This dissertation evaluates how lakes respond to changes in their environmental and climatic settings. This dissertation consists of two lake modeling projects and the environmental and paleoclimatic reconstructions of four lakes in southwestern Montana. Lakes and their associated biological communities respond to environmental and climatic change on rapid timescales. Lake modeling and lake core stratigraphies are complementary tools for exploration of how lakes and their biota respond to environmental or climatic perturbations. In one modeling approach for exploring these issues, a simple hydro-climatological lake model was developed that classifies lake sensitivity to climatic perturbations based upon lake area, catchment area, precipitation, and evapotranspiration. Using simple ratios of these commonly measured parameters the model classifies lakes into three domains: ephemeral, sensitive to vegetation change, and permanent. The lakes that plot within the sensitive to vegetation change domain should show water balance fluctuations in response to environmental change, and these lakes would make good targets for paleoclimatic studies.

Diatom records from four lakes, Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake, in southwestern Montana provide late-Holocene (past 3000 years) records of environmental and climatic variability. The lakes show similarities in the timing of major changes in the fossil diatom assemblages, suggesting regional climate forcings. Spectral analysis of the lake-core records suggest periodic fluctuations at spectral frequencies that are characteristic of oceanic influence on climate, such as the Atlantic Multi-decadal Oscillation and Pacific Decadal Oscillation.

The Crevice Lake core diatom record shows three distinctive diatom communities during the approximately the last 1000 years. The model DYRESM-CAEDYM was used in an inverse modeling approach to provide a means to estimate climate variables during these three stages. The model estimates of climate variables during the Medieval Period, the Little Ice Age, and the 20^th century, include incoming shortwave radiation, cloud cover, vapor pressure, and wind speed. The model results suggests that changes in spring seasonality, when the climate variables differ the most, is more important in affecting diatom community composition than total deviations from modern averages.