## 2 Methods
### Assessment of Regrowth Potential and Ratio of Fixed to Dissolved Nutrients  
Day 0 cell numbers were typically quantified as technical replicates from samples in AOC-free 100 mL Schott bottles (with the exception of hot water samples, which were sampled in 50 mL glass vials). After the quantification of cell numbers on day 0 (TCCday0 and ICCday0), the water samples were aliquoted into three AOC-free glass vials (50 mL, cat.nr. 7612150; Th. Geyer GmbH & Co. KG) containing 20 mL sample each. These vials were stored at 22 \({}^{\circ}\)C (\(\pm\)1 \({}^{\circ}\)C) for seven days (or indicated times in cases where the water was sampled from the hot water boiler), followed by repeated quantification of cell numbers by flow cytometry. Only ICC data are shown after regrowth. The ICC numbers obtained after seven days of incubation (ICCday7) represent the maximal intact cell concentrations that are supported by the nutrients contained in the original samples. The difference between ICCday0 and ICCday7 is referred to as the regrowth potential. Regrowth factors, on the other hand, are the ratio of ICCday7/ICCday0 and indicate how much the bacterial concentrations can increase on the basis of the given available assimilable nutrients.  
We supplemented 20 mL aliquots of samples subjected to ozonation or to high temperatures with a 1/1000 th volume (20 \(\mu\)L) of the corresponding untreated water (prior to the corresponding treatment step) to re-introduce bacteria capable of replication and to capture the correct regrowth potential. Samples without spiking were processed in parallel, and their regrowth was compared with that of spiked samples. Data with higher regrowth were selected. Samples from the chlorinated DWDS were not spiked as they still contained bacteria capable of replication  
ICCday0 and ICCday7 were used to calculate the ratios of biologically fixed nutrients (=biomass) and dissolved nutrients. The ICCday7 values were considered to represent the maximal concentrations of intact cells that the nutrient pool of the corresponding water supports. When the maximal cell concentrations were reached, it was assumed that all available dissolved nutrients had been biologically fixed in biomass and biological stability had been reached. The ICCday7 values were assessed in relation to the ICCday0 values that represent the intact cell concentrations that were actually present when taking samples. The ICCday0 values thus represent the proportion of the nutrient pool that was biologically fixed in the form of biomass at the time point of sampling. The remaining proportion of the nutrient pool, as a net value, was thus dissolved and available (i.e., not biologically fixed) and represents the nutrient reserve available for regrowth. The proportions are relative and do not provide information about the absolute nutrient concentrations. Also, the portion of the nutrient pool that was respired and, thus, not available for biomass production was not considered. In cases of raw water samples that showed \"negative growth\", meaning that the ICC values after 7 days of incubation were lower than on day 0 (e.g., through the die-off of phototrophic bacteria or changes in the microbiome), the calculation was performed as if all nutrients were biologically fixed on day 0.