Climate change and underwater light: Large‐scale changes in ultraviolet radiation transparency associated with intensifying wet–dry cycles

Abstract Ultraviolet radiation (UV) is the most energetic waveband of incident solar radiation and has wide‐ranging effects in the aquatic environment. Our analysis of an 18‐year record of underwater irradiance and related limnological variables in sub‐alpine, ultra‐oligotrophic Lake Tahoe revealed orders of magnitude changes in UV transparency associated with interannual climate perturbations. The large‐scale shifts between years were caused by pronounced changes in the loading of allochthonous particulate matter and colored dissolved organic matter associated with regional dry–wet cycles, while autochthonous factors explained the seasonal variations in UV under average weather conditions. Water clarity in the photosynthetically available radiation (PAR) waveband showed less variation, resulting in large interannual differences in the UV : PAR ratio. Clearwater lakes are likely to experience increasingly large fluctuations in underwater UV and spectral irradiance due to ongoing climate change and precipitation extremes, with potential impacts on their ecosystem structure and function.