Impact of a New Invader Ctenophore (Mnemiopsis Leidyi) on the Zooplankton Community of the Southern Caspian Sea

Abstract

The invasive ctenophore Mnemiopsis leidyi (Agassiz) which was transported from the Black Sea into the Caspian Sea at the end of the 1990s, has negatively affected the ecosystem of the Caspian Sea. Zooplankton abundance, biomass and species composition were evaluated on the Iranian coast of the Caspian Sea during 2001-2006. A total of 18 merozooplankton (13 species composed of larvae of benthic animals) and holozooplankton (4 Copepoda and 1 Cladocera) species were identified. The total number of zooplankton species found here was 50% less than in a previous investigation performed in the same region in 1996 before the introduction of M. leidyi into the Caspian Sea. Cladocera species seemed to be highly affected by the invasion of M. leidyi; only one species, Podon polyphemoides, remained in the study area, whereas, 24 Cladocera species were found in the study carried out in 1996. While Copepoda Eurytemora minor, E. grimmi, Calanipeda aquae dulcis and Acartia tonsa were abundant before the M. leidyi invasion, only A. tonsa (copepodites and adults) dominated the inshore and offshore waters after the invasion. The maxima in zooplankton abundance (22,088 ± 24,840 ind.m-3) and biomass (64.1 ± 56.8 mg.m-3) were recorded in December 2001 and August 2004, respectively. The annual mean zooplankton abundance during 2001-2006 was in the range of 3,361- 8,940 ind.m-3 which was 2-5 fold less than the zooplankton abundance in 1996. During 2001-2006, the highest abundance and biomass of M. leidyi were observed during summer-autumn coincident with warm temperatures and generally when abundance of other zooplankton organisms was low.