THE CASPIAN SEA 597 



were also observed in the relationship between individual groups : thus, for 

 example, in September 1934 Rotifera comprised 9 per cent, and in September 

 1 935 17 per cent, of the zooplankton biomass. A gradual drop of zooplankton 

 biomass takes place in October and November, accompanied by an increase 

 in the relative significance of Calanipeda. The average zooplankton biomass 

 for October 1935 was 92 mg/m 3 , 56 per cent of it being Calanipeda; for 

 November 1935 the average biomass was 35 mg/m 3 with 65 per cent Calani- 

 peda. 



This difference between the September data of 1934 and 1935 can be 

 explained by the hydrological conditions of the Northern Caspian in 1935. 

 In autumn 1935 the southern part of the Northern Caspian was exceptionally 

 enriched by plant food, brought, apparently, from great depths of the Central 

 Caspian and carried far to the north owing to the increased flow of Central 

 Caspian waters. This brought about an intensive Rhizosolenia bloom in the 

 southern and middle parts of the Northern Caspian and also, no doubt, 

 favoured zooplankton development. 



Zooplankton distribution is not uniform in the Northern Caspian (see Fig. 

 282). As early as 1921 N. Tchugunov distinguished there three plankton zones 

 characterized by their specific composition and the extent of quantitative 

 development of zooplankton, controlled primarily by salinity : (7) the mouths 

 of the Volga and Ural rivers with their lowered salinity of 0-3 to O4% ; (2) 

 the zone of mixing of the saline and fresh waters, with a salinity of 8 to 

 9% , approximately within the 12 to 18 ft bar of material carried down by the 

 rivers; (3) the saline zone, with a salinity of 10 to 12% exposed to the direct 

 influence of the Central Caspian, occupying the southern and central part 

 of the western half of the Northern Caspian. The boundaries between these 

 zones are naturally very unstable, change frequently, and can approach each 

 other depending on the amount of flood water, wind, etc. 



The zooplankton of the first zone is poor, and consists only of fresh-water 

 species. 



The next zone, richest in number and wealth of zooplankton, is populated 

 by typically brackish-water organisms. In early spring the average zooplank- 

 ton biomass of this zone is 16-5 mg/m 3 , say three times higher than in the 

 first zone ; by the end of May and the beginning of June it is 92 mg/m 3 , in 

 August 130 mg/m 3 , in September 160 mg/m 3 and in October* 154 mg/m 3 . It 

 was in this zone that the maximum phytoplankton development was recorded. 



The zooplankton of the third zone is considerably poorer both in numbers 

 and variety of species. Several species are not found here and the remaining 

 ones do not reach mass development. The average zooplankton biomass of 

 this zone in August 1935 was 20 mg/m 3 , in September 1935 27 mg/m 3 , and 

 in October 1935 only 13 mg/m 3 . 



The difference in zooplankton biomass in these zones is controlled by other 

 factors as well as salinity, which limits the range of one or another species ; 

 as has been shown by Kusmorskaya, plant food content in various areas is of 

 great significance in this respect. 



* The data for May, June, August, September and October are given only for the 

 western part of the Northern Caspian. 



