Annual Report Records Trend of Clearer Winters, Murkier Summers in Famed Blue Lake.
The clarity of Lake Tahoe’s famed blue waters in 2023 continued its years-long trend of improving during the winter and deteriorating during the summer. The annual clarity report, released today by the University of California, Davis–Tahoe Environmental Research Center, found that winter lake conditions were the clearest observed since 1983 and the 10th best on record, with visibility of 91.8 feet under the surface, compared with 72.2 feet in 2022.
Summer months tell a different story, marking the fifth murkiest on record with an average of 53.5 feet, compared with 68.9 feet in 2022. Overall, the annual average for lake clarity dropped to 68.2 feet from its 2022 value of 71.9 feet.
“It’s important to understand the short-term changes but even more important to be thinking about how this lies in the context of the longer-term trends.”
– Alexander Forrest, interim director of the UC Davis Tahoe Environmental Research Center
Plankton, plastic, and particles
The clarity report attributes 2023’s clearer winter months (December 2022 through March 2023) to deep mixing events that brought clear water to the surface from the bottom of Lake Tahoe. After the clear winter months, the report highlighted the role of runoff and plankton in the observed low summer clarity values.
Runoff from the heaviest winter snowfall in 70 years brought an influx of inorganic particles and a rapid drop in clarity in May. The report said the dominant phytoplankton species, Cyclotella, has decreased since 2017, while another, slightly larger algae, Synedra, is increasing. Changes among the phytoplankton and zooplankton communities within the lake also have the potential to impact clarity.
Phytoplankton are microscopic plants, while zooplankton are microscopic animals that often eat phytoplankton. These planktonic assemblages play critical roles in lake food webs and ecology, and their interplay with lake clarity presents ongoing research questions and opportunities.
“Phytoplankton assemblages have been changing in Lake Tahoe over the last decade, and we need further research to understand the full implications of this on Tahoe clarity,” Forrest said. Meanwhile, nonnative Mysis shrimp are beginning to return, while zooplankton species are also experiencing changes in their populations.
While algae and particles are known to influence clarity, less is known about the potential impact of microplastics.
“Research has confirmed the presence of microplastics in Tahoe, and we know small pieces of plastic have the potential to affect clarity, but we don’t know their relative contribution to Lake Tahoe,” Forrest said.
Search for clarity
UC Davis has measured clarity and other health indicators at Lake Tahoe since 1968. Clarity is measured as the depth to which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water. Lake Tahoe’s clarity is just one measure of the health of the watershed, but measurements of clarity loss in the 1950s and 1960s by the UC Davis Tahoe Environmental Research Center became central to efforts to protect the watershed from pollution and unplanned development.
UC Davis works with the Tahoe Science Advisory Council and partners across the Tahoe Basin to help inform policymakers and the community on strategies to protect the lake and stabilize the decline in clarity that occurred following the mid-20th century development boom. 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, set environmental standards, and better manage growth and development in the region. Under the Lake Tahoe Environmental Improvement Program or EIP, the states of California and Nevada, along with more than 80 public and private organizations, are actively working to restore lake clarity to its historic 97.4 feet. EIP partners have implemented hundreds of projects to restore the filtering function of wetlands and treat stormwater runoff.
“Thanks to the bi-state partnership, Lake Tahoe is one of the most protected watersheds in the nation. The science community has played a pivotal role in the drive toward environmental restoration, and we’re proud of the progress we’ve collectively made on Lake Tahoe’s clarity over the decades. What happens on the land here affects the lake, and we continue to look to science to inform and prioritize Tahoe’s restoration projects.”
– TRPA Executive Director Julie Regan
In 2023, UC Davis scientists took 25 individual readings at Lake Tahoe’s long-term index station. View the historic clarity readings from 1968-2023 in the clarity report at https://tahoe.ucdavis.edu/terc-publications.
Long-term Secchi Record
Lake Tahoe is known worldwide for its famed water clarity, eliciting awe from millions of visitors each year. UC Davis researchers have been monitoring the changes in lake clarity for the past 50 years using a Secchi disk (the white dinner plate). From the late 1960s through the turn of the century, there was a decline in clarity brought about by increased algae growth and the presence of very fine particles. Over the past decade, we have seen the rate of clarity decline ease, with winter clarity actually improving. Using technology beyond the dinner plate, researchers continue to refine their understanding of lake physics and ecology to determine the cause of annual clarity change, enabling management agencies to focus on the most pressing issues affecting Tahoe’s clarity.
The following is a statement regarding the 2023 Lake Tahoe Clarity Report from League to Save Lake Tahoe CEO Dr. Darcie Goodman Collins:
“A good way to interpret the most recent Lake Tahoe water clarity figures is ‘business as usual.’ Small fluctuations in Lake Tahoe’s annual average clarity—one or two feet up or down—are normal. The longer-term, multi-year data trends are more telling of the Lake’s health.
Two trends are highlighted in the report, which demand more study: winter water clarity is improving, while summer clarity is declining. In the summer, the impacts of a warming climate create the right conditions for aquatic invasive species and algae to grow, and they drive more visitors to Tahoe escaping the heat at lower elevations.
Though winter clarity gains are welcome news, it’s extremely important for everyone to make lake-friendly transportation decisions when in Tahoe, like riding Lake Link and TART Connect. These alternative transportation services keep cars off the roads, preventing tailpipe emissions, road erosion, and runoff from polluting the Lake.
Most importantly, the report sends a clear message that we need to double-down on investments in science as well as lake-saving initiatives, including improvements in alternative transportation and litter prevention, as well as restoration of Tahoe’s marshes, meadows, and wetlands which are the Lake’s natural pollution filters. When these ecosystems function as they should, the Lake can be resilient to the impacts of climate change. These initiatives are the League to Save Lake Tahoe’s priorities because they help Keep Tahoe Blue.
There are things everyone should do, like planning to use alternative transportation in Tahoe, refusing to buy or bring single-use plastics that often end up as trash in our environment, and volunteering at restoration events like Tahoe Forest Stewardship Days.
Clarity matters because the water is our biggest clue about how the Lake is doing. If we lose the clear water of Tahoe, we lose everything we love about it. So, let’s all do our part to Keep Tahoe Blue.”
