For the last five and a half decades, scientists have been measuring Lake Tahoe’s clarity by dropping a ten-inch white disk into the water. After submerging the tool, they note how far down it sinks before becoming impossible to see. Over time, the lake’s clarity has fluctuated: In 1968, for instance, the water was clear enough to see the disk at a depth of 102 feet, while more recently, the number has hovered between 60 and 70 feet.
Now, researchers report that Tahoe is the clearest it’s been in 40 years—and it’s thanks to tiny zooplankton that gobble up the particles that make the water cloudy.
Scientists with the Tahoe Environmental Research Center (TERC) at the University of California, Davis, found that the average clarity across the lake was 71.7 feet in 2022, an improvement from the 61 feet measured in 2021. But the most impressive clarity came at the end of last year, when researchers could see the white disk at a depth of 80.6 feet, according to an annual report published last week. Scientists were encouraged by the results, which suggest the food web has an impact on how clear the lake looks.
Famous for its stunning blue waters and massive footprint, Lake Tahoe is a popular tourist destination that straddles the state line between California and Nevada. Set high in the Sierra Nevada, this 191-square-mile freshwater body is the biggest alpine lake in North America. It’s also among the oldest and deepest lakes on the planet.
Tahoe’s clarity improvement happened primarily during the last five months of 2022. That’s largely because, in late 2021, the lake’s population of Mysis shrimp “unexpectedly crashed,” according to the report from TERC. Those non-native creatures feed on two types of microscopic zooplankton that help keep the lake clean, called Daphnia and Bosmina. When the shrimp suddenly declined, the zooplankton could flourish and act as a “natural clean-up crew,” per the statement.
Scientists don’t know what led the shrimp population to drop so sharply—possibly a fungal infection that devastated their main food source, reports KCRA’s Heather Waldman. But they do expect the crustaceans to eventually rebound, meaning the ultra-clear waters are likely to be temporary.
“It’s almost impossible to get rid of something entirely from a lake or any biological system,” says Geoffrey Schladow, TERC’s director, to Tahoe Quarterly’s Ryan Miller. As a result, the pristine clarity might only last for a year or so.
Still, understanding the relationship between these creatures and Tahoe’s clarity could provide useful insights for managing the lake ecosystem moving forward. What happened in 2021 with the decline of Mysis shrimp is a “fantastic experiment,” as Schladow tells the San Francisco Chronicle’s Gregory Thomas.
“This is a great opportunity for scientists to look at how the lake responds and then give the management agencies new ideas and tools for improving the clarity and health of Lake Tahoe,” he tells the publication.
Scientists and conservation groups are already pretty hands-on when it comes to managing Lake Tahoe’s health. They’ve successfully limited the amount of pollution, sediment, nutrients and fine particles that make their way into the water by maintaining roads and controlling erosion through various projects. To help make sense of what’s happening below the surface, scientists also monitor changes in the creatures who live in the lake, including zooplankton and fish.
In the near term, the unprecedented amounts of snow that fell on California over the winter will likely make Lake Tahoe murkier, as all that snowmelt will bring dirt with it into the water. And if there are large California wildfires this summer, the ash and smoke particles could also cloud up the lake, as they did in 2021, wrote the Los Angeles Times’ Tony Barboza and Anita Chabria at the time.
“It’s looking like there’s going to be megafires with smoke affecting the lake more frequently,” as Jesse Patterson, chief strategy officer with the nonprofit League to Save Lake Tahoe, told the L.A. Times.