<i>PSEUDOSOLENIA CALCAR-AVIS</i> (BACILLARIOPHYTA, CENTROPHYCEAEA) IN THE CASPIAN SEA
The diatom Pseudosolenia (Rhizosolenia) calcar-avis was introduced to the Caspian Sea in 1934. During a year it spread throughout the water area with salinity higher than 8‰ and dominated in phytoplankton community constituting 80–90% of the total biomass in the Middle and Southern Caspian Sea and 20–40% in the Northern Caspian Sea. After this introduction the role of small-celled diatom algae Rhizosolenia fragilissima and pyrrhophyte Exuviaella cordata dominated earlier, had sharply decreased. The first species was found in single instances, the biomass of the second one reduced substantially. The P. calcar-avis domination was observed until 2000 at all environmental changes connected with the sea level fluctuations. In 2000 small-celled algae, including R. fragilissima began to develop intensively and in 2001 the P. calcar-avis quantity in the total biomass reduced to 5–10% in the Middle and Southern Caspian. This happened after the comb jelly Mnemiopsis leidyi invasion, which sharply decreased the number of zooplankton phytophages that led to lessen the intensity of phytoplankton eating away. P. calcar-avis, which previously successfully competed with a native phytoplankton, thanks to that nobody feed on it because of it big sizes, had lost the advantage, and small-celled algae previously eaten away began to dominate. The grazing pressure decrease has facilitated penetration into the Caspian Sea of new phytoplankton species. In the 20 <sup>th</sup> century during 70 years only 2 species penetrated into the Caspian Sea; however, during 9 years after the comb jelly invasion at least 7 species appeared in the Caspian.