Remote high alpine lakes are sensitive indicators of atmospheric nitrogen (N) deposition. In the Pacific Northwest, N concentrations in alpine lake water are often naturally very low, which restricts growth of phytoplankton that are the base of the food web. Lakes have distinctive communities of species that have evolved under these conditions. Anthropogenic N deposition originating from the burning of fossil fuels and agricultural practices has increased concentrations of reactive N in lakes, specifically nitrate (NO3-). Reactive, inorganic, N is the only form that can be directly used by organisms. By increasing lake N, anthropogenic N deposition has potential to stimulate phytoplankton growth, and shift phytoplankton growth from N-limited to phosphorus-limited with resulting changes in species composition and ecosystem function.