Quantitative analysis of siliceous microfossils in 210Pb-dated Lake Michigan sediments shows five time zones in microfossil abundance and composition. Sediments deposited before 1885 contain low abundances comprising species associated with very oligotrophic lakes. Between 1885 and 1925 abundance increases slowly as does the ratio of planktonic diatoms to benthic diatoms and chrysophyte remains. These trends accelerate in sediments deposited between 1925 and 1964, most rapidly between 1954 and 1964. Maximal microfossil abundance occurs in sediments deposited ∼1964 and numbers, particularly of oligotrophic species with summer abundance maxima, decline thereafter. This sequence appears to reflect progressive eutrophication of Lake Michigan culminating in silica limitation during summer stratification after the mid-1960s.
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Stoermer et al. (1990) studied this question.