Ultraviolet radiation (UVR) damages most biota, yet little evidence exists for its long‐term effects on natural ecosystems. We used paleoecological techniques at three low‐elevation lakes to show that algal abundance in lakes was depressed 10‐fold by UVR during the first millennium of lake existence. Likewise, analysis of data from a lake near treeline showed that algal biomass declined 10–25‐fold both early in the lake histor...