Comprehensive year-round limnological investigations of Mountain Lake, Virginia were conducted from November 1992 through October 1994. Monthly physical, chemical, and biological measurements suggest that this ecosystem has shifted toward a more eutrophic condition. A trend in whole lake mean annual orthophosphate-phosphorus and inorganic nitrogen increases have occurred over the past decade, although primary productivity, cell densities, and chlorophyll a values indicate no consistent trends. Hypolimnetic oxygen deficits, however, have continued to increase over time, and in October 1994 the first measure of anoxic deep water was observed. Concurrently with the chemical changes, shifts in macrophytes and phytoplankton have occurred. Most notably were reductions in Nitella megacarpa and Dinobryon spp., an increase in Ceratophyllum sp., and a summer dominance of Sphaerocystis schroeteri and Aphanocapsa_ elachista. These two species represent algal classes, Chlorophyceae and Cyanophyceae, which have also increased in relative abundance and seasonal persistence in Mountain Lake. Nutrient addition studies conducted in 1994 indicated limiting concentrations of both phosphorus and inorganic nitrogen during most periods of thermal statification. Significant increases in primary productivity occurred within 48 hrs with the addition of K₂HPO₄ and/or NH₄NO₃ in the pico-, nano-, and microplankton size classes. The smaller size fractions (picoand nanoplankton) responded more strongly to the N enrichment, whereas the microplankton responded more with the P enrichment. In all cases, ammonium-N was the preferred inorganic nitrogen source during these Studies, as suggested from earlier studies of methylamine uptake in this lake. Studies of the importance of various phytoplankton size classes (pico-, nano-, micro-, and macro-) during thermal stratification indicated that the microphytoplankton were the most important size class in cell number, cell diversity, and contributed ~95% of the total primary productivity. The picophytoplankton were second in importance constituting ~5% of total primary productivity, whereas the relatively small numbers of nano- and macrophytoplankton contributed little primary productivity. This finding contrasts to many other oligotrophic ecosystems where the pico- and nanophytoplankton have demonstrated significantly greater importance. Increasing nutrient concentration, increasing hypolimnetic oxygen deficit, and shifting phytoplankton community structure suggest that Mountain Lake is entering the early stages of eutrophication. Continuation of increasing nutrient loads in this natural lake will potentially further shift this ecosystem toward eutrophic conditions. / Ph. D.