Abundance maps of diatom percentages from 174 Minnesota lake sediment surface sample show that many diatom species have centers of abundance in lake types from particular regions of the state. Small Stephanodiscus species characterize lakes in the southwestern prairies and in urbanized areas where trophic status is high. Aulacoseira granulata and Stephanodiscus niagarae are most abundant in the shallow, eutrophic lakes of southwestern Minnesota. These geographic associations result from environmental optima for species. Although correlations between particular species and environmental factors show high variance, clear relations can be demonstrated. In particular, the DECORANA program for ordination analysis shows that many species have clearly defined optima either in low- or high-alkalinity lakes. The relations discovered from the surface sample data set can be used to understand the fossil assemblages from Elk Lake, Minnesota. The dominance of small Stephanodiscus in Elk Lake suggests that the lake has been somewhat eutrophic for most of its history. The appearance of Aulacoseira ambigua and Aulacoseira granulata during the prairie period at Elk Lake implies that the lake was shallower or more turbulent at that time. DECORANA ordination shows that the fossil diatom assemblages of Elk Lake have not changed much since the lake was formed. Thus, environmental changes at Elk Lake were probably very subtle.
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