Impervious area percentage predicated influence of rapid urbanization on macroinvertebrate communities in a southwest China river system.

Urban areas and their development depress water physicochemical properties, river habitats and aquatic biotic integrity worldwide. Benthic macroinvertebrates at 61 sites representing three types of river reach locations (upstream rural areas, intermediate and downstream of urban areas) on the Liangtan River, a rapidly urbanized river with untreated sewage discharge in southwest China, were examined to compare their community composition and diversity. Their relationships with environmental variables were also explored. The results indicate a significant reduction in sensitive EPT (Ephemeroptera, Plecoptera and Trichoptera) species, dominance by a few tolerant taxa (Oligochaeta and Chironomidae), a significant decline in biodiversity and dominance of collector-gatherers at intermediate and downstream urban sites compared with aquatic macroinvertebrates at rural sites. The community composition of urban sites were different from those of rural sites and were quite similar to those of downstream sites, but maintained a much higher Oligochaeta density. The species-environment relation was predominately explained by the percentage of impervious area (PIA) and farmland at the catchment scale. At the local scale chemical oxygen demand (COD) and pH-value explained this relationship. Six community indicators, including richness, the Shannon-Wiener index, EPT(%), collector-filterers(%), scrapers(%) and collector-gatherers(%), had exponential relationships with PIA. Density of macroinvertebrates, Oligochaeta density, Oligochaeta(%), predator(%) and shredder(%) exhibited stepped threshold response patterns with PIA. Pielou's index had an approximately negative linear relationship with PIA. For exponential pattern, a threshold value of PIA between 10.6% and 14.4% was found. For stepped threshold response pattern, the first threshold was between 11.2% and 13.8%, and the second threshold was 23.8%. Our study indicates particularly great influences of urbanization with untreated sewage on macroinvertebrates and river ecosystems. PIA also showed to be a good indicator of urbanization and an accurate predictor of urban impacts on river macroinvertebrate integrity.