398 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



N. Chigirin (1930), the first worker to investigate the distribution of phos- 

 phates in the Black Sea, came to a number of interesting conclusions : phos- 

 phorus of dead plants remains mostly in the oxidation zone, while that of most 

 dead animals is driven into the reduction zone and accumulates there. Sixty 

 per cent of the total plankton phosphorus may consist of the latter ; the annual 

 amount of phosphorus brought in with river water forms about 1 per cent of 

 the total amount of phosphates dissolved in the oxidation zone. Hence some 

 definite amounts of phosphorus compounds are brought in from the reduction 

 to the oxidation zone. An alkalinity two to three times higher than that of the 

 open seas and considerably greater fluctuations in hydrogen ion concentra- 

 tion are also most characteristic of the waters of the Black Sea. 



Dynamics of organic matter 



All life in the Black Sea is concentrated in the upper layer, owing to its oxygen 

 and hydrogen sulphide distribution ; this layer is 150 to 200 m thick, forming 

 only 10 to 15 per cent of the volume of the Sea. The immense volume of the 

 deeper layers (85 to 90 per cent) is inhabited only by anaerobic bacteria. 

 Organic substances which reach the depths from the upper layer return to a 

 small extent and accumulate at the bottom. The feeble vertical circulation, 

 resulting in the accumulation of large amounts of organic matter in the depths, 

 also decreases the productive capacity of the Sea. As has been shown by 

 Danilchenko and Chigirin, the oxidation of nitrogen, ammonia and nitrites 

 to nitric acid ; of sulphur, sulphides, bisulphites, sulphites and hydrogen sul- 

 phide to sulphuric acid ; and the oxidation of ferrous and manganous com- 

 pounds into the ferric and manganic ones, takes place in the oxidation zone ; 

 there are no nitrites or nitrates in the reduction zone, which contains com- 

 paratively large amounts of ammonia and nitrogen, a smaller amount of 

 sulphates, and a larger one of carbonates and bicarbonates. Since hydrogen 

 sulphide is formed by the reduction of sulphates, in the deep layers of the 

 reduction zone their content is greatly reduced. In the hydrogen sulphide zone of 

 the Black Sea carbon, hydrogen, sulphur, phosphorus and silicon accumulate, 

 as well as nitrogen compounds. The combination of these conditions with the 

 existence of the hydrogen sulphide zone leads to a comparatively low general and 

 industrial productivity of the Black Sea in comparison with the Sea of Azov. 



Sediments 



The sediments of the Black Sea can be divided into two groups : those of the 

 oxidation zone (continental shelf) and those of the reduction zone (continental 

 slope and central depression). The shallow-water sediments were compre- 

 hensively investigated by S. Zernov in the first decade of the present century. 

 Deep-water sediments were thoroughly studied in the Soviet era (1924-33) 

 mainly by A. Archangelsky* (Fig. 191). The floor topography of the Black 



* In this work Archangelsky succeeded in obtaining, by means of so-called core tubes, 

 a bottom core in 4 m in length, and deep-water sediments of various parts of the Sea were 

 synchronized by them. On the other hand, the micro-lamination of these sediments, 

 which in Archangelsky's opinion is annual, gave him the possibility of expressing in 

 chronological order the duration of the deposition periods of each sediment. 



