Nutrient enrichment is followed by excessive algal growth (possible in toxic and harmful species); in Caspian Sea in recent decades. So aims of the present study were 1-study on potential of algal bloom at different transects and seasons, 2- determine correlation between potential of algal bloom and environmental parameters using statistical model, and 3- the role of sediment nutrients on algal bloom in the southern Caspian Sea in 2013-2014. The samples of water were collected from four transests (Bandar Anzali, Tonekabon, Bandar Nowshahr and Bandar Amirabad) and three depths (5, 10 and 20 meter) during four seasons. Surface sediments samples were collected at same depths and transects as well. Result showed that organic phosphorous and nitrogen of water were more than inorganic phophorous and nitrogen. Also, maximum and minimum water temperature was obtained at summer and winter, respectively. The values of DO and salinity were more than 5 ml/l and 12 g/l, respectively and pH of water were ranged from weak alkaline to alkaline. Maximum organic and inorganic phosphorous and nitrogen of water were distributed at different transects and seasons. The nitrogen was limiting factor accomponing with phosphorous during spring, summer and fall, but during winter shift to more nitrogen limitation. In addition, the ecosystem was experienced sillicon limitation during winter. Result of sediments showed that inorganic phophorous was maximum during different seasons, however, percent mean of residue-P which contain organic compounds and non-degrediable compounds, was less that 5 percent. Percent of Ca-P was higher than 90, whereas Bioava.-P was less that 10 percent. The order of different forms of phosphorous were registered Ca-P>Org-P>Fe-P>Al-P>Loosely-P. The mian role of adsorbed and desorbed of Loosely-P, Fe-P and Al-P were due to temperature, Eh and pH, respectively. Annual percent of TON was two folds than TIN and NH4/N was also three times than NO_3/N. Minimun and maximum of phytoplankton abundance were obserbed in summer (73±31 million cells/m3) and winter (505±55 million cells/m3), respectively. Statiscal analysis (ANOVA) showed that phytoplankton abundance significantly increased in winter compared to the other seasons (P1000 cells/ml) during spring. Maximum abundance of phytoplankton was mainly registered at Anzali and Amirabad transects.The re-existence of Thalassionema nitzschioides (as valuble food in food chain) in dominant species list (same as the stability year of ecosystem) and decreasing of percent abundance of harmful species to the dewell and native species are good sign of good quality of the Caspian Sea water. However, Pseudonitzschia seriata expansion of the cold season to other seasons (even in summer) is important point. Pseudonitzschia seriata as harmful algae and its ability to produce toxin and potential bloom, high percent frequency and abundance during the study has important role in environmental issues of the Caspian Sea.It seems that temperature changes in different parts of the Caspian Sea in each season (except fall season) was obtained in the range of dominant species needed, therefore, it had less importance compared to nutrients. Trophic status was meso-eutroph during fall and winter seasons which was higher level than spring and summer (mesotroph). The Stephanodiscus abundance increased at Anzali transet due to nutrients enrichment of water in spring. Excessive use of silica in the spring, summer caused dominant of non Diatoms species (Binuclearia lauterbornii) and low content of Diatoms species (Chaetoceros throndsenii). In fall, high abundance of Bacillariophyta (Thalassionema nitzschioides) accompanied with consuming of silicon and ammonium. The statistical analysis also showed significant correlation between Bacillariophyta and Thalassionema nitzschioides abundance and Si/N ratio. Subsequently, the excessive use of silica in autumn and winter (due to high abundance Bacillariophyta and dominant species including Pseudonitzschia seriata and Dactyliosolen fragilissima) cause severely declined dissolved silica, and for the first time in winter, silicon was limited factor for phytoplankton growth. In addition, the results showed that there were positive correlations between abundance of dominant species and nutrients of sediments in all seasons. This is shows that some species of phytoplankton, even though the limitation of nitrogen and phosphorus in the water, are able to supply the nutrients required from the surface sediments. It would be more importance in the present study, which higher concentration of different forms of phosphorus and nitrogen in sediments as compared to the deep water, provide transport of nitrogen and phosphorus from sediment to water column
