Several pelagic ecosystem models now allow flexibility in the elemental composition of the modeled plankton functional types, often based on some variant of the model for phytoplankton photoacclimation by Geider et al. (1998). This aspect of the models, however, remains poorly validated. Here we attempt a synthesis of how well physiologically-based models are able to describe observed changes in biomass stoichiometry, and how sensitive modeled fluxes are to assumptions on physiology. We do so by trying to reproduce both simple lab experiments and global field obervations. The global model that we use allows for flexible stoichiometry in both non-diatoms and diatoms, with an extension of the Geider model for diatom Si:C variations. We show both the Geider et al. (1998), and the Pahlow (2005) model are able to describe shifts on C:N stoichiometry over the course of a bloom, but both have problems describing the recovery from nutrient starvation. Globally, largest deviations of the C:N ratio from Redfield are modelled in the nutrient-depleted subtropical gyres, but they contribute little to vertical export. Lab-observed changes in C:Chl ratio are described well by the Geider model. Modelled global patterns in C:Chl are consistent with a number of field-and satellite-based observations. It remains a challenge, however, to distingish between variations in stoichiometry due to physiological acclimation and to community composition.