Specific features and origin of the Caspian Sea diatom flora

The systematic composition of the diatom flora of the Caspian Sea is original, but poor, especially in marine species. At present, 286 species, varieties and forms, belonging to 46 genera and 4 orders are known to exist in the Caspian Sea. Of this number, 77 species and varieties (27%) are planktonic and 209 (63%) benthonic. The phytoplankton of the Caspian Sea is wholly neritic. In regard to salinity, a characteristic of the Caspian diatom flora is that it is composed of three different groups: marine, brackish, and fresh-water species. The brackish water group is of great interest, as it includes many diatoms which are known only from this water body (endemisms). The poor species-composition of the diatom flora is accounted for by variations in the hydrogical conditions of the Caspian Sea in the geological past. The formation of the flora is based on such sources as the ancient flora remains of the Upper-Tertiary Caspian-Black Sea, the younger elements of the Pliocene Sea during its isolation period, and the immigrants entering the Caspian Sea during the short period of its communication with the Black Sea.