Urbanization and built infrastructures influence stream bacterial communities and pathogens along mountain to urban transitions
Stream bacterial communities are important in determining water quality by controlling biogeochemical transformations of limiting nutrients, degrading pollutants, and influencing pathogen abundance. However, bacteria enter streams in pristine montane regions, pass through multiple built infrastructures, and experience an immense variety of urban systems. Due to these vast differences in abiotic conditions and land use, it remains unclear the extent that bacterial communities are connected across these gradients or the degree that pathogens coincide with shifts in community composition. To address these concerns, we evaluated bacterial communities, Escherichia coli (a pathogen and fecal-matter indicator species) densities, and water chemistry in three watersheds along montane to urban gradients in UT, USA. The three watersheds share common water sources (snow-melt) but different levels of urbanization with the Provo River experiencing a rapid landuse transition from agriculture to urban; Red Butte Creek already highly urbanized, and the Logan River slowly transitioning in landuse from agriculture to urban. We used 16s rDNA target metagenomics and performed ANOVA and multivariate statistics in R to evaluate bacterial community composition in fall 2014. We also measured concentrations of E. coli monthly for one year, using IDEXX Colilert and Quanti-tray 2000.
The built environment, specifically reservoirs, and not the urban environment, strongly influenced bacterial community composition, while different levels of urbanization regulated E. coli densities. Reservoirs decreased bacterial diversity (one-sided t-Test, P <0.001), richness (P <0.001), and evenness (P <0.001) suggesting that environmental conditions unique to reservoir discharge or ambient conditions are strong drivers structuring communities. Based on a PCoA analysis, montane sites were clustered together, while urban sites in each watershed clustered independently. A PERMANOVA analysis supported these findings and indicated a significant interaction between watershed and elevation (F-value=3.4244, R2=0.22697, P<0.001). Log E. coli concentrations were 3-times higher at urbanized and agricultural sites than montane sites, particularly in summer and fall months (repeated measures ANOVA, F>62, P<0.0001), with the highest values measured in the most urbanized watershed, Red Butte Creek. Our results demonstrate that built infrastructures have the potential to disconnect communities along streams more than environmental changes associated with urbanization, and fecal matter and potential pathogen inputs were undetectable in montane systems and not influenced by reservoirs.