**8. Species composition of zooplankton**

182 Ecosystems Biodiversity

very small: up to 3.6 mm in the Southern Caspian and 4.2 mm in the Middle Caspian. It is well known that *Mnemiopsis* shrinks in unfavorable conditions; here, salinity, food, or a

In May, a few eggs and larvae were found in the Southern and Middle Caspian, but mass reproduction did not start yet because of scarcity of reproducing adults and probably, low

In June 2001, *Mnemiopsis leidyi* began to reproduce and continued its expansion towards the north: in the Southern and south Middle Caspian (Fig. 4), its average abundance was 680 ± 16.8 ind. m-2 (34 ± 2 ind. m-3), and biomass 88.3 ± 7.78 g m-2 (4.3 ± 1 g m-3) (Fig. 4). The highest abundance and biomass, found in the Southern Caspian, represented values of 2005 ± 1248 ind. m-2 (100 ± 62 ind. m-3) and 230 ± 197.66 g m-2 (10.2 ± 9 g m-3), respectively

spring temperatures (16°C in the Southern and 15°C in the Middle Caspian) (Fig.4).

 a) February- March b) August- September c) October- November Fig. 4. Seasonal distribution of the Caspian population of *Mnemiopsis leidyi* (ind.m3) in 2003

Investigations performed in the last decade indicate that there have been important changes in the zooplankton composition and structure in the Caspian Sea. However, contrasting events taking place in different regions of the Caspian Sea indicate a non-uniform structure of its ecosystem. Several fodder zooplankton species have either disappeared from or substantially decreased in number at different sampling sites of the Caspian Sea over the last decade. Some other species adapted to thrive in eutrophic conditions have either appeared or increased in quantity especially meroplankton. Meanwhile the biomass of the fodder zooplankton has also fluctuated considerably through the years. However, there seems to be a reverse trend in the long-term variation of fodder zooplankton between the shallow western and deep eastern areas. Over the last decade the abundance of fish larvae has decreased significantly when compared either to past records or with larval abundances

**7. A review of zooplankton investigations of the Caspian** 

combination of both may have been strongly suboptimal.

(Shiganova et al., 2004).

(shiganova et al., 2004)

A total of 18 zooplankton species (mero- and holozooplankton) were found. Among them there were 13 species of merozooplankton and only five species of holozooplankton. The latter belonged to Copepoda (four species) and Cladocera (one species) (Table 2). The only Cladocera species was *Podon polyphemoides*. Four copepod species were found, with the predominant calanoid *Acartia tonsa* present in all stations and every season. In 2006, a slightly higher diversity of Copepoda was seen; *Eurytemora grimmi*, absent in 2001–2005, was then observed for the first time at 50 m depth of the 100-m-deep station off Anzali (49N and 37E) in 2006.


Table 2. Species number of zooplankton before and after *Mnemiopsis leidyi* invasion in the Southern Caspian Sea.
