Shohei Watanabe collects data at Lake Tahoe for the UV study. (Image: Karin Higgins/UC Davis)

Usually the fact that the Lake Tahoe Basin can boast of hundreds of days of sunshine a year is a positive. Scientists are proving it’s not necessarily a good thing for the water.

Researchers with the UC Davis Tahoe Environmental Research Center along with peers at Miami University in Ohio analyzed 18 years of data through 2023 to determine the effect ultraviolet radiation has on Lake Tahoe.

They also used specialized equipment to measure UV radiation underwater.

The study was published this year in the Association for the Sciences of Limnology and Oceanography (ASLO).

“In a wet year, UV radiation penetrates shallower. In a drought year, more UV radiation is slicing through the water. It’s a bit like reducing the lake’s ‘sunscreen,’ making it susceptible to severe sunburn,” the study’s lead author, Shohei Watanabe, said in a statement. He is an associate project scientist with TERC.

The study found UV radiation affects the carbon cycle, how fish and zooplankton behave, and suppresses photosynthesis, which damages the underwater food web.        

The study revealed a hundredfold difference in transparency between the severe summer drought of 2015 and the extremely wet year of 2023.

“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,” the study says, pointing out the UV issue is not limited to Lake Tahoe.

Researchers believe understanding the impact of UV light and monitoring underwater radiation could help those charged with protecting the health of Tahoe and other lakes.

UV light has actually intentionally been used at Tahoe to kill aquatic invasive species. Artificial ultraviolet-C light was first tested at Lakeside Marina in 2017, and has since been used in the lagoons in Tahoe Keys.

UVC from the sun is blocked from the ozone layer, while it’s UVA and UVB that penetrates Earth’s atmosphere.

Note: A version of this story first appeared in the Tahoe Mountain News.