The population of Stephanodiscus rotula in a temperate eutrophic lake was studied over a 2 month period (January to early March). Although during the study period nutrient concentrations in the lake water remained far in excess of phytoplankton requirements, no notable increase in chlorophyll levels was recorded. This suggests that algae were limited by shortage of light and low temperatures. Frequency distribution analysis of cell diameters showed a dramatic size reduction at the end of winter, followed by restoration of higher values by early spring. Electron probe X-ray microanalysis spectra from Stephanodiscus cells routinely showed peaks of Ca, K, Si, P, S, Fe, Al, Mn, Mg, Na and Cl, with substantial variation in elemental concentrations both between and within samples. End-of-winter reduction in the cell size coincided with a considerable depletion of intracellular chemical levels of Si, P, Cl and K and could be related to the concurrent decrease in dissolved organic C and increase in intracellular Al. Correlation and factor analysis of intracellular elemental concentrations showed that statistical elemental associations within Stephanodiscus cells were mainly determined by three factors, with P, Cl, Si and K showing higher loadings on the first, Ca, Mn and S on the second, and Fe, Ca and Al on the third factor. Significant correlations among the elements of the first association may indicate the importance of P (ATP), K (through involvement in P metabolism) and Cl (possibly charge balance) in the active Si uptake during the study period.