state that over most of the Southern Ocean, the suppressing effect of 

 this factor does not appear. 



The absence of light suppression of photosynthesis is obvious from 

 numerous data on primary production vertical distribution (El-Sayed, 

 1968a). 



The thickness of the euphotic layer is determined by the position 

 of the compensation point. In works on primary production, it has been 

 the practice to consider it to be located at the depth to which 1% of 

 the incident light penetrates. In the Southern Ocean, this depth varies 

 during the summer season from 7 to 95 m (Hasle, 1969; El-Sayed, 1968a, 

 1970b; Steyaert, 1973), depending on the height of the sun over the 

 horizon and the quantity of phytopl ankton present (Sverdrup, 1953; Hart, 

 1962). In 78°^ of cases, the compensation point is at a depth of less 

 than 50 m. This means that the production upon which the population of 

 the entire mass of the ocean feeds is formed in this very thin layer. 



The beginning of the light season is a necessary, but hardly sufficient 

 condition for the development of phytoplankton. In the spring, when the 

 entire layer of surface water down to a depth of 100-200 m is well 

 mixed, cells are drawn away from the surface into the unlit depths, 

 where respiration predominates over photosynthesis. Therefore, in order 

 for the development of phytoplankton to begin, it is very important for the 

 Summer pycnocline to form, limiting the vertical extent of the convection 

 layer, and for stable stratification to occur, helping the cells to remain 

 for a longer period of time at the surface. 



Changes in the density structure of the water occur as a result of 

 thawing of ice or the spring rise in temperature over the water area which 

 is free of ice. Both of these processes lead to development of a layer of 

 low density, the thickness of which depends on the intensity of wind mixing. 

 Usually, it increases within the limits of the Antarctic from 10-20 m in 

 the south to 80-100 m in the north, being 40-60 m in most regions, i.e., 

 near the thickness of the euphotic layer. The lower boundary of the 

 summer transformed water is characterized by an increase in the density 

 gradient (Makerov, 1956). The vertical stability, reflecting the character 

 of the change in density with depth, is great throughout the entire surface 

 layer during the warm season in the high latitudes. Further north, there 

 is a fairly thick mixed layer. The development of the mixed layer is 

 characteristically lower and the region with high stability has a greater 

 longitudinal spread in the Atlantic sector than in the eastern portions of 

 the Pacific and Indian Ocean sectors. For example, at the 0° meridian, 

 the value of E'lO^ > 1000 in the 0-50 m layer extends from the edge of the 

 ice right up to 55°30'S, while at 78°W, it extends only to 65°S, and at 115°E-- 

 to 63°30'S (Ishino, 1963). This is apparently related to the significant 

 differences in the conditions of formation of the summer stratification. 

 Since the changes in the density structure of the water between the -2 C 

 and the +2 C isotherms are determined almost entirely by salinity, differences 

 in the ice content of the individual water areas become decisive in their 

 significance (see Fig. 12). In the Atlantic sector, the maximum winter 

 extent of the icepack reaches 54°S. while the summer ice rim lies at around 

 70°, but in the eastern parts of the Indian and Pacific Ocean sectors the 

 ice does not extend as far to the north, and the seasonal movements of the 



82 



