The water quality and phytoplankton of Ufiobodo and Ebonyi River reservoirs were studied for nine months from March to November, 2019 to evaluate the suitability of the reservoirs for domestic water supply and habitat for aquatic organisms. Temperature, pH, conductivity, total dissolved solid (TDS), transparency, dissolved oxygen (DO) and depth
were measured in situ, while water samples were collected for determination of nitrate, iron, silicate and phosphate in the laboratory. Phytoplankton samples were also collected concurrently using plankton net of mesh size 45μm and mouth diameter (0.26m) and identified in the laboratory using standard keys. The results showed that mean conductivity
(32.33µS/cm), TDS (16.00mg/l), nitrate (33.99mg/l), depth (10.36m) and phosphate (4.81mg/l) were higher at Ebonyi than Ufiobodo Reservoir but DO was lower at Ebonyi Reservoir (2.77mg/l) than at Ufiobodo Reservoir (6.67mg/l). Mean DO level was below the permissible limit for drinking water at Ebonyi Reservoir while nitrate, phosphate and iron concentrations exceeded permissible limits in both Reservoirs. Five phytoplankton Phyla were encountered, predominated by Chlorophyta (107 individuals/l, H=2.80 and d=3.85) and Bacillariophyta (81 individuals/l, H=2.39 and d=2.50) at Ufiobodo and Ebonyi Reservoirs, respectively. Principal Component Analysis (PCA) showed that temperature, conductivity, TDS, depth, DO, nitrate and phosphate were the major factors influencing phytoplankton abundance and diversity of the reservoirs. Thus, the reservoirs should be protected to maintain/improve the water quality for domestic use and to support the socio-economic and ecological services they provide.