RECENT PALEOLIMNOLOGY OF 3 LAKES IN NORTHWESTERN MINNESOTA

A paleolimnological study was undertaken to investigate changes in 3 Minnesota [USA] lakes over the last 100 yr and to demonstrate the stratigraphic effects of cultural eutrophication in 2 of them. The study combined the analysis of the lake sediment from short cores with stratigraphic analyses of pollen, plant macrofossils, mollusks, diatoms and certain other algae, chydorid Cladocera and Daphnia ephippia. The rise of Ambrosia type pollen (ragweed) marks the onset of interference with the landscape by European man, which can be closely dated. Calculations of sedimentation rates from this base gave reasonable correlations of other stratigraphic events with historical events. Elk Lake is considered unpolluted today and was chosen as a control. Man''s effects are limited to logging some of the surrounding forest and to the construction of a dam. Small changes in the lake''s fauna and flora are demonstrated, showing the sensitivity of the lake to changes in its catchment area. Lake Sallie and St. Clair Lake, in the same watershed as the city of Detroit Lakes, are affected by logging and by addition of nutrients from agricultural runoff and sewage effluent. Considerable responses by the lake organisms are apparent. In Lake Sallie the changes were gradual, but in St. Clair Lake they were very abrupt because the lake was partially drained at the same time and the water volume was thereby reduced. The merits of such an integrated study, the types of information gained from the analyses of the various fossils, and the wider application of the results are discussed.