Benthic macroinvertebrate community changes along an organic pollution gradient in the Bagmati River

The composition of macroinvertebrate assemblages at 6 different stations of the Bagmati River was studied by field sampling in February 2012. A total of 2583 benthic macroinvertebrates representing 10 orders and 29 families were recorded in the river. Diptera formed the major part of the benthic invertebrates with 45.1% followed by Ephemeroptera, 24.5%, Oligochaeta 15.9% and Trichoptera 5.6%. The abundance of the macroinvertebrates varied significantly between the stations. The EPT index and EPT to Chironomidae ratio showed that highly sensitive taxa were abundant in the upstream sites of the river whereas the pollution tolerant taxa were abundant at the downstream sites. The biotic indices (BMWP, ASPT, FBI, CLI, and EPT) revealed that the ecological condition of the river was good at the upstream sites and very poor in the downstream direction. The relationship between biotic indices and dominat taxa of the sampling stations of the Bagmati River with the physico-chemical parameters was highly significant. This study shows that distribution of benthic macroinvertebrates in river reaches with differing pollution can function as biological indicators in the bioassessment of the river and can also be used in the evaluation of the water quality in other freshwater localities in Nepal