Mountain ponds in the northeastern US generally have few direct anthropogenic impacts, so may act as a regional background signal for changing atmospheric deposition and climate change. This talk will present long-term trends on changing geochemistry in mountain lakes in the Northern Appalachians and Adirondack Mountains, using data collected from 257 ponds between 1978-2019. This regional dataset revealed several widespread and significant trends, including declining sulfate as “acid rain” was regulated in the Clean Air Act and increasing dissolved organic carbon. To further explore interactions between water chemistry, temperature, and biological communities, we also present data from more comprehensive monitoring at 8 ponds in the western Maine mountains. While mountain lakes have some characteristics that are distinct from larger or lower elevation sites, we argue that these ponds provide important evidence for regional changes and an opportunity to explore the mechanisms of physical and biological interactions.