Figure S1. Ambient temperature and disinfectant types of sampling periods and water temperature. In panel A, red lines indicate periods of stable disinfectant application. The types of disinfectant are labeled to the lines. In panel B, water temperature data were retrieved from the monitoring data of Illinois American Water. Symbols indicate monitoring sites N1, C0, S1, S2, and N2 within the distribution system.

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Figure S2. Description on distribution system conditions associated with water meter sites. (A) Installation Years of water mains that supplies the sample sites. (B) Installation year of service lines connected to the water meters. (C) Water main materials. (D) The correlation of water main material to water main installation year. (E) The distribution system configuration. (F) water age.

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Figure S3. Rarefaction curves (A) and rank abundance curves (B) in WM biofilms and water samples before sub-sampling.

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Figure S4. Alpha diversity comparison between water and water meter samples.

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Figure S5. Multidimensional scaling of community similarity based on Bray-Curtis distances for all the biofilm samples (A) and those from seasons receiving partial free chlorine disinfection (fall 2009 shown in Panel B and winter 2010 shown in Panel (C). Figures supports that community structure were not strongly affected by disinfectant type.

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Figure S6. Absence of correlation between Bray-Curtis similarity and route distance. Black lines indicate the regression results.

Figure S7. (A) Cluster analysis on the group centroids of 10-day temporal bins. Bins containing 3 or less samples were excluded from the analysis. Results showed similar trend as the cluster analysis from 20-day bins. (B) Canonical analysis of principle coordinates for Bray-Curtis distance between water meter biofilm samples, with seasons (summer, fall and winter) as the constraint variables. The analysis was done as discriminant analysis and the number of PCO axes was chosen to maximize a leave-one-out allocation success to groups. The proportion of variation explained by the chosen principle coordinates is 92.6%; the size of the first squared canonical correlation is 0.743, and the second is 0.466. Percentage of allocation success is 80.86%.

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Figure S8. Seasonal fluctuation of alpha diversity in WM biofilms. ANOVA on both groups showed significant between-group differences. Groups with significant difference (p<0.05) in Fisher’s test are marked on the plot.

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