Meet the ‘Natural Clean-Up Crew’ Restoring Lake Tahoe’s Clarity

Lake Tahoe – the iconic lake shared by California and Nevada – is the clearest it’s been in 40 years.

The reason? Partly a comeback from the “natural clean-up crew” of the native zooplankton Daphnia and Bosmina, the University of California (UC), Davis Tahoe Environmental Research Center (TERC) said in a Monday report. 

“Daphnia and Bosmina largely disappeared from the lake after they were grazed down following the introduction of the Mysis shrimp in the 1960s,” UC Davis TERC Director Geoffrey Schladow said in a university press release. “In late 2021, the Mysis population unexpectedly crashed, and it took 12 months for the Daphnia and Bosmina to build up their numbers and start their natural cleansing.”

UC Davis has been measuring the lake’s clarity since 1968. To do this, it uses a device known as a Secchi disk. The 10-inch disk is tossed into the lake, and researchers measure the farthest depth at which they can still see it. 

In 2022, the lake’s clarity made a major comeback. 2021 was the second worst year for lake clarity on record, with the disc visible from a depth of just 61 feet, The Sacramento Bee reported. It also was the highest year on record for the number of particles entering the lake, potentially due to wildfires. 

In 2022, however, average clarity surged to 71.7 feet. And during the last five months of the year, it extended even farther–to 80.6 feet, UC Davis said. That’s the clearest the lake has been since the 1980s.

[It] is, I believe, totally unprecedented,” Schladow told the San Francisco Chronicle. “We’ve never plotted data like this, where the last five months of the year were totally different.” 

Daphnia and Bosmina can play such an important role in clarifying the lake because they eat the size of particles most likely to cloud it, including silt, clay and algae, UC Davis explained. Their reappearance now – and the lake’s sudden clarity – helps to bolster a hypothesis that the lake’s food web has a major impact on its clarity when compared to other factors like runoff, lake warming and winter mixing. This year, only the return of the phytoplankton could explain the sudden change after 20 years of relative stasis, the report concluded.

“In some ways this, what we have unfolding in front of us, is a huge natural experiment,” Schladow told KCRA. “It’s not an experiment that was designed, but it’s presenting an opportunity for us to finally understand what role the food web plays in the clarity of Lake Tahoe.”

How long will this exceptional clarity last?

“We expect the impact of Daphnia and Bosmina to grow over 2023, and clarity may return to 1970s levels—despite the expected large runoff from this year’s record snowpack,” TERC boat captain and Secchi disk observer Brant Allen said in the press release. 

However, eventually the Mysis shrimp population is expected to resurge, gobbling up the phytoplankton, though Schladow said in the press release that future attempts to manage lake clarity should consider controlling their numbers.

Still, Julie Regan – who heads the Tahoe Regional Planning Agency that California and Nevada established in 1969 to restore and protect the lake–found hope in the findings amidst the strain that the climate crisis has put on the Lake Tahoe ecosystem through fires, storms and warmer temperatures.

“The lake’s resilience must continue to be supported by regional investments in water quality, forest health and aquatic invasive species prevention and control,” Regan said. “We will continue to work with regional science partners to better understand the role native species play in promoting clarity.”