The Impact of Impoundment on Downstream Macroinvertebrate Communities at Koga Irrigation Dam, West Gojam, Ethiopia

Despite its vital importance, river regulation affects downstream lotic environment and aquatic fauna. In this study, downstream effects of reservoir were evaluated using macroinvertebrate bio-assessment techniques. Macroinvertebrates community structure and physicochemical parameters were assessed at upstream and downstream sites relating to dam impact. Samples from four sites were collected using standard dip-net (500-μm mesh size) and identified to family level. Rapid Bio-assessment Protocol-II metrics and scoring criteria was used to quantify changes in macroinvertebrate community structure and water quality. Downstream of reservoir; total abundance, Chironomidae abundance and Hilsenhoff Biotic Index values increased. However, richness, Ephemeroptera Plecoptera and Trichoptera taxa and Ephemeroptera abundance decreased. Trichoptera were absent from reservoir and downstream sites, but the number of gatherers and filter-feeder individuals were higher than reaches above the reservoir. Physicochemical parameters were significantly different (p < 0.05) among sites except temperature. A post-hoc test for Shannon Diversity Index indicated a significant difference (p < 0.05) between Site 1 and Site 4. Habitat and water quality changes have disrupted downstream continuum of macro-invertebrate communities. Generally, impoundments have profound impacts on macroinvertebrates within reservoir and below it as indicated by this study; therefore, needs an appropriate conservation measures.