Assessment of Benthic-Macroinvertebrate structures in relation to Environmental Degradation in some Ethiopian Rivers

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Addis Ababa University


Surface water monitoring programs rely on biological, chemical, and habitat information to make science-based judgments on aquatic life use-support designations. Urbanization and extensive agriculture within and adjacent to stream corridors can seriously impact aquatic species and their habitats. This study assessed biological impairment to macroinvertebrate communities in some rivers/ streams primarily disturbed by extensive agricultural activities, industrial and urban land use in Ethiopia. These primary land uses are considered to be the most important threatening factors to aquatic ecosystem. A total of 15 sites were sampled for macroinvertebrates and environmental parameters. These data were collected between August 2005 and June 2006. Sites were categorized a priori into three groups (reference, rural, and urban) based on the predominant land use upstream of the sampling reach. Macroinvertebrate sampling was conducted in accordance with Rapid Bioassessment Protocol. Physicochemical parameters (by using standard methods) were collected and habitat features were scored with the EPA Rapid Bioassessment Protocol (RBP) Habitat Assessment procedures. Streams data were compared to reference conditions. Sites were assessed with the Benthic Index of Biotic Integrity (B-IBI), an aggregate index that incorporates 14 metrics. Exploratory box plots and scatter plots were viewed along with Pearson correlation coefficients and linear regression to evaluate relationships between environmental and biological data. Multivariate techniques such as principal components analysis (PCA), correspondence analysis (CA) and cluster analysis were used. Significance tests were performed on environmental and biological Parameters with the student t- test. There were significant differences in most of the environmental variables (p<0.05) between most categories, but reference and residential sites were not significantly different in some parameters. The dispersion of disturbed sites in PCA ordination space clearly demonstrated that environmental factors deviated from the reference condition. Taxonomically, visual inspection of the CA ordination suggested that reference communities were highly similar to each other. However, substantial departure of urban sites from the reference site array indicated very different community makeup. This analysis also demonstrated distinct separation of assemblages from rural versus urban sites. Streams from the urban categories had significantly lower B-IBI and positive metrics scores, and significantly higher negative metrics scores than reference sites (p<0.05). The B-IBI and its associated metrics were significantly correlated (p<0.05) to most physiochemical parameters and RBP total habitat scores. The dramatic decrease of EPT taxa at urban sites indicated that these organisms are especially sensitive to excessive nutrient and organic loading. Overall, the B-IBI indicated that nearly all urban sites were impaired. The data presented here indicated that macroinvertebrate communities are sensitive and vulnerable to urban/industrial land uses. To best characterize and monitor ecological conditions of these rivers, regular sampling of all variables and development of a single mutlimetric index developed from biological and environmental variables is suggested.