THE CASPIAN SEA 561 



winter is about 10 per cent of the whole volume of water. Intensive'early 

 summer flooding, which freshens this part of the sea, considerable seasonal 

 fluctuations of water-exchange with the Central Caspian, sharp fluctuations 

 in the amounts of plant nutrients brought in with fresh waters, all these 

 factors make the Northern Caspian saline conditions unstable and change- 

 able. The large river-mouth areas of the Sea become almost completely 

 freshened under the effect of river water. Salinity increases in the south, and 

 on the boundary between the Northern and Central Caspian (along the line 

 Chechen' Island-Mangishlak peninsula) the average salinity is 5T% by 

 chlorine* (the general salinity being 12-1% ). Salinity decreases to the north 

 of this line. The Northern Caspian mainly has a salinity of 8 or 9% : only in 

 the estuaries of the rivers Volga and Ural does the salinity drop sharply. 

 Huge amounts of plant nutrients are brought into the Northern Caspian with 

 the fresh water ; they are found in amounts maximal for the whole of the 

 Caspian Sea just to seaward of the Volga and Ural delta : up to 40 mg phos- 

 phorus, up to 2,800 mg silicon, up to 250 mg nitrogen as nitrates per 1 m 3 . 

 The junction of the fresh waters rich in plant nutrients with the more saline 

 waters is a place of huge plankton development (up to 2,000 to 4,000 mg/m 3 ). 

 A kind of powerful phytoplankton filter is created and only a very small 

 quantity of plant nutrients passes through it, so that outside it the quantity 

 of plankton diminishes sharply to a few or a few tenths of mm 3 per 1 m 3 . 

 Hence, since the plant nutrients are almost completely used up by the great 

 gatherings of plankton just seaward of the deltas, the Sea is supplied with it 

 not directly from the river inflow, but only from detritus plant nutrients and 

 from the diluted organic, and probably colloidal, compounds. The bottom 

 deposits of the zone situated to seaward of the deltas play the role of a store- 

 house for a definite period. The distribution of huge silt deposits in the 

 Northern Caspian, forming wide bands in front of the Terek, Volga and Ural 

 estuaries, is in complete accord with this. 



Changes in depth of vertical circulation 



Being distributed throughout the whole Sea, plant nutrients drift finally into 

 the deep depressions of the Central and Southern Caspian ; return from there 

 is difficult and rare. However, there is another factor which influences the 

 return of plant food from the deep depressions, which act as huge store- 

 houses. 



The upper layer of the Sea would get either more or less saline as a result 

 of an increase or decrease of river inflow, which would also cause either a 

 rise or a fall of the level of the Caspian. However small the salinity fluctua- 

 tions of the upper layer of the Sea they would affect the vertical mixing of 

 waters. With increase of salinity in the upper layer the lower limit of vertical 

 circulation goes deeper (possibly only by a few tens of metres), especially in 

 winter ; deeper layers of the sea rich in plant nutrients will then be drawn into 



* The usual Knudsen formula for the determination of the total salinity of marine 

 water from the chlorine numbers cannot be applied to the Caspian Sea, and the co- 

 efficient 2-38, established by Lebedintzev, is used instead — S% = Clx2-38. 



2N 



