The seasonal abundance and distribution of phytoplankton along the Tanintharyi coastal waters were investigated for 24 months from June 2013 to June 2015. A wide fluctuation in cell abundance 72,450-714,396 cells/l at Kawthaung, 47,416-947,501 cells/l at Myeik, 8,930-28,439 cells/l at Kampani, 8,976-17,888 cells/l at Ye and 5,162-16,986 cells/l at Setse were noted during the study period. Amongst, Kawthaung and Myeik stations had the highest phytoplankton abundance, whereas Ye and Setse stations showed remarkably lower abundance. It was noted that Ye and Setse stations were much influenced by freshwater discharge from the Thanlwin River, which deposited huge tons of sediments. The water clarity was lower at these stations compare with others. Therefore, turbidity may affect the occurrence and species abundance of phytoplankton. A clear seasonal trend was found at all five stations with a sharp increase in the pre-monsoon months and a gradual decrease in the monsoon and post-monsoon periods. Keywords: phytoplankton, Tanintharyi, water clarity, turbidity,

A total of 69 taxa of dinoflagellates at Rakhine coastal waters, 8 taxa at Mon coastal waters, and 29 taxa at northern Tanintharyi coastal waters were recorded in the present study. Regarding the percentage composition of dinoflagellates, Gonyaulacoid and Peridinoid were noted as the most abundant groups in dinoflagellates. A wide range of salinity fluctuations was noticed in the present study due to the effects of the monsoon cycle in the Indian Ocean. The maximum salinity, 34‰ was recorded in pre-monsoon months whereas the minimum value, 14‰ was detected in monsoon season. The highest pH value, 9.0 was observed in monsoon months. The concentrations of PO4-P, NH3-N, and NO3-N were normally high during pre-monsoon months. The cell abundance of dinoflagellates at all stations was high during pre-monsoon months indicated that a mesotrophic environment favor species richness and dense.

Regarding the spatial and seasonal variations of dinoflagellates in different salinities regimes, Prorocentrum rostratum showed strongly correlated with high salinity (≥ 29ppt.) but P. micans indicated a negative correlation with salinity. In Dinophysoids, Dinophysis caudata showed wide salinity tolerance than other species in the group. D. miles, Ornithocercus magnificus, O. steinii and O. thumii showed a strong correlation with salinity. In Gonyaulacoids, Ceratium furca, C. fusus, C. horridum, C. trichoceros, C. tripos, Gonyaulax polygramma, G. spinifera, and Pyrophacus magnificus showed a strong correlation with salinity. In Peridinoids, Protoperidinium depressum, P. oblongum, P. oceanicum, P. pyreforme, and Podolampus palmipes showed a strong correlation with salinity. In Gymnodinoid and Noctilucoid, Gyrodinium estuariale and N. scintillans showed a strong correlation with salinity.