Biodiversity, distribution and abundance of zooplankton in the Iranian waters of the Caspian Sea off Anzali during 1996-2010

The mesozooplankton of the south-western Caspian Sea, off Anzali, sampled from 1996-2010, had undergone severe changes, especially after the year 2001, when the invasive ctenophore Mnemiopsis leidyi bloomed for the first time. Native species vanished or decreased, while invasive species such as Pleopis polyphemoides, Acartia tonsa, M. leidyi and the larvae of Balanus sp., plus Hediste diversicolor dominated the zooplankton. It could be stated that the increasing amount of nutrients since the early 1980s led to a decrease of endemic species and smoothed the way for opportunistic invader species, which outcompeted and depleted the endemic species in their turn. However, the major changes in the zooplankton community and possibly the blooming of M. leidyi during 2001-2002 were triggered by changing weather patterns, when a period with heavy rain at the end of the 1990s was followed by a prolonged drought (2001-2002). It is not clear to what extent M. leidyi was responsible for the disappearance of endemic Copepoda and Cladocera species such as Eurytemora grimmi, Limnocalanus grimaldii, Cercopagis pengoi, and Polyphemus exiguous, because the mesozooplantic invader species seemed to be more successful competitors than the endemic Caspian Sea fauna. Compared with other areas of the Caspian Sea, the development of the M. leidyi stock was moderate in the area under investigation. Mnemiopsis leidyi numbers and biomass increased from the beginning of sampling in 2001 to about 600n.m(-3) and 40-60g wet weightm(-3) until 2003. Since then the stock oscillated in this range until 2010.