Changes in the elemental composition of Asterionella formosa during the diatom spring bloom

Changes in the elemental composition of the diatom Asterionella formosa within mixed phytoplankton samples were determined over the spring bloom period using energy dispersive X-ray microanalysis. Using a 10 kV electron microprobe, X-ray information from the top 1–2 μm of the cell revealed overall mean concentrations of: Si (4636 mmol kg–1 dry wt), P (82), S (54), Cl (71) K (94) and Ca (90). Concentrations of all elements showed wide variation within each date sample, with unimodal frequency distributions approximating to a Normal distribution. Correlation of the entire data set demonstrated clear statistical associations between elements, including Si, P and K. Uptake of Si during the course of the diatom bloom led to a major fall in lake water concentration (1.0 to 0.07 mg l–1), which correlated with a decrease in the mean cell Si concentration (6735 to 3107 mmol kg–1 dry wt) during the final phase of the bloom. Mean cell concentrations of P and K also showed marked decreases with time, in close parallel with cell Si. These changes in P and K were attributed to the high level of internal correlation with Si and not to any significant decrease in P and K availability in the environment.