The response of macro invertebrates for habitat quality and anthropogenic disturbance among rivers within gilgel gibe watershed, south west Ethiopia

Abstract

Rivers are important pathways for the flow of energy, matter, and organisms through the landscape. River water quality monitoring programs rely on biological, chemical, and habitat information to make science-based judgments on aquatic life support designations. Of the biological indicators, macroinvertebrates are those commonly used worldwide. The patterns of benthic macroinvertebrate distribution are related to the natural and human influenced variation of environmental factors. The main objective of this study is to investigate the response of macroinvertebrates for habitat quality and human disturbances. A cross-sectional study of Physicochemical, biological and hydromorphological components of rivers/streams in Gilgel Gibe watershed of the Omo Gibe river basin, were carried out during April 2014 to assess its ecological status. Investigation sites were pre-classified into three landuse/cover zones, (Forested sites, Agriculture and urban impacted sites). A total of 28 stations (considering the pool/riffle sub-habitat status) were sampled. Macroinvertebrate samples were collected to provide a description of the community composition at each sub habitat of the sampling site. Water samples were collected for analysis of selected physicochemical parameters following water quality assessment protocols. Habitat quality information was collected using rapid bioassessment protocol. Macroinvertebrates were identified to the family level and water quality was analyzed using standard methods while the habitat quality class was quantified using the qualitative habitat evaluation index(QHEI). Multivariate statistical analysis and different biological indices and metrics were computed to describe the macroinvertebrate assembledge. A total of 2210 individuals within 46 taxa belonging to 14 orders were identified from the 28 sampling sites. The most abundant orders were: Ephemeroptera 31%, Trichoptera 18%, Odonata 15% and Diptera 10% represented by 14 families. The results revealed that Macroinvertebrate distribution varied considerably with the change in anthropogenic activities and habitat conditions in the study area.Upstream (forested) sites were found to have significantly better ecological water quality than downstream sites based on indices and metrics. The sites most impacted by urbanization, had the poorest water quality scores than Agricultural and reference sites. Decreasing habitat quality resulted in a decline of diversity and richness of organisms. Sites characterised by riffle-good and pool-good habitat conditions demonstrated high diversity and hence good ecological water quality than the sites with bad habitat condition. Differences in the richness and II diversity of the macroinvertebrate assemblages were attributed to both habitat quality and land use. This study highlights the importance of quantifying both local habitat and land use parameters of watersheds in stream biological assessments to understand the response of benthic communities to disturbances. In addition this study proved that macroinvertebrate based indices and metrics are robust in capturing both habitat degradation and water pollution if used in regular monitoring programs. Furthermore, this study indicates the need of action by land managers and policy makers towards improving agricultural practices so as to improve the habitat condition of the rivers and excercising integrated water quality management.