We combine field and lab work with biogeochemical models to understand the control of catchment and climatic variables on headwater lake biogeochemistry. Headwater lakes are ideal study systems because they are sensitive to external influence. My students, colleagues, and I develop and apply biogeochemical systems models with field data, new sensor-based measurements, and geospatial information to pose and answer questions about how and why lakes change. This work ultimately allows us to answer questions about applied issues such as the risk of low oxygen events, harmful algal blooms (HABs), lake ‘brownification’ (due to increased dissolved organic matter from catchments) and safe provision of drinking water under different climate scenarios.