Long-term transformation of the mesoplankton structure from the coastal waters of the Northeastern Black Sea under the impact of the ctenophores-invaders

Results of the long-term observations of the variability of mesoplankton in Golubaya Bight (south of Novorossisk) are analyzed. The variability is related to the appearance in the ecosystem of the ctenophore-invader Mnemiopsis leidyi in the late 1980s and then, at the end of the 1990s, another invader, Beroe ovata. The changes in the structure and seasonal dynamics of zooplankton during two periods, namely, from 1991 to 1998 and from 1999 to 2001, are estimated. These data are compared with those referring to the period preceding the appearance of Mnemiopsis leidyi in the ecosystem. In the period from 1992 to 1998, the mean annual biomass of the forage mesoplankton was one order of magnitude lower than that before the invasion of Mnemiopsis leidyi (2.6 instead of 65 mg/m(3)). Due to the invasion of Beroe ovata feeding on Mnemiopsis leidyi, the structure and concentration of mesoplankton began to be restored. In 3 the period from 1999 to 2001, the mean annual biomass of the forage mesoplankton increased up to 14 mg/m(3), i.e., approximately fivefold as compared with the preceding period. The variability of all the principal components of the forage zooplankton is discussed.