Climate extremes alter how sunlight travels through clear lakes

Lake Tahoe sits high in the Sierra Nevada, straddling the border of California and Nevada. For more than half a century, researchers have tracked its temperature, chemistry and clarity.

In 2006, scientists at the UC Davis Tahoe Environmental Research Center added a new line of surveillance. The team installed precise sensors that measure how far sunlight of different colors, including energetic UV rays, can travel through the Lake Tahoe’s water column.

Eighteen years of those measurements, analyzed with colleagues at Miami University and Laval University, now reveal startling swings in underwater UV exposure that mirror the region’s increasingly extreme climate jolts.

Measuring sunlight beneath Lake Tahoe

Scientists visited the lake every two weeks and lowered a multichannel radiometer that records UV-B (305 nm), UV-A (320 nm and 380 nm) and visible light used by plants for photosynthesis (PAR, 400–700 nm).

Researchers calculated the depth at which only one percent of the surface light remains – a yardstick for how deeply each wavelength penetrates.

Between 2006 and 2023, that one-percent depth for 320 nm UV light sometimes reached more than 95 meters in drought summers, yet barely 10 meters in the wettest springs.

The experts found up to a 100-fold difference in UV radiation between a wet and a dry year.

When drought removes the sunscreen

Lake Tahoe’s water is famously clear because it holds very little dissolved organic matter and few suspended particles. Those substances behave like a natural sunscreen: they absorb or scatter incoming UV.

During years of exceptional drought, streamflow slackens and the lake receives scant deliveries of colored dissolved organic matter or fine mineral grains. As a result, more UV light slices through the water.

Shohei Watanabe, an associate project scientist with the UC Davis Tahoe Environmental Research Center, is the lead author of the study. “It’s a bit like reducing the lake’s sunscreen, making it susceptible to severe sunburn,” he said.

In contrast, during winters and springs dominated by powerful atmospheric river storms, torrents of runoff flush soil and organic stains into the lake. That cloudier mix limits UV penetration, drawing the effective sunscreen closer to the surface.

Sunlight shapes lake life

UV rays are the most energetic part of sunlight. At modest doses they help sterilize surface waters, break down dissolved carbon compounds, and keep certain invasive fish at bay.

In excess, however, UV rays damage DNA, inhibit photosynthesis, and alter the behavior of tiny crustaceans that feed fish.

The Tahoe study shows that the ratio of UV to the visible light plants need (PAR) also changes sharply with the climate rhythm, shifting ecological balances throughout the food web.

Because clear alpine and polar lakes share Lake Tahoe’s optical character, the team warns that clear-water lakes are likely to experience increasingly large fluctuations in underwater UV and spectral irradiance. These shifts are expected to intensify as precipitation extremes become more frequent.

Monitoring underwater radiation

The researchers found that UV transparency tracks regional hydrology more sensitively than traditional clarity measures such as the Secchi disk.

In Lake Tahoe, the depth at which a white disk disappears to the eye changed far less between wet and dry years than the UV measurements did. That makes underwater light a potential early-warning signal for managers watching for climate-driven disturbances.

Thus, monitoring underwater radiation can serve as a sentinel. It alerts authorities to water quality, fisheries, and recreation stresses long before they appear at the surface.

Global trends and local impacts

Maintaining an unbroken observational line was crucial. “To understand what’s really happening in nature, the long-term dataset is quite important,” Watanabe said. “One or two years of data couldn’t reveal this kind of huge fluctuation related to climatic perturbations.”

The record also shows how global trends translate into local impacts. A future of alternating megadroughts and deluges could cause Tahoe and other clear lakes to swing between extremes.

Some years may bring hyper-sterilizing UV pulses, while others may see UV-muted, nutrient-rich waters that fuel algal blooms.

Protecting the clarity of lakes

The findings stress curbing erosion and runoff of fine sediments and colored organics to protect Lake Tahoe’s clarity. These measures can help restore Lake Tahoe’s natural UV and sunlight sunscreen, even during wet years.

At the same time, understanding that drought may expose deeper layers to damaging radiation could influence habitat protection for plankton, juvenile fish, and amphibians.

The study blends painstaking fieldwork with a simple message: climate change does not just warm lakes; it changes the color and character of the light that reaches their depths.

In Lake Tahoe’s shifting shades of ultraviolet, scientists can read both the scars of recent droughts and the fingerprints of storms still to come.

The study is published in the journal Limnology and Oceanography Letters.

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