The model Rostherne represents the first attempt to apply SEM XRMA (scanning electron microscopy X-ray microanalysis) data to lake ecosystem modelling. It considers subsystems proved to be most important for Rostherne mere (Cheshire, UK) with incorporation of uptake dependency of one nutrient upon internal deficiency in another. The model showed a reasonable fit (R-2 = 0.87, P < 0.001) between measured data and simulation curves for most of the considered variables (i.e. P, Si, chlorophyll-a and algal concentrations in the lake water, nutrient mass fractions of algal cells, etc.) and could, therefore, have been used to estimate some parameters and variables which were not measured otherwise (e.g. sedimentation and growth rates, etc.). The possibility of incorporating alternative expressions for the processes considered is discussed and tasks for future research in relation to coupling of Various submodels with the proposed submodel of nutrient uptake are envisaged. (C) 1998 Elsevier Science B.V. All rights reserved.