SUMMARY
An effluent receiving stream and Opa River to which it drains within Obafemi Awolowo University, South-Western Nigeria, were studied to ascertain the level of physico-chemical and biological pollution. Water samples were collected and analysed using various standard methods for planktonic abundance and selected physico-chemical parameters over a period of seven months. Most of the water quality parameters investigated showed statistically insignificant differences between the stream and Opa River, except of conductivity, nitrate and turbidity. Notable parameters were nutrient level indicator parameters (conductivity, nitrate, organic matter, COD and planktonic abundance) whose mean concentrations were higher in the effluent receiving stream than in Opa River. Even more, the seasonal variations of some of the investigated parameters (sulphate, organic matter and conductivity) at both sampled stations revealed the effect of anthropogenic run-off as additional source of dissolved nutrient and chemical enrichment rather than the effluent only. Highest occurrence of Chlorophyceae and abundance of Bacillariophyceae in the studied water bodies revealed a level of organic pollution, while the low occurrence species diversity of copepod, cladocera, diptera and protozoa further connotes the unfavourable physico-chemical condition of the water bodies. However, this effect was observed to be reduced at the point where the stream emptied into Opa River. Hence, the effect of the effluent discharge may not pose a health risk to several rural communities downstream which rely on the receiving water body primarily as their source of domestic water. Nevertheless, due to the level of organic pollution recorded, it could be suggested that the oxidation pond effluent discharge should be treated and/or recycled before discharge into this natural body of water.