Our findings indicated that many of the reviewed studies assumed wrongly that the system was at steady state or could be described by a single-pool approach. To select the correct model, exact knowledge of C input rates and several data points are needed from the beginning of the experiment. For steady-state conditions an apparent temporal change of MRT computed from a single-pool model is thus a clear indicator that a two-pool approach must be chosen. The errors made by the wrong choice of model varied with the length of the experiment and usually resulted in an over-estimate of MRT by a factor of 1.15 for some data published on physical size separates, but by a factor of up to 11 for individual microbial biomarkers such as muramic acid.