Numeric models simulating the principal components in lake ecosystems do not, at the present time, satisfactorily describe the dynamics of the phosphorus in the water. This is primarily due to the great complexity of the exchanges that take place between the PO4 and the other mineral or organic fractions, and to the difficulty in quantifying these exchanges. Furthermore, the rapid dynamics of the exchanges involve timescales lesser than those used in the models to describe an ecosystem, which essentially use a one day step. Lastly, the analytic methods used provide a measure of forms of phosphorus which do not correspond to the functional entities simulated by the models. This paper summarizes present general knowledge of the phosphorus cycle in the microbial loop, of what it is normally possible to measure, and what we currently know how to model.