18S: Contrasting Urban Heat and Cool Islands Reveal Chlorine Residual, Not Heat Gradients, Drive Drinking Water Microbial Stability and Pathogen Dynamics
Urban heat island (UHI) effects and climate warming threaten drinking water quality by increasing temperatures during distribution, potentially favoring opportunistic pathogens. We compared seasonal microbial dynamics in two district metered areas (DMAs): an UHI-DMA with stronger in-network warming and a cool island (CI)-DMA with greater thermal buffering. Integrating hydraulic modeling, physicochemical and microbial monitoring, pathogen quantification, and 16S/18S community profiling, we disentangled source-water influences from in-network processes. Summer warming increased microbial abundance within both systems (e.g., +0.40?0.66-log10 ICC, +1.32?2.68-log10 Legionella spp.), with larger increases in the CI-DMA driven by depletion of already low chlorine residuals, whereas residuals in the UHI-DMA remained protective despite higher relative consumption. Nontuberculous mycobacteria often declined from high entry-point levels and showed no consistent relationship with chlorine, indicating distinct ecological controls. Bacterial community composition was primarily structured by free chlorine (R^2=0.31-0.63) opposing microbial parameters associated with summer samples, with stronger effects in the CI-DMA indicating seasonal shifts driven by weaker chlorine stress. In contrast to bacteria, eukaryotes responded directly to temperature (R^2=0.16-0.26), suggesting temperature-driven protistan succession consistent with the summer expansion of amoebal diversity. Site-specific detection of Legionella and Mycobacterium ASVs suggested biofilm-associated origin, with bulk-water occurrence likely reflecting episodic biofilm detachment under depleted chlorine conditions. Collectively, microbial instability was driven not by warming alone, but by interactions among temperature, upstream water quality, disinfectant decay, and biofilm-associated processes, highlighting that urban greening mitigates thermal stress but effective residual disinfectant management remains essential to limit microbial risks during heat events leading to elevated water temperatures.
Urban heat island (UHI) effects and climate warming threaten drinking water quality by increasing temperatures during distribution, potentially favoring opportunistic pathogens. We compared seasonal microbial dynamics in two district metered areas (DMAs): an UHI-DMA with stronger in-network warming and a cool island (CI)-DMA with greater thermal buffering. Integrating hydraulic modeling, physicochemical and microbial monitoring, pathogen quantification, and 16S/18S community profiling, we disentangled source-water influences from in-network processes. Summer warming increased microbial abundance within both systems (e.g., +0.40?0.66-log10 ICC, +1.32?2.68-log10 Legionella spp.), with larger increases in the CI-DMA driven by depletion of already low chlorine residuals, whereas residuals in the UHI-DMA remained protective despite higher relative consumption. Nontuberculous mycobacteria often declined from high entry-point levels and showed no consistent relationship with chlorine, indicating distinct ecological controls. Bacterial community composition was primarily structured by free chlorine (R^2=0.31-0.63) opposing microbial parameters associated with summer samples, with stronger effects in the CI-DMA indicating seasonal shifts driven by weaker chlorine stress. In contrast to bacteria, eukaryotes responded directly to temperature (R^2=0.16-0.26), suggesting temperature-driven protistan succession consistent with the summer expansion of amoebal diversity. Site-specific detection of Legionella and Mycobacterium ASVs suggested biofilm-associated origin, with bulk-water occurrence likely reflecting episodic biofilm detachment under depleted chlorine conditions. Collectively, microbial instability was driven not by warming alone, but by interactions among temperature, upstream water quality, disinfectant decay, and biofilm-associated processes, highlighting that urban greening mitigates thermal stress but effective residual disinfectant management remains essential to limit microbial risks during heat events leading to elevated water temperatures.
Polytechnique Montreal