THE CASPIAN SEA 559 



The upper zone is the area of phy toplankton activity, with intensive photo- 

 synthesis proceeding mainly in the 25 to 50 m layer. Below 100 m there is 

 accumulation of organic matter and plant nutrients caused by the sinking 

 down of the remains of dying plankton, while vertical circulation is not 

 sufficiently strong to bring them up in any considerable quantities; thus 

 accumulation is greater than consumption. Within the zone of impoverish- 

 ment of plant nutrients only the 25 to 50 m layer (on the average 35 m) is 

 characterized by intensive photosynthesis (the subzone of photosynthesis). 

 Deeper down, sunlight does not penetrate in the amounts required for intensive 

 phytoplankton development. The nitrites are accumulated below the photo- 

 synthesis subzone as a result of the decomposition of plankton organisms, 

 which sink into this subzone (the nitrites subzone). The two subzones of the 

 upper zone are divided in summer time by a layer with a sharp temperature 

 drop and are hardly mixed at all. The upper zone is intensively mixed when 

 the surface water is cooled, and plant nutrients, which had disappeared from 

 the upper layer in the summer, are distributed throughout its whole column. 

 Within the accumulation zone the oxygen content decreases while the plant 

 nutrients increase with depth. A considerable accumulation of nitrates, mostly 

 at depths of 200 to 400 m, is characteristic of the upper part of this zone ; 

 at a greater depth (below 400 m) ammonia nitrification becomes impossible 

 owing to a shortage of oxygen and the process stops at the ammonia stage. 

 A sharp decrease of oxygen content is characteristic of the lower boundary 

 of the nitrate subzone. 



Mean data along the cross section Kurinsky Kamen'-Ogurchinsky Island 

 for August 1933 are given in Table 233 and in Fig. 269. The letters and Roman 

 figures correspond to Brujevitch's zones and subzones in Table 232. 



Naturally none of these boundaries remains constant, especially in differ- 

 ent parts of the Caspian Sea, and some of them frequently do not coincide 

 with each other. Sharp changes in quantities of phosphates do not coincide 

 with the boundary of the accumulation of nitrates and silicic acid, etc. The 



