Principal component analysis and taxonomic interpretation of environmentally related variation in silicification inStephanodiscus(Bacillariophyceae)

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Abstract
Principal component analysis (PCA) was performed on the correlation matrix of 16 characters measured on each of 129 specimens of the freshwater centric diatom Stephanodiscus niagarae from 14 lentic North American habitats. There were two major independent components to overall silicification. Firstly, larger valves were more heavily silicified than smaller ones, regardless of environment of development. Secondly, specimens from waters of higher ambient silicon: phosphorus ratios (Si : P) were more heavily silicified than specimens of similar size from low Si : P waters. Si concentrations alone were only weakly correlated with silicification. S. superiorensis and S. yellowstonensis are morphologically similar to S. niagarae. They are as heavily silicified as S. niagarae under similar Si : P conditions but allocate different amounts of silica to different valve features. S. yellowstonensis and S. niagarae were analysed with PCA in a previous study. The first two PC axes in both the present and previous studies are similar in the way they describe variation in overall morphology including silicification. However, the previous study found a PC axis which described silicification in a unique way (as ratios of characters that separated S. yellowstonensis and S. niagarae). That the variation described by the third axis is only observed when S. yellowstonensis and S. niagarae are analysed together suggests a genetic basis for the differences in silicification between the two species.