Biological assessment of the headwater rivers of Opa Reservoir, Ile-Ife, Nigeria, using ecological methods

Background: Headwater streams and small rivers within a catchment basin contribute greatly to the overall physico-chemical and biological quality of downstream larger freshwater systems; hence, there is a need to continually assess the water quality of such smaller systems. In this study, the three major tributaries (Obudu, Opa, and Esinmirin rivers) of a tropical reservoir were assessed for their water quality using some selected water and sediment parameters, as well as their macroinvertebrate faunae. Results: All the measured water parameters were found to be within the recommended standards for freshwater life in the three rivers, except for PO43- and dissolved oxygen in Opa River which was possibly due to anthropogenic factors. The bottom sediment of the rivers was predominantly sandy and generally low in chemical characteristics. A total of 17 species of macroinvertebrates were recorded in this study: 14 species in Obudu River, 12 species in Esinmirin River, and 11 species in Opa River. Diversity (Margalef and Shannon-Wiener) and Pielou's evenness indices were all low and indicative of an impaired freshwater system, with the lowest indices recorded in the Esinmirin River. Some macrobenthic environmental indicators of poor water quality (e.g., Tubifex sp., Tipula sp., Chironomus sp., Bulinus globosus, and Eristalis sp.) were exclusively recorded in the Esinmirin and/or Obudu River. Conclusion: The study concludes that the reservoir's headwaters were moderately polluted and had a tendency to become severely polluted by anthropogenic activities along the rivers. Measures should be put in place to reduce environmental impact on the quality of the headwaters and by implication, the reservoir.