. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE CASPIAN SEA 555 organic substances, either brought by the rivers (allochthonous), or gathered in the Sea itself as the remains of dead animals and, still more, dead plants (autochthonous). Such regions are most frequent in the Northern Caspian, often in very shallow places. In the eastern part of the Northern Caucasus, in the Kaidak and Mangishlak areas, and in Krasnovodsk Bay large areas of the bottom are covered by muds many metres thick, the so-called batkaki, rich in organic substances with
. Biology of the seas of the Marine biology -- Soviet Union; Hydrology -- Soviet Union. THE CASPIAN SEA 555 organic substances, either brought by the rivers (allochthonous), or gathered in the Sea itself as the remains of dead animals and, still more, dead plants (autochthonous). Such regions are most frequent in the Northern Caspian, often in very shallow places. In the eastern part of the Northern Caucasus, in the Kaidak and Mangishlak areas, and in Krasnovodsk Bay large areas of the bottom are covered by muds many metres thick, the so-called batkaki, rich in organic substances with a thick bacterial crust, evolving huge amounts of methane and hydrogen sulphide. According to A. Sadovsky (1929) a 4 cm. SOUTHERN CASPIAN Fig. 265. Distribution of nitrate nitrogen in the Caspian Sea waters (mg/m3) in cross section (Brujevitch, 1934). layer of mud is accumulated there annually. Throughout the northern shore of the Northern Caspian, in Agrakhansk Bay and in Krasnovodsk Bay, we find similar zones of huge deposits of decaying organic matter. They also fill the central part of the Ural trench. Under certain conditions, when the water in these shallows gets thoroughly mixed by a gale, the top layer of the soil may be washed away and hydrogen sulphide may enter the water. These pheno- mena may sometimes become acute and lead to suffocation. It is a purely local phenomenon, linked with the occurrence of muds rich in hydrogen sulphide in very shallow areas. Let us now consider the content of nitrogen, phosphorus and silicon com- pounds in the Caspian Sea waters (Figs. 265, 266 and 267). Nitrogen Ammonia nitrogen content in the Caspian is about the same as that of the Baltic and North Seas, higher than in the ocean, but in deeper layers much lower than that in the Black Sea. Its amount in the Caspian fluctuates within a few tens (20 to 50, and in the Southern up to 70 mg/m3). The nature of the distribution of nitrites is similar to that of other seas. In winter the nit
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