448 



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

 Table 182 



III. The northwestern zone embraces a vast shallow (less than 150 m deep) 

 part of the Sea between the Crimea and the western coast. This zone, in con- 

 trast to the previous one, is the least saline part of the Sea, as a result of dilu- 

 tion by river waters. It is also the coldest in the winter. Huge accumulations 

 of Leophora are concentrated here ; forms tolerant of considerable loss of 

 salinity are abundantly represented (the molluscs Corbulomya maeotica, 

 Solen marginatus, Bamea Candida var. pontica, and others) ; on the other hand 

 many common Black Sea forms are absent (Patella, Littorina and Pecten 

 among the molluscs ; Amphioxus, Phoronis, Lygia, Saccocirrus, etc.). 



The lower limit of benthos. As mentioned above the lower limit of plankton 

 distribution in the Black Sea slopes from west to east. The same is observed 

 for benthos. V. Nikitin has shown (1938) that the lower limit of benthos runs 

 at different depths in different areas (Fig. 215) {Table 183). 



Only in the Bosporus area does the lower limit of benthos go down to a 

 depth of 170 to 200 m. The area occupied by benthos is about 2,900 km 2 . 

 Hence in the areas of circular currents plankton penetrates deeper than ben- 

 thos by about 25 to 40 m. The total area occupied by benthos in the Black 

 Sea is 95,360 km 2 or a little more than 23 per cent of the total Sea area. The 

 lower limit of bottom-life is related to a considerable decrease of oxygen- 

 content (2 to 5 per cent) and an increase of carbon dioxide (pH 7-7 to 7-6). 



Fig. 215. Lower limit of zoobenthos in Black Sea (Nikitin, 1938). 



