Fossil diatom assemblages in short cores of lake sediment from nine lacustrine environments (seven lakes) in Minnesota and South Dakota show the ecological reaction of freshwater diatoms to limnologic changes associated with enrichment following European-American settlement. The time of settlement is stratigraphically determined by an increase in the proportion of Ambrosia (ragweed) pollen, which signals late 19th century land clearance and cultivation in this region. In some cases other stratigraphic profiles, such as phosphorus or mining wastes, are used to date settlement activities near the lakes. Marked changes in the diatom stratigraphy frequently correlate with the time of settlement around the lakes, and in most cases it has been possible to interpret these changes in the context of limnologic modifications caused directly or indirectly by the settlement activities of man. Initially, diatom diversity decreases as the lakes become enriched by increased erosion and (or) by disposal of municipal wastes. Littoral (epiphytic and benthic) diatoms become underrepresented in the sedimentary record compared to predisturbance times, perhaps because of excessive shading by blue-green algae, or simply because they are numerically swamped out by massive blooms of planktonic diatoms. As enrichment increases, the more or less even distribution of spring, summer, and fall planktonic diatoms changes to a planktonic diatom flora dominated by species that bloom in the early spring—sometimes even under the ice. Foremost among these are Stephanodiscus minutus and S. hantzschii, the latter characterizing the most eutrophic lakes studied. Apparently the summer and fall diatom plankton cannot compete with the massive blooms of floating, blue-green algae that occur in the warmer seasons. Only in the shallow turbulent lakes do the heavy summer and fall diatom plankters maintain sizable populations that effectively compete with the buoyant blue-green algae. The stratigraphic record of the blue-green algae is inferred by the stratigraphy of Chydorus sphaericus, a normally littoral cladoceran that utilizes blue-green algal filaments to suspend itself in the limnetic regions of a lake, thus vastly increasing its habitat and population size. There are several variations on this theme, depending on the initial trophic state of the lake and other limnologic characteristics such as basin morphometry, but overall, the diatom stratigraphy of lake sediment is an effective way to assess man’s impact on the lake ecosystem.