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Diatom assemblages were selected as indicators of lake condition and to assess historical lake water quality changes in 257 lakes in the northeastern United States. The "top" (surface sediments, present-day) and "bottom" (generally from >30 cm deep, representing historical conditions) samples from sediment cores collected from lakes and reservoirs were analyzed for diatom assemblages. The distribution of diatom species was closely related to several environmental variables, primarily lake water pH, total phosphorus, and chloride. Using weighted-averaging calibration and regression approaches, we constructed predictive models to infer these variables from the diatom assemblages. The diatom-based inference models were then used to assess the current status of lake water quality and to assess historical changes in lake water conditions in natural lakes over the past 150 years. Changes were also assessed in reservoirs. Population estimates of historical changes in limnological variables were made for all lakes of the northeast United States and also for lakes in the Adirondacks, New England Uplands, and Coastal Lowlands/Plateau ecoregions. The extent of cultural impact has been quite variable among the ecoregions, with marked water quality deterioration occurring in hundreds of lakes. Chloride and phosphorus levels have increased, especially in lakes that currently have high concentrations. Low-pH lakes have become more common in all three ecoregions. The maximum abundance of low-pH lakes was recorded in the Adirondacks, an area receiving the highest acidic precipitation in the northeast. In the Coastal Lowlands/Plateau, there has been a clear increase in eutrophic lakes, as inferred by total phosphorus. This was accompanied by a marked increase in the number of lakes with high chloride levels.

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