In this paper we present a novel approach for modelling spatial distributed biochemical and environmental processes like the growth of plants and the related biochemical reactions. One of the main challenges for modelling of spatial distributed phenomena is the estimation of the model parameters. The physical phenomena like flow and mass transport can be described by PDEs of fluid dynamics, but for effects like growth rates often no analytic models are available. However, in many cases experts have knowledge about the system behaviour that can be formulated by a set of if-then-rules. As this kind of knowledge can easily be handled by socalled Fuzzy models we propose the coupling of FEM models with such Fuzzy models. By this means one or more parameters of the classical PDEs are estimated by Fuzzy Models. Besides the natural inclusion of expert knowledge a second benefit of this approach consists in the fact that Fuzzy Models can describe even very nonlinear phenomena. The proposed approach is applied for modelling the growth of algae of Orbetello lake in Italy. Index Terms— Finite Element Method, Fuzzy Model, Expert Knowledge, Eutrophication