Widespread synchrony in phosphorus concentrations in northern lakes linked to winter temperature and summer precipitation

In recent years, unexplained declines in lake total phosphorus (TP) concentrations have been observed at northern latitudes (> 42°N latitude) where most of the world's lakes are found. We compiled data from 389 lakes in Fennoscandia and eastern North America to investigate the effects of climate on lake TP concentrations. Synchrony in year-to-year variability is an indicator of climatic influences on lake TP, because other major influences on nutrients (e.g., land use change) are not likely to affect all lakes in the same year. We identified significant synchrony in lake TP both within and among different geographic regions. Using a bootstrapped random forest analysis, we identified winter temperature as the most important factor controlling annual TP, followed by summer precipitation. In Fennoscandia, TP was negatively correlated with the winter East Atlantic Pattern, which is associated with regionally warmer winters. Our results suggest that, in the absence of other overriding factors, lake TP and productivity may decline with continued winter warming in northern lakes. Scientific Significance Statement Phosphorus is one of the most important elements in lakes, and frequently serves as the limiting nutrient for aquatic plants and plankton at the base of the food web. The effect of a changing climate on phosphorus concentrations in lakes at northern latitudes is not well understood. By combining data from almost 400 lakes in 18 regions in North America and Northern Europe, we were able to observe similar patterns of year-to-year variability in phosphorus both within and among regions, indicating that concentrations of this important nutrient are being controlled by large scale climate factors. Using a machine-learning model, we identified winter temperature and summer precipitation as the most important climate influences on phosphorus, and the role of winter climate was reinforced by looking at the relationship of regional median TP with large-scale atmospheric patterns. Taken together, our results suggest that climate is important for lake phosphorus, and that winter warming may be contributing to observed declines in lake TP at high latitudes.