Abnormal algal bloom is a global issue. So the Caspian Sea such as the other aquatic environment is at risk of algal bloom. Thus the present study in 2013-14, was conducted to get goals, firstly determination of temporal and spatial variation of phytoplankton abundance and biomass with emphasis on dominant species with bloom potential and secondly detemination of chlorophyll-a concentration. Mostly sampling conducted in mid-seasons in 4 transects (Anzali, Tonkabon, Nowshahr and Amirabad) at surface, 10 and 20m layers in depths of 5, 10 and 20m. Based on the results the annual mean±SE of phytoplankton abundance was 219 ± 33 million cells/m^3. The minimum and maximum valus of abundance recorded in summer (73 ± 31 million cells/m^3) and winter (505 ± 55 million cells/m^3) respectively. Totally 147 phytoplankton species identified which classified in 7 phyla and small phytoflagellates group. The major portion of abundance formed by Bacillariophta (81%). The minimum and maximum seasonal phytoplankton biomass were in summer (90 ± 20 mg/m^3) and winter (1777 ± 201mg/m^3) respectively. The annual median of chlorophyll-a concentration was 2.43 mg/m^3 at surface layer and its seasonal value in spring, summer, fall and winter recorded 1.54, 1.18, 5.81 and 2.59 mg/m^3 respectively. Therefore algal blooms started in fall and it was followed with lower intensity in winter. Mean of phytoplankton abundance and biomass in winter were 3-6 folds to other seasond. But chlorophyll-a concentration in winter was low and closed to the values in spring and summer. Probably, decreasing of ligh availibity and low chlorophl content of dominant species showed important roles in decreasing of chlorophyll-a concentration in winter. As conclusion, the abundance of Stephanodiscus socialis, Binuclearia lauterbornii and Thalassionema nitzschioides classified in the fair level of bloom (100-1000 cells/ml) in spring (in Anzali transect), summer and fall (in Amirabad transect) respectively. In winter, Dactyliosolen fragilissima،(in all transect except Nowshahr) and Pseudonitzschia seriata were in this group. The presence of Thalassionema nitzschioides (as valuble food in food chain) in dominant species list (same patern to the stability year of Caspian Sea) and decreasing of percent abundance of harmful species from 68% (in 2008-2009) to 43% (in this study) are good signs of water quality from Caspian Sea. On the other hand, Pseudonitzschia seriata as a harmful alga (ability to produce toxin and bloom forming) has important role in environmental issues. In current study, Pseudonitzschia seriata expanded from cold season to other seasons (even in summer). Also, it had high percent frequency (100%) and abundance (294 ± 30 million cells/m^3) in winter which indicate to unsuitable condition of water quality in Caspian Sea.
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