Extended Data Fig. 1: Map of the sampling locations.

Panels show Fennoscandia (a) and northern North America (b). Lake locations are shown in blue. Orange dots denote ‘trend’ lakes with more than 15 years of bottom-water DO observations. Coloured regions mark terrestrial biomes³⁷. The biome ‘Temperate Forest and Grasslands’ includes ‘Temperate Conifer Forests’, ‘Temperate Broadleaf and Mixed Forests’ and ‘Temperate Grasslands, Savannas and Shrublands’. World map data is from the ‘rnaturalearth’ R package⁸⁶. The ESRI shapefile of the terrestrial biomes³⁷ can be accessed at https://storage.googleapis.com/teow2016/Ecoregions2017.zip.