Ecological impact of river impoundment using benthic macro-invertebrates as indicators

A study of the benthic macroinvertebrates of a fourth order stream in Southern Nigeria was carried out to assess the ecological impact of impoundment. Three stations 1, 2, and 3 were selected from upstream of the impoundment, the impoundment (reservoir) and its downstream, respectively. There was no significant difference (P>0.05) in the values of the physical and chemical parameters between the upstream and downstream stations. Water level, transparency, current velocity and dissolved oxygen were, however, significantly lower within the reservoir. The total number of benthic macroinvertebrate taxa and their overall density were significantly higher in the control station (upstream) than within the reservoir and the downstream station. The density of the three dominant taxonomic groups, Diptera, Oligochaeta and Ephemeroptera differed significantly (P<0.05) among the stations. While chironomid dipterans dominated the up- and downstream stations, Oligochaeta was dominant in the reservoir. The density of Ephemeroptera was significantly reduced within the reservoir and the downstream station. Analysis of faunal similarities showed that the upstream station 1 was significantly different from stations 2 and 3. Stream impoundment, therefore, alters not only the river hydrology but also the faunal characteristics of downstream stations. This study further strengthens the viewpoint that river management consideration should be implemented to lessen the impact on the downstream ecology. Furthermore, when considering impounding a river, the ecological implication must also be considered.