S. Bagheri
U. Niermann
J. Sabkara
A. Mirzajani
H. Babaei

 Mnemiopsis leidyi which was accidentally introduced into the Caspian Sea in ۱۹۹۹ and since then has colonized extensively. The horizontal distribution of M. leidyi and dominant mesozooplankton species was investigated in the south western Caspian Sea during February, May, July and November ۲۰۰۸. The average number and biomass of M. leidyi were in the same range (ca ۲۰۰ individuals.m-۳ (۲۰۰۰ ind.m-۲) and ۱۶ g wet weight.m-۳ (۱۸۰ g.m-۲) in comparison with previous surveys. As in previous years the population consisted mainly of individuals <۱ cm. The decline in mesozooplankton species observed since ۱۹۹۶ continued in ۲۰۰۸. Only two species of the previously recorded ۲۴ Cladocera species were found in ۲۰۰۸. Of five Copepoda species recorded in ۱۹۹۶, only one, Acartia tonsa, was found in ۲۰۰۸ and even here adult individuals have reduced ۳-fold since ۱۹۹۶. Bivalve larvae have declined by one order of magnitude since ۱۹۹۶. Among the dominant species, only the numbers of Cirripedia larvae and in part the numbers of Pleopis polyphemoides (Cladocera) were in the same range as in ۱۹۹۶. 

Mnemiopsis leidyi, Acartia tonsa, Mesozooplankton, Long-term fluctuation, Caspian Sea