Percentage analyses of the diatoms from a core of sediment show the changes in the community since the formation of Pickerel Lake. The lake basin is in calcareous drift from the Wisconsin glaciation and is located on the Coteau des Prairies upland.

This work supplements earlier studies of the pollen, plant macrofossils, and mollusks made by Watts and Bright (1968), in which they presented evidence for four vegetational stages: (1) spruce forest, (2) deciduous woodland, (3) bluestem prairie, and (4) continued open prairie with the return of some woodland. The diatom core was correlated with the nearby pollen core by pollen analysis.

The diatom succession begins in diatom zone I, with an assemblage dominated by Fragilaria and Mastogloia species and containing many taxa of alkaline waters. It also includes several taxa that generally prefer acid environments and only occur in the lower part of this core. This diatom assemblage is correlated primarily with the presence of spruce forest and its associated soils; the change in the diatoms occurs when the deciduous woodland that follows is partly replaced by prairie openings. It is suggested that the changes in the type of material (mainly soil) washed into the lake produced a more alkaline environment, which excluded some of the acidophilous taxa.

Diatom zone II is largely transitional to zone III, which is dominated by Melosira species and includes many indicators of a more eutrophic (nutrient-rich) environment, for example Fragilaria crotonensis. More brackish-water taxa also indicate an increase in the mineral content of the water. All of these features are consistent with evidence from the pollen stratigraphy, which indicates that this is the time of maximum prairie development in this area, along with maximum soil erosion and lake-water evaporation.

Diatom zone IV is largely transitional, corresponding to the time when woodland was once again developing around the margins of the lake basin. Diatom zone V features increasing percentages of forms characteristic of surf zones in lakes, indicating perhaps a shallowing of the site as a result of sedimentation. There is some indication of recent enrichment, concurrent with an increase in Ambrosia pollen (ragweed), which is possibly due to disturbance by man.

The use of a constant volume of material in the diatom preparation makes it possible to draw a diagram of diatom abundance for each level. The diatoms are scarce in samples from the lower part of the core, and four radiocarbon dates bear out a suggestion that more inorganic material was deposited at the core site during the early history of the lake than subsequently.

A total of 207 taxa is reported, and ecological notes on all are included in an appendix. A diatom diagram shows curves for 71 of the taxa.

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