The A/C (Araphidineae/Centrales) ratio in high and low alkalinity lakes in eastern Minnesota

SUMMARY. Diatoms from surface sediment samples from lakes on both sides of a glacial drift border in eastern Minnesota show that Araphidinate diatom genera like Fragilaria, Asterionella and Synedra are more common in lakes in drift with lower CaCO3 whereas Stephanodiscus and Melosira species are more abundant in those with high CaCo3. In a lake sediment core from low carbonate drift Fragilaria crotonensis and Asterionella formosa increased when pioneer farmers arrived. These results confirm speculation that high A/C ratios are associated with low‐alkalinity eutrophic lakes.