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Lakes & ponds--Minnesota; Glacial lakes


ABSTRACT-Most lakes in Minnesota owe their origin directly or indirectly to glacial deposition or erosion 10,000 to 20,000 years ago. The lakes' shapes have since been modified by waves and currents near the shores and by the deposition of sediment off-shore-principally the sediment produced by growth of algae and other organisms. This sediment is a receptacle for pollen grains blown into the lake from the surrounding vegetation, and the stratigraphic succession of pollen grains records the postglacial vegetational and thus climatic history of the area. The sediment also preserves the fossils of microorganisms that reveal by their chemical composition the record of past changes in salinity, which in tum is related to water levels and thus to climate. Knowledge of the natural prehistoric processes in lakes and landscapes as recorded in lake sediments provide a perspective for evaluating the effects of modem land use and pollution on the chemical and biological processes in lakes, and it may assist in plans for improving their water quality and management.

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