A macroinvertebrate-based Index of Biotic Integrity (M-IBI) was developed to monitor ecological integrity of the Nyando River and its tributaries. Macro-invertebrates were sampled with a scoop net between September and December 1999 from 12 stations selected to correspond with different human activities in the catchment. The samples were hand sorted in the field, preserved in 70% alcohol before transportation to the laboratory for identification and counting. The stations were grouped into three condition categories (reference, moderate and impaired) according to the level of water and habitat quality. Twenty metrics representing the structural and functional organization of macroinvertebrates were evaluated for range, response to disturbance and stream size differences and redundancy with each other. Ten metrics met the test criteria and were used to provide the final scores for developing the M-IBI. Metrics values at both reference and impaired sites were used to establish the scoring criteria using inter-quartile ranges. The metrics displayed variability across the sites and the final index was able to separate reference sites from moderately impaired and impaired sites. There were variations in ecosystem integrity among stations and this was reflected in community composition and structure of resident macroinvertebrates. The index developed can, therefore, be used to monitor the integrity of streams within the Nyando River Basin.
Key words: Bioindicators, bioassessment, metrics, water quality, habitat integrity.
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