In this paper the authors construct nonlinear observers in order to discuss the validity of biological models. They consider a class of systems including lots of classical models used in biological modeling, They formulate the nonlinear observers corresponding to these systems and prove the conditions necessary to their exponential convergence, They apply these observers on the well-known Droop model which describes the growth of a population of phytoplanktonic cells. The validity of this model is discussed to the light of the performance of the observers working on experimental data.