Author(s): Afia Zinat, Md. Abu Sayed Jewel, Bithy Khatun, Md. Khayrul Hasan Nayan and Saleha Jasmine

Abstract: In the present study, phytoplankton community structure in relation to hydrological parameters were studied based on three seasons over a period of 12 months from March 2020 to February 2021 in the Pasur river estuary, Bangladesh. Water quality parameters namely water temperature, DO, pH, alkalinity, TDS and Salinity were determined between 23.05 to 32 °C, 4.93 to 8.78 mg/L, 7.12 to 7.99, 109 to 175 mg/L and 6.20 to 14.68 ppt respectively. Concentrations of nutrients viz. nitrate (0.05 to 1.02 mg/L) and phosphate (0.03 to 0.85 mg/L) also fluctuated seasonally. A total 68 species of phytoplankton belonging to 42 genera were identified with 40 belong to the class Bacillariophyceae, 12 to Dinophyceae, 8 to Cyanophyceae, 6 to Chlorophyceae and 2 to Euglenophyceae. Relatively higher total cell density of phytoplankton was observed during post-monsoon season (48.204 × 10³ cells/L) and lower during monsoon season (37.301 × 10³ cells/L). The percentage composition of phytoplankton was recorded as Diatoms > Dinoflagellates > Blue greens > Greens > Euglenoids respectively. Generally, diatoms formed the dominant group in term of total species and cell numbers during the study period. Species diversity, richness and evenness index ranged from 1.45 to 3.80, 0.25 to 0.98 and 0.48 to 0.96 with highest values were recorded during post-monsoon coinciding with the stable hydrographical conditions with a higher concentration of nutrients in the water columns.

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