THE BLACK SEA 



411 



of thousands as one moves away from the coast, and especially in zones with 

 strong influence of coastal run-off. The largest number of micro-organisms is 

 found at depths of 10 to 75 m. 



Bacteria biomass changes with the same regularity. The minimum number 

 of micro-organisms is found at depths of 75 to 1 50 m, where the bacterial 

 population of the upper layer is replaced by the community of the hydrogen 

 sulphide zone (mainly by a particular group of filamentary micro-organisms). 

 The number of micro-organisms in the hydrogen sulphide zone is considerably 



DISTANCE FROM THE COAST, MILES 



Fig. 195. Distribution of micro-organism population in Black Sea and the alteration 

 of its density with the distance from the coast (Kriss). 



higher than that of the surface oxygenated zone. Another sharp rise in the 

 number of micro-organisms is observed in the upper layer of the sea-bed, 

 where it reaches 1^ to 3 milliards per 1 g of the wet weight with a biomass of 

 3 to 6 g/mA The effect of river discharge on the number of micro-organisms 

 in the water is clearly shown by M. Lebedeva (Fig. 196). The quantitative 

 range of micro-organisms changes in winter, owing to a considerable fall in 

 temperature : their main mass is then concentrated in the to 50 m layer. The 

 number of bacteria decreases sharply deeper down, only increasing again in 

 the hydrogen sulphide zone (Fig. 197). Kriss gives comparative values of 

 phyto- and zoo-plankton biomass as an illustration to his data {Table 163). 

 According to these data the biomass of bacteria is considerably higher than 

 the quantity of plant and animal plankton, and if we take into consideration 

 a much greater biological activity of the micro-organisms, their immense 

 importance will become evident both for the phytoplankton and zooplankton 

 of the surface zone and for the biochemical processes in the hydrogen sulphide 

 zone. In the oxygenated zone a direct connection can be observed between 

 the quantity of micro-organisms and the plant and animal population, both 

 in the main mass of water and in the sea-bed. 



Kriss determined the rates of multiplication of micro-organisms by a direct 





