Monovalent—divalent cation ratios and the occurrence of phytoplankton, with special reference to the desmids

SUMMARY. We have analysed data from ninety-nine Scottish freshwater lochs, to explore the relationship between water chemistry and phytoplankton assemblages. Our results confirm that there are strong correlations both between the phytoplankton quotient and divalent cation concentrations, particularly Ca⁺⁺, and also between the phytoplankton quotient and the (Na⁺+ K⁺)/(Ca⁺⁺+ Mg⁺⁺) ratio. However, the latter is evidently a spurious relationship, arising from the former association, together with an association between Ca⁺⁺ and the (Na⁺+K⁺)/ (Ca⁺⁺+ Mg⁺⁺) ratio. The observed correlation does not persist if account is taken of concomitant variations in the Ca⁺⁺concentration. This conclusion is suggested both by relatively informal analyses (median polish, partial correlation coefficients) and by more formal modelling and testing (stepwise regression, all-subsets regression).