Author(s):

Abu Sayeed Muhammad Sharif, Md. Shafiqul Islam, Md. Nesarul Hoque and Md. Simul Bhuyan

Abstract: The spatial and temporal effects of environmental variables on phytoplankton abundance and distribution of the lower Meghna River Estuary was studied at Sandwip, Hatiya, Bhola, Barisal and Chandpur during monsoon and post-monsoon. A total of 28 phytoplankton genera were identified of which 22 genera occurred during monsoon and 16 genera occurred during post-monsoon season. During the annual cycle Nitzschia was common genera at all five sites however Coscinodiscus, Biddulphia, Nitzschia, Thalassiothrix and Triceratium were dominant genera in the study area. During the study water was slightly acidic in nature with very low salinity in major part of the estuary. Salinity, H-CO₃, pH, secchi depth, TSS, water temperature and silt were responsible for the variations in phytoplankton community structure but the effect was not significant (p>0.05). During the monsoon season CCA analysis shows, Euglena and Zygnema showed a positive relation with salinity while Clostridium, Fragilaria and Scenedesmus showed close affinity to water temperature, secchi depth, TSS and pH. Again Navicula, Stauroneis, Pediastrum, Rhizosolenia, Ankistrodesmus etc. genera exhibited strong affinity to H-CO₃. On the contrary during the post-monsoon season CCA analysis shows, Nitzschia, Sphaerogoum and Melosira revealed strong relation with H-CO₃ with the exception of pH, TSS, secchi depth. The dendogram of the phytoplankton genera, based on phytoplankton abundance pattern with different seasonal parameters showed Sandwip, Hatiya, Bhola and Barisal formed cluster. The phytoplankton diversity (H') did not show much variation (1.18±0.22 and 0.87±0.40 for the monsoon and post-monsoon seasons respectively) temporally. The highest phytoplankton diversity (H') was recorded at Sandwip (1.517) during the monsoon and the lowest value was recorded at Sandwip (0.35) during the post-monsoon. Again, the evenness (J') of phytoplankton showed less variation with seasonal environmental changes (0.80±0.23 and 0.58±0.18 monsoon and post-monsoon). The maximum evenness value was found at Bhola (0.99) during monsoon and it was minimum at Chandpur (0.36) during the post-monsoon season.

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