As most wastewater systems run using mesophilic organisms - we have a normal temperature range from 10 - 40 deg C (according to the text books). However, we can further break this down into groups based on narrower temperature ranges. For example, Pseudomonas fluorescens - a very nice wastewater organism with excellent metabolic capabilities - thrives from 5 - 32 Deg C. So in colder weather, P. fluorescens and other lower temperature mesophiles increase as a percentage of the population. The same drift in populations is seen at higher temperatures as we see increased predominance of high temperature microbes as we go above 37 Deg C.
Knowing that mesophiles can be further divided into low, medium, and high temperature groups, we need to look at where temperature growth pressures do not clearly favor any group and tend to cause upsets as the biomass adjusts.
In lagoons we often see stress as we drop below somewhere between 15 - 20 Deg C. Even many activated sludge units experience high turbidity in this temperature range. As the cold temperature group of mesophiles increase in population, turbidity drops and the biomass reaches a new equilibrium. Below 5 - 10 Deg C, we experience another fluctuation as true psychrophiles start to establish in the biomass.
On the high end, as temperatures go above 37 - 42 Deg C we see another change in biomass dynamics favoring higher temperature mesophiles. Above 45 Deg C, the changes and stress become very apparent with many systems not functioning to needed efficiency and more thermophilic strains start to appear.
Again, my values are based on observation of municipal and industrial wastewater systems. Each system has its own range where the combination of setup, retention times, influent makeup, and operational parameters dictate the temperature fluctuation points.