Siliceous microfossil succession in Lake Michigan

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 mid1960s. Acknowledgments Contribution 5 19 of the Center for Great Lakes and Aquatic Sciences, University of Michigan. This research was supported by NSF grant OCE 86-146 19 and grant R-813831 from the Office of Exploratory Research, U.S. EPA. The role of various factors in driving changes in phytoplankton community structure in the Great Lakes remains a topic of considerable interest and debate. Some time ago we (Schelske and Stoermer 197 1, 1972) advanced the hypothesis that reduction in available silica concentration was a major factor in Lake Michigan. The silica depletion hypothesis rests on two major assumptions: that increased phosphorus loading results in increased production of diatoms and other phytoplankton which require or utilize silica and that some fraction of the siliceous remains of these organisms is sequestered in sediments. In systems such as Lake Michigan where hydraulic renewal times are relatively long (m 100 yr) and the lake surface constitutes a significant (N 30%) portion of its total drainage basin (Schelske 1975), relatively modest increases in silica storage can result in decreased concentrations of available silica in the water column.