Canadian lakes show contrasting phosphorus trends across climatic regions

Phosphorus regulates lake productivity and water quality, yet long-term changes in lake phosphorus and their climatic drivers remain poorly understood at continental scales. Here we show how total phosphorus changes across 15,240 Canadian lakes > 5 km2 from 1984 to 2025 using a deep neural network trained with 35,452 field measurements and applied to 386,727 Landsat images. Arctic tundra watersheds show increasing phosphorus concentrations of 0.15 μg/L yr−1, whereas boreal forest, temperate forest, agricultural and urban watersheds show declines of 0.07, 0.11, 0.16 and 0.12 μg/L yr−1, respectively. Path analyses link northern increases to ground warming and soil erosion, consistent with permafrost-related phosphorus mobilization. Southern declines are associated with vegetation greening, hydrological change, and reduced human phosphorus inputs, while surface runoff remains important in agricultural and urban systems. These findings reveal contrasting climate-sensitive phosphorus trajectories and show that climate change can reinforce or counteract nutrient management gains, highlighting the need to integrate region-specific climate adaptation into lake management. Lake phosphorus increased in Arctic Canada but declined across other climatic regions, with northern increases linked to ground warming and soil erosion, according to a study that combined field measurements with deep learning analysis of Landsat imagery from 15,240 lakes between 1984 and 2025.