Author(s): Md. Tarikul Islam, Sayeed Mahmood Belal Haider and Abu Sharif Md. Mahbub-E-Kibria

Abstract: Phytoplankton assemblages in the south eastern coastal area of the Bay of Bengal, Bangladesh was studied from surface water for a period of 12 months (July 2020 to June 2021) in relation to environmental variables like, water temperature, pH, dissolved oxygen, salinity, conductivity, transparency, rainfall, TDS and nutrient contents-including nitrate, nitrite, phosphate and silicate. A total of 137 phytoplankton species were identified whereas, 115 species of diatoms from 44 genera, 15 species of dinoflagellates from 8 genera, 2 species of green algae from 2 genera, 4 species of cyanobacteria from 3 genera and 1 species of silicoflagellate were identified. The highest phytoplankton abundance was found at Shaplapur about 12245 cells/l and the lowest at Saint Martin’s Island about 1250 cells/l. Phytoplankton showed complete dominance of diatom genera namely Asterionella japonica & Thalassionema nitzschiodes in post-monsoon and Odontella rhombus in pre-monsoon. Other frequently occurring diatoms were Coscinodiscus perforatus, Actinocyclus normanii, Thalassiothrix fraunfeldii, Ditylum brightwelli, Rhizosolenia alata, Chaetoceros affinis, Thalassionema nitzschoides etc. respectively. Species diversity was observed to be maximum in post-monsoon (67 species) followed by pre-monsoon (42 species) and monsoon season (28 species). The average annual environmental variables values among the stations were salinity (27.09±2.27 PSU); water temperature (27.22±0.40 ºC); DO (4.78±0.15 mg/l); TDS (21.33±1.93 g/l); Conductivity (41.40±3.49 mS/cm); Transparency (3.09±2.02 ft); Rainfall (87±0.00 mm); NO₃-N (0.38±0.07 mg/l); PO₄-P (0.07±0.01 mg/l); SiO₄ (0.06±0.01 mg/l) respectively. Correlation studies of phytoplankton composition to physicochemical variables indicated significant negative relation with water temperature, salinity and pH but positive relation with nitrate, nitrite, silicate and phosphate of the water body. Nitrate was found to be the limiting factors for phytoplankton growth during pre-monsoon periods whereas, the role of silicate and phosphate remained insignificant in these perspectives.

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