Phytoplanktons play pivotal role in deciding water quality. The presence of various algae impairs the taste and odor of water. Algae consume a number of elements from water during their growth. Their excessive growth increases BOD, increased acidity and high pH. The industrial effluents are rich in nitrates and phosphates which promote the growth of phytoplankton. The present study focuses on the impact of phytoplankton and pollution on the taste and quality of water of Daha river in Siwan, Bihar. The water quality was investigated for a period of six months from January 2021 to June 2021. The surface water samples were collected monthwise, from three sites of the River Daha. The physiochemical parameters analyzed were temperature, pH, acidity, alkalinity, BOD, COD and their corelations with the phytoplankton and nearby water of the River. The species of phytoplankton identified belonged to class chlorophyta, Bacillariophyta (diatoms) and Cyanophyta. Diatoms were found to be present at all the three sites. The Study revealed that the presence of Oscillatoria, Euglina, Volvox (fishy odor), Staurastrum (grassy odor) detoriated the quality and odor of the water of Daha river. The analysis of physio-chemical parameters indicated that the water quality of Daha river and nearby places were deteriorated by the industrial effluents and human interventions. Keywords:- Phytoplankton, Interventions, BOD, COD, Fishy Odor, Physio-chemical.

Phytoplankton, Interventions, BOD, COD, Fishy Odor, Physio-chemical.