A decision support system and method can be used to control a water treatment or distribution system. The decision support system collects data from multiple water system operators and analyses the data for a selected water system according to one or more rules or algorithms. The system returns data, optionally including alerts or predictions, to the system operator. Optionally, the decision support system uses machine learning applied to (i) historical data from a selected water system and/or (ii) data from other water systems to modify the rules or algorithms used to analyze current data from a selected water system. In some embodiments, the data collected includes microbial population data such as ATP data, optionally including derivatives of microbial population data; microbial speciation information; or, metagenomic data.