Development of biological water quality categories for streams using a biotic index of macroinvertebrates in the Yangtze River Delta, China

Biotic index (BI) based on macroinvertebrate tolerance value (TV) is widely used for water quality bioassessment and environmental management. Most regional used BI versions were adapted from the original Hilsenhoff BI and Hilsenhoff TVs. Here, we aim to build a BI classification of water quality for streams in the Yangtze River Delta using statistical methods to set quality class boundary. We determined reference sites using a combination of priori objective screening criteria of total nitrogen (TN, <= 1.409 mg/L), total phosphorus (TP, <= 0.035 mg/L), and electrical conductivity (EC, <= 249 mu s/cm) and the most-disturbed sites with all TN, TP, and EC values higher than 75th percentile across the 233 sites sampled during 2004 to 2011 in Yangtze River Delta. We applied the 75th percentile of BI values across 19 reference sites as the benchmark of high class and then equally divided the range between the benchmark and the expected highest BI values into four categories. We then proposed five biological classes of water quality: high (BI < 3.91), good (3.91-5.43), moderate (5.44-6.95), poor (6.96-8.47) and bad (BI > 8.47) with a bias of 0.22 for each class. The BI performed well for the discrimination of the most-disturbed sites with accuracies of 88.24% and test sites with accuracies of 81.22%. BIs showed the highest remarkable linear relationship with a surrogate of comprehensive gradient of seven measured environmental variables. In addition, BI was significantly related with community taxa richness, Simpson and Shannon diversity indices. A total of 69.53% investigated sites had a quality above the good class, indicating that most streams were in good condition in Yangtze River Delta. Our study highlighted that the BI is a useful index for water quality biomonitoring and assessment in the Yangtze River Delta. We suggest that local government agencies apply the BI to their regular water quality assessments and even use it in their annual water quality reports to advance the shift in environmental management.