Lake Tahoe’s clarity the best since 1980s, but is more needed to keep Tahoe blue?

LAKE TAHOE, Calif./Nev. - For the last five months of 2022, Lake Tahoe was the clearest it has been since the 1980s and much of that is due to the resurgence of the lake’s native zooplankton, said the U.C. Davis Tahoe Environmental Research Center (TERC).

TERC and Tahoe Regional Planning Agency released the annual report on lake clarity Monday, a report showing praise for the tiny zooplankton for providing a natural clean-up crew to help restore the lake’s famous blue waters.

In 2022, Lake Tahoe’s average annual clarity was 71.7 feet compared to 61 feet in 2021. The key finding from 2022 was the great improvement in lake clarity from August through December when the average Secchi depth was 80.6 feet. This coincided with the highest numbers of the zooplankton Daphnia and Bosmina, said TERC in its report. Clarity is measured as the depth to which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water. Both California and Nevada are actively working to restore lake clarity to its historic 97.4 feet

The League to Save Lake Tahoe says to be careful of just a few months of positive changes though, and this report is not a complete signal that Lake Tahoe’s water clarity is now on a path to recovery (see below).

Zooplankton a clear factor

Zooplankton are small, microscopic animals. Some zooplankton, particularly Daphnia and Bosmina, are specialized to consume particles in that critical size range.

“Daphnia and Bosmina largely disappeared from the lake after they were grazed down following the introduction of the Mysis shrimp in the 1960s,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “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.”

Other factors are known to influence year-to-year changes in clarity. These include the magnitude of the runoff, the warming of the lake surface and the depth to which the lake mixes in the previous winter. The report examined all these factors and concluded that only the change in the zooplankton community could account for the magnitude of this year’s change.

For a limited time only

The researchers emphasize that the process is still in its early stages, and they expect the clarity improvement to continue through 2023.

“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. “These events support the hypothesis we put forward several years ago that the food web is a major factor in controlling lake clarity.”

However, the assistance provided by Daphnia and Bosmina may be only short-term. Mysis shrimp populations are expected to rebound. As they consume Daphnia and Bosmina, clarity will return to what we have seen in the past 20 years.

“Future management actions should explicitly look at incorporating ways of controlling the Mysis population,” Schladow said. “We have a brief window of time to monitor the lake in the absence of Mysis and then track the impacts of their return on lake clarity.”

This would be in addition to the strenuous efforts that are taking place to keep fine particles and nutrients out of Lake Tahoe. Management agencies in the basin report more than 500,000 pounds of fine sediment and other clarity-harming pollutants are being kept out of the lake every year through roadway maintenance and erosion control projects.

TERC scientists are currently monitoring zooplankton communities through donor funding. They are also working with local fishing guides to monitor changes to fish. Kokanee salmon, for example, are expected to be larger in 2023, as Daphnia are their preferred food source.

League to Save Lake Tahoe

The following is a statement on the findings from League to Save Lake Tahoe CEO Dr. Darcie Goodman Collins:

“It’s encouraging to see that water clarity improved in 2022 compared to the year prior. Yet, a better annual average for a single year shouldn’t be taken as a signal that Lake Tahoe’s water clarity is now on a path to recovery. The data from 2022 tells a nuanced story and reminds us there is much more we need to understand about Lake Tahoe if we hope to keep – and restore – its blueness.

For example, in January of 2022, a wind-driven upwelling event led to a clarity reading of 137.8 feet, the second-largest Secchi depth ever recorded at Lake Tahoe since measurements began in 1968. That single extreme data point added 1.1 feet to the annual average, moving the needle from 70.6 to
71.7 feet. This detail is important, considering that a foot of clarity gain or loss can be interpreted as triumph or catastrophe.

The context of 2022 is also important. Last year, Tahoe was not shrouded in wildfire smoke and ash for long periods as it was the prior two years. Nearby wildfires deposit clarity-reducing particles in the Lake. Also, streamflows did not contribute a significant amount of runoff in 2022, which also degrades clarity. So, with a relative lack of things falling on or flowing into Lake Tahoe, what’s happening in the water itself may offer an explanation for relative improvements in clarity.

The Clarity Report does note coincidental changes in the populations of organisms living in the Lake, mysis shrimp and two species of zooplankton, which certainly warrants further funding and scientific study. So too do physical processes, like deep water mixing, which occurred fully earlier this year (outside the range of the 2022 report) for the first time in four years. Changes in clarity should not be attributed solely to a single factor, such as a change in the food web or physical processes.

Science, funding and implementation of conservation policies are crucial to protect Tahoe and bring its clarity back. The League is focused on combating the threat of aquatic invasive species and improving regional transportation to prevent ecological damage and pollution. To help bring back clarity that’s been lost, we work with land managers and volunteers to restore Tahoe’s meadows,

Protecting Lake Tahoe

UC Davis has measured clarity and other health indicators at Lake Tahoe since 1968, helping to inform policymakers and stakeholders on strategies to protect the lake and stabilize the decline in clarity that dates back to the region’s development boom in the 1960s.

In 1969, the states of Nevada and California created the Tahoe Regional Planning Agency (TRPA) to lead the collaborative effort to protect and restore Lake Tahoe and better manage growth and development in the region. TRPA Executive Director Julie Regan says the emerging trend is welcome news for Lake Tahoe at a time when the ecosystem is experiencing more extreme storms, wildfire, 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.”

In 2022, UC Davis scientists took 28 individual readings at Lake Tahoe’s long-term index station. View the historic clarity readings from 1968-2022.

The clarity report is funded by the Tahoe Regional Planning Agency.