Oil and Natural Gas Corporation (ONGC) is leading national oil company refines crude oil and natural gas etc. Large amount of water is used in refining process. Water used in the process is non-consumptive and come out as wastewater and after treatment the treated wastewater is stored in cemented pond of 0.05 ha before discharging into Tapi river.

Diversity of phytoplankton was investigated in same cemented pond. Water quality parameters including color, temperature, total solids, total dissolved solids, total suspended solids, pH, total hardness, total alkalinity, chloride, phosphate, silicate, nitrate, DO, BOD, COD, and ammonia were monitored.

Total seventy three genera of phytoplankton belonged to four classes viz., Euglenophyceae, Chlorphyceae, Bacillariophyceae and Cyanophyceae were identified. Chlorophyceae group was dominated among the four classes. Levels of oxygen (5.678±0.218 mg/L), nitrate (4.089 ± 0.926 mg/L), phosphate (0.257 ± 56.786 mg/L) and silicate (0.218 ± 0.029 mg/L) showed direct relationship with the diversity of phytoplankton.