

 time of the peak is increasingly delayed (January-February), and the 

 second maximum drops off in the intermediate zone, then disappears completely 

 in the southern zone. The correctness of these concepts, in their general 

 features, has been confirmed repeatedly (Vinogradov, Naumov, 1961; Hasle, 

 1969; Steyaert, 1973; Voronina, Zadorina, 1974). 



Extremely sparse materials are available for the Subantarctic. The 

 vernal maximum of plankton in the Atlantic sector is observed there primarily 

 in October-November (later in the south than in the north); the second peak, 

 apparently, occurs in March-May. In the Pacific Ocean sector, where the 

 boundary of the Subantarctic is shifted far to the south, the bloom is 

 observed in late December (Cassie, 1963; Hasle, 1969). 



This picture is quite schematic, and the limits of possible fluctuation 

 of individual indices are great. Among the factors defining the onset of 

 biologic spring (according to Bogorov, 1938), only changes in solar radiation 

 are constant with respect to time. The depth and intensity of the pycnocline 

 depend on the force and duration of the wind (Makerov, 1956); a decrease in 

 temperature may cause ice formation in the high latitudes even in mid-summer; 



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