A HEALTH ASSESSMENT USING A BENTHIC-INDEX OF BIOTIC INTEGRITY IN GANJIANG RIVER BASIN

Benthic index of biotic integrity (B-IBI) is one of the most widely-used aquatic ecosystem health assessment indices. In the present study, benthic macroinvertebrates were sampled and collected from 60 stream sites in Ganjiang River basin during the period of 2009 to 2010, among which 15 sites were considered as reference sites and 45 impaired sites. Seventeen candidate metrics were selected for the establishment of B-IBI system, which were Shannon-Wiener index, Margalef index, Total number of taxa, (Crustacea+Mollusca) taxa, EPT taxa, Chironomidae taxa, Intolerant taxa, Dominant taxon %, Three dominant taxa %, Chironomidae %, Tubifex %, (Crustacea+Mollusca) %, Intolerant taxa %, Tolerant taxa %, Biotic Index, Predators % and Filterers %, respectively. The analyses of index value distribution, dis-crimination ability and Pearson correlation were then performed on these candidate metrics. After the analyses, four biological metrics, e.g. total number of taxa, (Crustacea+Mollusca) taxa, (Crustacea+Mollusca) % and Biotic Index were selected as the indicators of tolerance towards water pollution. B-IBI was obtained by summing up all these three indices after which were transformed into a uniform score by using the ratio scoring method. The criteria of health ranking were finally determined based on the 25% percentile of B-IBI value in reference sites. I-IBI values were assessed with these criteria, and results showed that among the 60 sites in Ganjiang River basin, 19 sites were health, 19 sub-health, 14 fair, 8 poor and no very poor. In the whole, rivers in Gangjiang River basin were in the state of health or sub-health. Tributaries were in health in its upstream such as Mianshui River, Gongjiang River, Shangyoujiang River and Taojiang River, while Zhangshui River, Lianshui River, Meijiang River and Pingjiang River were in the poor state; all most tributaries were in health or sub-health in the mid-stream except Wujiang River (poor); tributaries were also in health or sub-health in the downstream; main stream was in health through the river health assessment.