Seasonal variation in the coastal water phytoplankton communities and their environmental responses at upstream and downstream of the steep Naf River in the south-western Bay of Bengal

As a multinational river, the Naf River flows into the Bay of Bengal in the Indian Ocean, between the Cox's Bazar district of Bangladesh and the Rakhine state of Burma. In a multidisciplinary approach, several experiments were carried out to understand the seasonal diversity of the phytoplankton community structure. A total of four layers of water was sampled from four depths in the Naf River during monsoon (September) and winter (December) of 2016. 41 species of phytoplankton were identified, and 3 different dominant groups (Cyanobacteria, Diatoms, and Dinoflagellates) were found. Diatoms and cyanobacteria alone were found to be most prevalent. Higher species diversity was observed in the monsoon season, with Synedra sp. (1.84×105 cells L-1, 18.76%) and winter with Microcystis sp. (1.41×105 cells L-1, 17.74%), respectively. In monsoon, NO3-N and PO4-P were both higher than winter (450.9 and 34.4 µg L-1, respectively) especially, at downstream Naf River. Moreover, high diversity indexes (richness) of phytoplankton were recorded along with these estuarine stations. Significant correlations (P<0.01) of nutrients with phytoplankton may liable behind these scenarios. An analysis of principal component analysis (PCA) and linear regression supported this correspondence. In the monsoon season, the concentration of Chlorophyllα reached the highest level (165 µg L-1) at a depth of 1.5 m, in Station-D. Cluster analysis based on the nutrient content of the Naf River was found two (upstream and downstream) mentionable zones during the winter and monsoon seasons. The results of the present study indicate that estuarine downstream areas are more productive than upstream areas of the Naf River at the southwest coastal zone of the Bay of Bengal.