**9. Zooplankton frequency**

In general, low zooplankton abundance and biomass (wet weight) were observed in summer months from 2001 to 2006. The highest abundance and biomass of zooplankton along the whole water column were not regularly found in the same season each year (Fig. 5). The maximum abundance recorded was 22,088 ± 24,840 ind.m3 (average of stations and depths) in December 2001, whereas the highest biomass was 64.1 ± 56.8 mg.m3 (average of stations and depths) in August 2004. Monthly variations of zooplankton biomass were similar to the fluctuations in abundance except in some summer–autumn periods when large-sized specimens dominated. The minimum zooplankton abundance and biomass were 397 ± 567 ind.m3 and 1.8 ± 2.6 mg.m3, respectively, in September 2002. The annual mean zooplankton abundance varied between 3361 and 8940 ind.m3 during 2001–2006. The

*Mnemiopsis leidyi* Invasion and Biodiversity Changes in the Caspian Sea 185

Fig. 6. Spatial distribution of zooplankton abundance in different seasons (depth averages of available months) in the Southern Caspian Sea during 2001–2006. (A) Spring. (B) Summer.

(C) Autumn. (D) Winter.

average zooplankton abundance and biomass for all months and years were calculated as 7015 ± 11.959 ind.m3 and 32.8 ± 57.6 mg. m3, respectively.

Fig. 5. Monthly variations in spatial and depth averages of Copepoda. Cladocera merozooplankton and total zooplankton abundance and biomass in the Southern Caspian Sea during 2001–2006. TZA, total zooplankton abundance; TZB, total zooplankton biomass

The maximum seasonal mean of zooplankton abundance was recorded in spring and the minimum in summer (Table 3, Fig. 6A–D). In spring, the greatest zooplankton abundance was observed at the Sephidroud River inlet (9×104 ind.m3) (Fig. 6A). In summer, the abundance of zooplankton decreased compared with the values reported in spring, and an almost even distribution was found along the coastal regions (max. 8– 11 · 103 ind.m3) decreasing towards the open sea (Fig. 6B). In autumn, zooplankton concentration was slightly greater than in summer; again the highest abundance (2×104 ind.m×104) was reported at the Sephidroud River inlet of 5 m depth (Fig. 6C). In winter, abundance was greater than in autumn and the maximum was observed at the Babolsar stations (3 ×104 ind.m3, Fig. 7D).

Comparison among different groups of zooplankton showed that Copepoda accounted for the maximum abundance and biomass every year from 2001 to 2006 (Fig. 6). Among Copepoda, different developmental stages of the calanoid species A. tonsa dominated during the study period. Copepoda, Cladocera and merozooplankton constituted 88%, 4% and 8% of total zooplankton abundance, respectively.

Hossieni et al. (1996) reported 36 zooplankton species (86% holoplankton and 14% meroplankton) in the southern Caspian Sea, consisting of 24 species of Cladocera, seven species of Copepoda and meroplankton such as larvae of Bivalvia and Balanidae (Table 3).

average zooplankton abundance and biomass for all months and years were calculated as

Fig. 5. Monthly variations in spatial and depth averages of Copepoda. Cladocera

Sea during 2001–2006. TZA, total zooplankton abundance; TZB, total zooplankton

biomass

ind.m3, Fig. 7D).

and 8% of total zooplankton abundance, respectively.

merozooplankton and total zooplankton abundance and biomass in the Southern Caspian

The maximum seasonal mean of zooplankton abundance was recorded in spring and the minimum in summer (Table 3, Fig. 6A–D). In spring, the greatest zooplankton abundance was observed at the Sephidroud River inlet (9×104 ind.m3) (Fig. 6A). In summer, the abundance of zooplankton decreased compared with the values reported in spring, and an almost even distribution was found along the coastal regions (max. 8– 11 · 103 ind.m3) decreasing towards the open sea (Fig. 6B). In autumn, zooplankton concentration was slightly greater than in summer; again the highest abundance (2×104 ind.m×104) was reported at the Sephidroud River inlet of 5 m depth (Fig. 6C). In winter, abundance was greater than in autumn and the maximum was observed at the Babolsar stations (3 ×104

Comparison among different groups of zooplankton showed that Copepoda accounted for the maximum abundance and biomass every year from 2001 to 2006 (Fig. 6). Among Copepoda, different developmental stages of the calanoid species A. tonsa dominated during the study period. Copepoda, Cladocera and merozooplankton constituted 88%, 4%

Hossieni et al. (1996) reported 36 zooplankton species (86% holoplankton and 14% meroplankton) in the southern Caspian Sea, consisting of 24 species of Cladocera, seven species of Copepoda and meroplankton such as larvae of Bivalvia and Balanidae (Table 3).

7015 ± 11.959 ind.m3 and 32.8 ± 57.6 mg. m3, respectively.

Fig. 6. Spatial distribution of zooplankton abundance in different seasons (depth averages of available months) in the Southern Caspian Sea during 2001–2006. (A) Spring. (B) Summer. (C) Autumn. (D) Winter.

*Mnemiopsis leidyi* Invasion and Biodiversity Changes in the Caspian Sea 187

In the present study, a total of 226 phytoplankton species were identified. While diatoms constituted 45% of the total species number, chlorophytes, cyanophytes, dinoflagellates and euglenophytes formed 20, 17, 11 and 8% of phytoplankton species, respectively (Fig. 7). Number of species in spring (91 species) and summer (101 species) were higher than in

Fig. 7. Variations in species number of different phytoplankton groups in the southern

The highest monthly mean phytoplankton abundance and biomass were 396 × 106 ± 299 × 106 cells m-3 in January 2002 and 1,789 ± 1,761mg m-3 in May 2002 (Fig. 8). Minimum abundance and biomass values were observed in August 2003 (1 × 106 ± 1 × 106 cells m-3 and 7 ± 5 mg m-3) (Fig. 8).The overall average cell abundance and biomass of phytoplankton during 2001–2006 were 64 × 106 ± 76 × 106 cells m-3 and 250 ± 360 mg.m-3, respectively. While diatoms were the most abundant phytoplankton group during 1996, after the introduction of *M. leidyi* the abundances of cyanophytes (in autumn) and dinoflagellates (in winter) exceeded diatom abundance in 2001 and 2002 (Fig. 8 and 9). Excluding 2005, diatom abundance was again high during 2003–2006. An unprecedented bloom of the toxic cyanophyte *Nodularia* sp. was observed between the second half of August and the end of

**10. Phytoplankton population** 

autumn (86 species) and winter (77 species).

Caspian Sea during 2001–2006 and 1986–1994



Table 3. Annual variations (A) in abundance (ind.m3) and (B) in biomass (wet weight, g.m3) of total zooplankton, Copepoda, Cladocera and merozooplankton species in the Southern Caspian Sea (before and after Mnemiopsis invasion). (C) Seasonal variations in abundance and biomass of zooplankton species in 1996 from Hossieni et al. (1996). (D) Full list of Cladocera species reported by Hossieni et al. (1996).
