The biomass of North Caspian zooplankton. The seasonal changes in the 

 composition and numbers of North Caspian zooplankton are very marked. 

 In winter zooplankton is very poor in both numbers and variety, comprising 

 only four forms {Calanipeda aquae dulcis, Halicyclops sarsi, Ectinosoma sp. 

 and Synchaeta sp.). 99-7 per cent of its biomass consists of C. aquae dulcis 

 and is on the average only 9-5 mg/m 3 . There are 350 specimens per 1 m 3 . A 

 rapid qualitative and quantitative increase of zooplankton is observed in 

 April and May: its average biomass in April is 15 mg/m 3 and in May 58 

 mg/m 3 . The number of species found rises to 30 in April and 40 in May. This 

 is connected primarily with its multiplication, which begins in the spring. 

 The main mass of zooplankton, as regards specimens, is composed of Hali- 

 cyclops sarsi, Harpacticoida and Lamellibranchiata in April ; there are then 

 few Rotatoria and Cladocera. At the end of May and the beginning of June 

 the flood waters move much farther south and therefore the number of fresh- 

 water Rotatoria and Cladocera increases considerably. Intensive multi- 

 plication of almost all plankton forms proceeds simultaneously, chiefly that 

 of Calanipeda, which by that time occupies first place in the biomass. Further 

 growth of zooplankton biomass takes place in the summer, and in the mouth 

 of the river Volga it increases by 70 per cent by August as compared with 

 April. The intensive multiplication of the majority of planktons continues, 

 and in this respect the August plankton does not differ much from that of May. 

 In August the mean biomass in the western half of the Northern Caspian 

 (less productive in zooplankton than the eastern half) constitutes 60 mg/m 3 , 

 the average number of specimens being 6,950 per 1 m 3 . An extinction of zoo- 

 plankton was observed in September 1934. It could not be considered a 

 consequence of strong changes in the hydrological conditions of the Sea, 

 since in this respect the difference between August and September is slight. 

 It may be that the food resources of the Sea were exhausted by September. 

 The zooplankton biomass dropped at that time to 15 mg/m 3 . In September 

 1925 zooplankton was found to be much richer than in the previous year. 

 The average zooplankton biomass then was 100 mg/m 3 . Some differences 



