These data show that the scale of time changes in the quantity of 

 phytoplankton is very great. Unfortunately, there are but limited 

 materials available for a judgment concerning the length of the maximum 

 period. In the coastal waters near the Southern Orkney Islands, the 

 time of maintenance of high concentration of chlorophyll "a" is about 3 

 weeks and the high indices of primary production last for about 2 weeks 

 (Home et al . , 1969), while in the area of Mirnyy, the duration of the 

 period of intensive production was less than 10 days (Klyashtorin, 

 1964). All of this indicates the very short duration of the periods of 

 high concentration of phytoplankton. The drop in phytoplankton occurs 

 during the period with a high content of nutrients in the water, and 

 cannot be explained by exhaustion of them. Probably, the reason for the 

 phytoplankton drop is the increased consumption of algae by the developing 

 generations of herbivores (Hart, 1942; El-Sayed, 1968b; Home et al . , 

 1969). However, no comparison of the rates of production and consumption 

 has ever been performed; the content of trace elements and vitamins has 

 never been determined, although the influence of these factors on the 

 development of algae may be very great (Provasoli, 1963; Belser, 1963). 

 Therefore, the concept of the connection between the seasonal decrease 

 of phytoplankton and the press of herbivores is as yet but a hypothesis. 



Vertical distribution . During the production period, the maximum 

 of phytoplankton population is located in most cases in the upper 10 or 

 20 meter layer (Hart, 1942; Hasle, 1969). It lies frequently not at the 

 surface, but rather at a depth of 5-10 m. The quantity of algae decreases 

 more or less rapidly with increasing depth below this layer. Usually, a 

 clear dependence is observed between the degree of the density stratifica- 

 tion of the water and that of the phytoplankton (Hasle, 1969). With 

 high stability of the surface layer, the algae are found in the uppermost 

 level, and most of their populations are in the euphotic zone, where 

 illumination is optimal. In the Southern Ocean, where nutrient salts 

 are not limited, we should not expect any negative influence of stability 

 on the production at the end of the season, as occurs in the northern 

 temperate regions (Riley, 1946). With stronger vertical motions of the 

 water, the distribution is more uniform, conditions of photosynthesis 

 deteriorate and the quantity of phytoplankton decreases. This is observed 

 in the area of the Antarctic convergence (El-Sayed et al . , 1964; El- 

 Sayed, Mandelli, 1965; Mandelli, 1967) and in places with intensive 

 upwelling of water, e.g., in the region of the cyclone near the divergence 

 to the north of Enderby land (Beklemishev, 1959), but not along the 

 entire zone of the divergence, as has been sometimes thought (Beklemishev, 

 1960; Kozlova, 1964). The influence of factors facilitating or preventing 

 an increase in productivity of phytoplankton is felt through the influence 

 on its vertical distribution. 



In many places, particularly in less stratified water, more than 

 half of the population is found in the hypophotic zone (Mandelli, Burkholder, 

 1966; Home et al . , 1969; El-Sayed, Dill, 1972). Sometimes even the 

 maximum population is located deeper than the compensation point. This, 

 apparently, is a result of its submergence from the higher levels, and 

 indicates a late stage in the seasonal succession. 



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