The population of Ceratium hirundinella in a freshwater lake was studied both by traditional limnological methods and X-ray microanalysis during the final phase of stratification. Correlation and Factor analysis of intracellular elemental concentrations revealed a clear pattern of statistical relationships mainly determined by three factors, with K, P and Mg demonstrating higher loadings on the first, Ca and Si on the second, and S, Cl and Na on the third factor. S also showed significant correlations with K and P, elements belonging to the first association. The results of the statistical analysis are summarised in a structural model of intracellular elemental associations. Consideration of the latter in the mathematical description of nutrient uptake and plankton population dynamics should prove useful in aquatic modelling. Development of the population resulted in a dramatic decrease in transparency and nutrient concentrations in the epilimnion water. The recorded peak of the population coincided with minimum levels of ambient PO4.P and DIN and maximum intracellular levels of a number of important chemicals (Mg, K, P, S). The decrease in the epilimnion concentrations of orthophosphate did not match the simultaneous increase in the total amount of P contained within the population of Ceratium. Ambient nutrient scarcity, therefore, could have been alleviated by Ceratium's capacity for vertical movements, heterotrophic consumption of organic materials and/or partial entrainment of the hypolimnion water during periods of colder weather. Following the demise of the population, chemicals accumulated inside the cells were removed from the epilimnion with easily sedimented biomass. The results clearly showed the importance of Ceratium in transforming nutrients between different levels of the Rostherne ecosystem. Considering the high lake water retention time, the geochemical consequences of Ceratium's ecochemistry and population dynamics may considerably delay changes in the lake's trophic status expected after sewage diversion.