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Mnemiopsis leidyi population activities first were recorded during the coastal observations in 2001 in which its population considerably increased afterward and now sustained the southern Caspian Sea. Maximum summer-autumn M. leidyi abundance was recorded in euphotic layer in 2002 (851±85 ind.m^-3) and maximum biomass was in 2001 with 48.1±14.4 g.m^-3) while minimum were in aphotic layer. In years 2003 to 2011, M. leidyi abundance and biomass sharply declined to 1-843 ind.m^-3 and 0.07-37.7 g.m^-3, respectively. The length-frequency distribution of Mnemiopsis showed larval, juveniles and transitional stages that made up 98.6% of the total population. In the southern Caspian Sea, Acartia tonsa was the highest number of mesozooplankton species (36 species) which decreased tenfold after Mnemiopsis invasion during 2001-2010. Fecundity experiments of Mnemiopsis showed the average of 8 eggs. day^-1, with a maximum of 35 eggs.day^-1which increased with increasing body length. The main food was larger amounts of zooplankton prey such as copepodites and adult of A. tonsa. It seems the sharp decreased of Mnemiopsis abundance could be related the decreasing of ova reproduction; and food ability is the main reasons of M. leidyi sustained in the southern Caspian Sea. The objective of present study was to provide information about parameters influencingthe M. leidyi decreased after initial bloom invasion in the southern Caspian Sea.

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The zooplankton diversity of Helleh estuary, Busheher, Persian Gulf, and its relationship with some characteristics of water quality was investigated. Samples were taken using plankton net (mesh size of 140 µm) with vertical towing from 5 fixed-stations in mid-season from summer 2011 to spring 2012 for one year period. The mean (±SE) of biodiversity indices of Simpson, Shannon-Wiener, and Margalef were, respectively 0.84±0.10, 3.01±0.20, and 1.90±0.10 in summer; 0.63±0.06, 2.20±0.20 and 1.87±0.08 in autumn; 0.73±0.04, 2.22±0.10 and 1.43±0.10 in winter; and 0.87±0.01, 3.44±0.30, and 2.38±0.30 in spring. The seasonal zooplankton similarity based on Jaccard’s and Sørensen’s coefficients were highest (0.61 and 0.76) for summer-autumn, 0.56 and 0.72 for autumn-spring while these value were lowest for winter-autumn (0.27 and 0.43) and spring-winter (0.30 and 0.47). In addition, there were significant correlation between zooplankton density with salinity, dissolved oxygen and chlorophyll a, Simpson and Shannon- Wiener with temperature, and Margalef with pH.