The Effect of the Ctenophore Mnemiopsis leidyi (Ctenophora: Lobata) on the Population Density and Species Composition of Mesoplankton in Inshore Waters of the Caspian Sea

摘要

This paper examined the qualitative and quantitative of food composition of the ctenophore Mnemiopsis leidyi, as well as its feeding intensity and predatory impact on selected mesoplankton species in the inshore waters of the southern Caspian Sea. The ctenophore Mnemiopsis leidyi initially increased during 2001-2002 with the abundance and biomass of 381-480 ind.m~(-3) and 15.1-27.7 g.m~(-3), respectively, and during recent years (2012-2013) its population and biomass reached at a low level with an average of 5-46 ind.m~(-3) and 0.6-4.0 g.m~(-3). The highest summer-autumn average of M. leidyi abundance was observed in 2002 with 1338 ± 334 ind.m~(-3) and the maximum biomass in 2001 with 77.5±19.4 g.m~(-3). Over the years, 2003-2012, M. leidyi population did not reach to its initial value and the mean range of abundance and biomass 5-25 ind.m~(-3) and 0.6-19.2 g.m~(-3). Interannual and interspecies differences in the intensity of predatory impact on zooplankton were found. Copepod nauplii of Acartia (Acanthacartia) tonsa, the cladocerans Podon (Pleopsis) polyphemoides, experienced the highest pressure from ctenophore predation. Zooplankton species composition showed there were 36 species before the M. leidyi introduction, while now days it is 12 species. Food spectra of M. leidyi consisted of zooplankton in which the copepods were the main in the southern Caspian Sea. The mass development of M. leidyi defines the qualitative and quantitative composition of zooplankton community through the control of abundance of copepod and cladoceran nauplii, which, in turn, results in changes in the trophic structure of the whole food chain.