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

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Abstract
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.

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