Impact of Urbanization Gradients and Stormwater Input on the Microbial Community Structure within Impaired Low-Order Streams

As more of the world’s population continues to concentrate in urban areas, the influence of urbanization on surrounding freshwater ecosystems has become stronger. Many studies have shown the negative effects that urbanized land use can have on the chemical composition and macroecology of neighboring streams as a result of unpermitted discharges of wastes and toxins, as well as contaminated stormwater input. However, few studies have shown how urbanized land use and storm events modify the complete microbial community structure within urban stream environments. In this study, 16s rRNA amplicon sequencing was conducted for baseflow and wetflow samples of two locations on Second Creek, an urban impaired stream in Knoxville, TN. This approach allowed us to gain insight into how an urbanization gradient affects the microbial community structure within an urban stream, as well as how the microbial community structure within an urban stream shifts in response to storm events. These results were compared against fecal indicator bacteria (FIB) quantities to assess the relevance of Escherichia coli (E. coli) as an indicator of fecal contamination in Second Creek. Once patterns were established for Second Creek, the sampling scope was broadened to include six other streams within Knox County, each impacted by various levels of urbanized land use. Results from Second Creek suggest that urbanization introduces a clear fecal signature and a reduction in microbial diversity further downstream, as well as substantial changes in microbial community composition following storm events. However, wetflow results call into question the applicability of E. coli as an indicator of fecal contamination after storm events. While the other streams show similar results to Second Creek during wetflow, many baseflow patterns are not shared between all streams. These results suggest that a “one size fits all” approach is not appropriate when determining microbiological contamination in urban streams. Additionally, results suggest that fecal contamination does not increase during wetflow and that E. coli may be regarded as an adequate indicator of fecal contamination under baseflow conditions only.