Ordination techniques were used to analyze phytoplankton and water chemistry data collected between 1927 and 1978 from Lake Michigan at Chicago. Ordination analysis summarized the phytoplankton data and illustrated the progression from oligotrophy to eutrophy and the subsequent reversal of cultural eutrophication after 1970 in the nearshore waters of Lake Michigan at Chicago. The analysis highlighted a significant correlation between blue-green algal biomass and Na+ concentration. The increase in mean annual Na+ concentration in Lake Michigan at Chicago and the experimental evidence implying a Na+ requirement for blue-green algae suggested that the increase in blue-greens, although influenced by P enrichment and by other factors, such as CO2 availability, allelopathic effects and N: P ratios, may also be linked with increases in Na+ concentration
