## 3 Results and Discussion
### Ecological Conditions of the Caspian Sea  
The Caspian Sea is the largest inland, cross-border international water body lapping the shores of five Caspian states. It is also the recipient of a huge volume of runoff from many rivers that carry various contaminants into the world largestinland lake. The uniqueness of the Caspian Sea as the world's largest habitat for surgeon species of fish and marine mammals, the endemic of this reservoir--the _Pusa caspica_, brings its problems not only to the interstate, but also to the global level. According to the nature of the underwater terrain, the Caspian Sea is sharply divided into three parts: North, Middle and South. They are almost equal in area, but they are very different in water volume. The northern part accounts for about 1/100 of the total water volume, the middle part 1/3 and, and the southern part 2/3. The northern Caspian Sea, in turn, is divided into two parts: the eastern and western, the border between which runs along the line connecting Kulaly Island with Novinsky one, i.e., the eastern part of the Northern and Middle Caspian is completely located in Kazakhstan (Figure 7). Figure 7: Kazakh part of the Caspian Sea with environmental observation stations (Source: [52]). The length of the coastline of the Kazakh coast is 2320 km or 39% of the total coastline of the Caspian Sea, equal to 5970 km. Kazakhstan's sector is located on the shelf, in a relatively shallow zone with depths of mostly 4-8 m. Shallow water, small water volume, and a limited level of hydrodynamic processes are reasons for low intensive self-purification processes in this water body. It is 8 times slower than in the deep-water bodies of the Middle and Southern Caspian Sea. The average depth of the northern Caspian Sea and the area of its water surface significantly depend on water level fluctuations, the scope of which was 3 m in the 20 th century. At the sea level--28.0 m b.s.l., the average depth of the northern Caspian is 4.5 m. The area of the water surface is 90 thousand km\({}^{2}\), and the water volume 397 km\({}^{3}\). At a level--27.0 m b.s.l, close to, the area of the Northern Caspian is 105,000 km\({}^{2}\), and the water volume is 442 km\({}^{3}\)[43]. In terms of biological productivity, the Northern Caspian Sea is the second largest water body in the world (after the Azov Sea). A highly productive photosynthesis activity of the phytocenosis has formed a natural habitat with a powerful food base for fish and animals. Thus, there are special natural conditions in this flat water body that are most sensitive to external influences. In comparison with the rest of the deep-Caspian Sea part, the negative impact of toxic compounds there is many times stronger [43; 44]. Currently, under the influence of a number of powerful anthropogenic factors, the water body is being destabilized. The contamination sources of the Caspian Sea are diverse, and they are spread on the territories of all Caspian states, including their water bodies. The development of oil and gas fields in the Caspian region is currently taking place at an increasing pace. This causes contaminations of the lake water with oil products and their accompanying toxic compounds. Under these influences the influx of contaminants along the Volga and Ural Rivers, as well as some other anthropogenic factors, the Caspian Sea water body has been destabilized. Rivers are the main source of oil contamination in the Caspian Sea. They carry 75,000 tons/year oil products to the Caspian Sea, according to the Caspian Thematic Center for Contamination Control (Baku) [44]. This is 47.0% of the total amount of hydrocarbon contamination entering the lake. 95.5% (71,600 tons/year) of the river flow of petroleum hydrocarbons is provided by the Volga River. Part of the Volga runoff extends to the East, largely determining the background concentration of toxicants in the North-Eastern Caspian Sea.