. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE CASPIAN SEA 617. S%„/ 2 3 4 5 ? 7 8 9 10 11 12 13 14 2 4 6 8 10 12 14 16 18 20 22 24 26 28m SALINITY DEPTH Fig. 290a. Distribution of benthos Fig. 290b. Distribution of benthos biomass biomass of Northern Caspian accord- of Northern Caspian according to depth ing to salinity (Birstein, 1939). (Birstein, 1939). of sea-water. In calm weather a definite vertical stratification is observed in this zone, since the fresh waters of the Volga and Ural flow over the saline sea-water. When the processes
. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE CASPIAN SEA 617. S%„/ 2 3 4 5 ? 7 8 9 10 11 12 13 14 2 4 6 8 10 12 14 16 18 20 22 24 26 28m SALINITY DEPTH Fig. 290a. Distribution of benthos Fig. 290b. Distribution of benthos biomass biomass of Northern Caspian accord- of Northern Caspian according to depth ing to salinity (Birstein, 1939). (Birstein, 1939). of sea-water. In calm weather a definite vertical stratification is observed in this zone, since the fresh waters of the Volga and Ural flow over the saline sea-water. When the processes of decomposition of the organic substances of the sea-bed become intensive owing to conditions of vertical stratification, oxygen is used for the oxidation of the soil and the bottom layer loses much of its oxygen. This oppresses many benthic animals and the heap of wrack becomes inhabited by euryoxybiotic forms which can live in water deficient in oxygen. This problem was discussed above when dealing with brackish water. In the Northern Caspian the main benthos biomass is found at depths of 12 to 16 m, whereas its average depth is only 6 m (Fig. 290b). The relationship between the benthos biomass and the nature of the sea-bed is just as indica- tive. It has been shown for many seas that on mobile hard floor (gravel, shell gravel, large-grain sands) the fauna becomes scarce and sometimes dis- appears. This, however, is not so in the Caspian Sea. Low biomass indices are found on soft beds, situated chiefly along the 12 to 18 ft of wrack {Table 261). This is Table 261 Sea-bed Mean biomass, g/m2 Shell-gravel 79-0 Sand-shell-gravel 32-2 Sand 23-9 Sand-shell-gravel-ooze 26-7 Ooze-shell-gravel 18-8 Ooze 8-8 Ooze with hydrogen sulphide 0-9. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Zenkevich, L. A. (Lev Ale
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