Invasion of the Black Sea by the ctenophore Mnemiopsis leidyi and recent changes in pelagic community structure

A short synthesis of the present state of the ctenophore, Mnemiopsis leidyi, invasion in the Black Sea is given, together with a brief review of its status in other areas of the Mediterranean basin. The impact of M, leidyi on the main components of the pelagic community, mesozooplankton, ichthyoplankton and fish resources, based on published data and new field studies (1992-1997) are analysed. This assessment showed sharp fluctuations in the interannual abundance of M. leidyi. The main factors controlling the spatial distribution of M. leidyi were temperature and, to a lesser degree, salinity, whereas its abundance was controlled by food availability. An analysis of the main constituents of the pelagic ecosystem of the Black Sea before the M. leidyi outbreak showed that a reduction in numbers of planktivorous fishes, the main competitors of M. leidyi, could be a possible reason for the upsurge in abundance of M. leidyi. Following the increase of M. leidyi, there was a decline in the abundance and species diversity of ichthyoplankton and mesozooplankton. An assessment of data collected during the period 1992-1997 showed that the number of fish eggs and larvae and of zooplankton was negatively related to M. leidyi abundance. After the recent decrease of M. leidyi in the period 1995-1997, there has been an increase in abundance and diversity of fish eggs, fish larvae, and zooplankton, which together with an increased catch of planktivorous fish indicates that there has been a recovery of the ecosystem.