This project has been started as monitoring study since 2001 that in first phase has lasted for two years and because of its importance, it was decided that this study should be continued for two other years. In the early 1980s, an alien ctenophore Mnemiopsis leidyi already known as a gelatinous zooplankton was transported (likely via ballast waters) to the Black Sea from its native waters of the western Atlantic. The possibility of Mnemiopsis leidyi introduction into other sensitive, neighboring ecosystems, notably the Caspian Sea, had been mentioned during the GESAMP meeting in 1994. And, as expected, this ctenophore was reported to be present in the Caspian Sea by November 1999. In this report, spatial and temporal distribution of M. leidyi had been investigated in southern Caspian Sea (Iranian waters) in 6 transects consisted of Lisar, Anzali, Sefidroud, Nowshar, Babolsar and Amirabad at 24 stations during 2003 and 2004. As a whole, 818 samples had been analyzed included 236, 302, 136 and 144 belong to ctenophore, phytoplankton, zooplankton and benthos. Meantime 811 physicochemical parameters had been measured. Water temperature varied between 7.8 to 29.3 co. Transparencies also fluctuated from 0.20 to 10.5 m. Salinity in different regions was from 7.80 to 13.90 ppt and the average was 12.09 ppt. pH was between 7.2- 8.6. Demand oxygen (O2) has been measured very high in water surface (12.89 µg.l) and it decreased at minimum level in deep down to 4 µg.l. Nutrients measurements such as NO_2, NO_3 and NH_4 were 0.10-15.17 , 3.80- 133.40 and 0.36- 86.40 µg.l, respectively. Total nitrate and phosphate were measured between 337.00- 1635.00 and 14.90-103.40 µg.l, respectively. Comparison of M. leidyi distribution in the southern Caspian Sea shown that maximum abundance recorded in warm summer months (e.g. August) with 11627 ind.m^2 and its biomass maximum was in autumn months (September and October) about 446.11 gr.m^2 . Mean abundance and biomass of M. leidyi were 2504 ind.m^2 and 142.42 gr.m^2, respectively. Maximum size of the ctenophore recorded as 70 mm while the less than 10 mm length frequency consisted 92.2 per cent of total population. Spatial distribution of M. leidyi in different regions shown that Sefidroud not only had the maximum Mnemiopsis abundance but also consisted higher biomass in the southern Caspian Sea in 2003-4. At present study, 142 phytoplankton species were identified which belongs to Chrysophyta (67, 48), Chlorophyta (23, 16%), Cyanophyta (22, 15%), Pyrhophyta (21, 15%) and Euglenophyta (9, 6%). Mean phytoplankton abundance and biomass were 11137032 ind. m^3 and 40.26 mg. m^3, respectively. Chrysophyta (diatoms) consisted 42 and 47% of whole phytoplankton abundance and biomass. In the other hand, this group included 50 % of phytoplankton population. In zooplankton study, there were identified 21 species in which 10 species were only meroplankton (zoobenthic) and 11 species was holoplankton. These species belongs to Copepoda (4, 37%), Rotatoria (4, 36%), Cilliophora (2, 18%) and Cladocera (1, 9%). Monthly zooplankton abundance and biomass data shown that its population completely effected by Copepoda. Among copepoda, Acartia clausi was the main species duri ng 2003-4. As a whole, maximum abundance recorded in spring (March) and autumn (September) about 11497 and 11321 ind. m^3 while its maximum biomass occurred in summer (August) with 48.95 mg. m^3. Zooplankton minimum abundance and biomass were in wintertime about 3894 ind. m^3 and 10.16 mg. m^3. Benthic fauna in recent study consisted 27 species included Gammaride (8, 29%), Cumacea (7, 26%), Polychaeta (4, 15%), Bivalvia (3, 11%) and Oligochaeta, Balanidae and Xantidae each one (4%) species. Mean benthic abundance and biomass were 1215 ind. m^ 2 and 20.09 g. m^2, respectively. Polychaeta and Oligochaeta were established maximum frequency about 49 and 34% of total fauna while Bivalvia consisted 82.8% biomass overall.
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