Areal Distribution and Stratigraphy of Diatoms in the Sediments of Lake Sallie, Minnesota

The patterns of modern diatom sedimentation in presently eutrophic Lake Sallie, Minnesota, USA, have aided in elucidating the trophic changes in the lake caused by man's activities since the 1870s. The modern sedimentation patterns were determined from a transect of surface sediment samples taken through littoral and profundal zones of the lake and the trophic history is interpreted from the diatom content of two lake—sediment cores. The transect shows that, in this lake, diatom valves are mostly deposited in the area of their living habitats and are not homogeneously redistributed by lake currents. Planktonic diatoms Stephanodiscus sp. cf. S. hantzschii, Stephanodiscus niagarae, and Melosira granulata dominate in the profundal sediments, and sessile Fragilaria species characterize the sediments beneath the littoral habitat. Fragilaria construens var. binodis prefers nutrient—rich water entering the littoral zone near the inflow of the Pelican River, a stream carrying secondary sewage effluent from the town of Detroit Lakes. The diatom stratigraphy indicates that the benthic and planktonic diatom communities have changed in species composition and probably in productivity as a result of cultural eutrophication. Spring—blooming Stephanodiscus sp. cf. S. hantzschii dominates the post—settlement record. A core near the Pelican River inflow shows a marked increase in Fragilaria construens var. binodis that probably reflects increased nutrient levels during the most recent history of Lake Sallie