Climate change and underwater light: Large-scale changes in UV transparency associated with intensifying wet-dry cycles

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 largescale shifts between years
were caused by pronounced changes in the loading of allochthonous
particulate matter and colored dissolved organic matter (CDOM) associated
with regional dry-wet cycles, while chlorophyll a combined with
autochthonous CDOM controlled 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. Clear-water 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.