Vertical and longitudinal phytoplankton distribution was conducted (February 2019 - January 2020). Transparency, pH & TSS were high at photic water but decreased gradually into aphotic except pH that was opposite. The chemical parameters were low at the surface and increased with deeper water. The physical parameters except pH were slightly lower during the wet season but conductivity and total phosphorous were slightly higher during the dry season. There were six phytoplankton groups which comprised Chlorophyta(37%), Pyrrhophyta(24%), Bacillariophyceae(20%), Cyanopyta(17%), Chrysophyta(1%) & Charophyta(1%). A total of 1,472 species were recorded from 57 genera. The mean values of phytoplankton (mg/L) in Riverine, Transitional and Lacustrine were 18.79, 13.46 & 24.68 and 2.88, 2.55 & 2.00 for photic & aphotic respectively. The ANOVA results showed significant diference between longitudinal (photic & aphotic) with regards to phytoplankton at P=0.05. This means that the higher phytoplankton density reported at photic shows more availability of nutrients and the water's stability and better health status of the Reservoir. Phytoplankton encountered in the water body reflect the average ecological condition, and therefore, can be used as an indicator of water quality.