Population dynamics of Mnemiopsis leidyi in the Caspian Sea, and effects on the Caspian ecosystem

The ctenophore Mnemiopsis leidyi was introduced to the Caspian Sea, most probably from the Black Sea. In 2000, it spread across all areas of the Caspian with a minimum salinity of 4.3parts per thousand. In 2001-2002 its population size expanded to reach a critical wet mean biomass of about 1 kg. m(-2) (55 g m(-3)). In 2001 (allowing a coefficient 2 to compensate for imperfect catchability), its abundance was in excess of the highest values ever recorded in the Black Sea. Yet, in 2002, its biomass again doubled. Effects on the ecosystem were faster and stronger than in the Black Sea. In 2001, repercussions were felt at all trophic levels, including that of the top predator, the Caspian Seal. The functioning of the ecosystem changed in the same way as that of the Black Sea. Cascading effect occurred at the higher trophic levels, from a decreasing zooplankton stock to collapsing planktivorous fish to vanishing predatory fish and seal. Similar effects occurred at lower trophic levels: from a decrease in zooplankton stock to an increase in phytoplankton, released from zooplankton grazing pressure. Part of these effects were top-down, part were bottom-up. In conclusion, the Caspian Sea example provides yet another illustration of the fact that a lower gelatinous carnivore, well adapted to rapid expansion, can suppress whole ecosystems and their functioning.