The population of Stephanodiscus rotula in Rostherne Mere (Cheshire, UK) was studied during the spring of 1996 using light microscopy, hydrochemical analyses and scanning electron microscopy X-ray microanalysis (SEM XRMA). Data obtained were statistically analysed to reveal relationships between the elements within the cells and in the lake water. The results confirmed that algal biomass production was mainly controlled by availability of Si. The depletion of ambient Si coincided with changes in the intracellular elemental composition, particularly a considerable reduction of intracellular Si whose depletion was also reflected in changes of intracellular elemental correlations and ratios. A simple structural model of elemental relationships in S. rotula cells is proposed to explain the pattern of intracellular elemental associations. This model can be used as a reference for species response to changes in environmental parameters. S. rotula provided a significant contribution to overall biogeochemical cycling due to the removal of nutrients from the water column and transporting them to bottom sediments following the culmination of the bloom. The role of S. rotula was particularly prominent in the biogeochemical cycle of silicon, as the diatom's spring growth may account for more than 20% of its total annual loss to bottom sediments. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.