Generalist top consumers integrate across the various basal energy sources that support food webs at the ecosystem scale. In lake ecosystems, it is largely unknown to what extent fish biomass is supported by terrestrial vs. internally produced organic matter (OM). Using stable hydrogen isotopes, we estimated that terrestrial OM contributed 12-57% to fish biomass in 30 boreal lakes. Terrestrial reliance increased with increasing dissolved OM concentration and light attenuation in the water, suggesting an overall control by colored terrestrial OM. Based on light attenuation and surface area distributions of >3500 lakes, we estimate that one quarter of lake fish biomass in boreal Sweden is supported by terrestrial OM. This first quantification of energy supply to lake food webs at a large geographic scale emphasize the importance of terrestrial-aquatic coupling and suggests that ongoing changes in export of terrestrial OM will affect the energy support of fish populations in boreal lakes.