By applying the Benthic-Index of Biotic Integrity (B-IBI) to benthic macro-invertebrate and water quality data, we evaluate the health of Wenyu River and explore the relationship between water quality and B-IBI. Macro-invertebrate and water samples are collected from 22 river sites, where 4 are unimpaired or minimally impaired sites and 18 are impaired sites. We mainly consider twenty candidate metrics of macro-invertebrate and twelve water quality indexes. These twenty candidate metrics are total number of taxa, EPT taxa, Crustacea and Mollusca taxa, hironomidae taxa, total biomass, dominant taxa individual relative abundance, the first three dominant taxa individual relative abundance, Trichoptera individual relative abundance, Ephemeroptera individual relative abundance, Tubificida individual relative abundance, Chironomidae individual relative abundance, Hirudinea individual relative abundance, Crustacea and Mollusca individual relative abundance, number of sensitive taxa, tolerant taxa individual relative abundance, sensitive taxa individual relative abundance, biomass percentage of tolerant taxa, biomass percentage of sensitive taxa, BI index and clinger individual relative abundance and twelve water quality indexes are dissolved oxygen (DO), temperature, PH, total suspended solids (TSS), conductance, COD_mn, BOD₅, total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH₃), nitrate (NO₃^-) and nitrite (NO₂^-). The main results are given as follows:
1) Among these 20 cross box-plots of candidate metrics between reference site (unimpaired sites) and impaired sites, 12 metrics have the higher IQ (interquartile ranges, IQ ≥ 2). They are total number of taxa, EPT taxa, Crustacea and Mollusca taxa,total biomass, dominant taxa individual relative abundance, the first three dominant taxa individual relative abundance, Ephemeroptera individual relative abundance, number of sensitive taxa, sensitive taxa individual relative abundance,biomass percentage of tolerant taxa, and clinger individual relative abundance.
2) Since BI index does not follow the normal distribution, we use Pearson's correlation coefficient to evaluate the relationship between the remained 11 metrics. Metrics that are strongly correlated (r > 0. 75) indicate the overlapped information, and we choose one of them as a surrogate.. Based on the above analysis, the B-IBI of Wenyu River including total number of taxa, total biomass, relative abundance of the first dominant taxa individuals, number of sensitive taxa and relative abundance of clinger individuals, are constructed.
3) We use 25% percentile of B-IBI value in reference sites to determine the criteria of health ranking and divide the distribution range below 25% percentile into 4 quarters. The criterion for the benthic macro-invertebrate in Wenyu river is B-IBI > 1.821, 1.366-1.821, 0.910-1.366, 0.455-0.910 and 0-0.455, which are corresponding to health, sub-health, fair condition, poor condition and very poor condition, respectively.
4) We find that 27.3% of the river reaches are in health condition, 9.1% are in sub-health condition, 13.6% are in general condition, and 50% are in poor and very poor conditions.
5) The correlation coefficient between B-IBI value and water quality of Wenyu River is -0.549, which indicates that the biological index, such as benthic macro-invertebrate, should be taken into account for water environmental assessment.