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After steep declines in water clarity through the 1990s, annual average clarity appears to have plateaued, although with high variation. Winter trends have stabilized or improved, but summer trends remain concerning. To understand why Lake Tahoe's clarity is not improving, we suggest that targeted research is needed into the exact nature of the clarity-reducing particulates and their potential aggregates.

In 2024, the annual average clarity remained comparable to previous years. The long-term trend shows annual average Secchi depth decreasing through the late 1990s and leveling off to the present day, with continuing high variation. The annual average for 2024 was 19.0 m with a 95% confidence interval (CI) of 17.0 – 20.2 m, which makes it statistically similar to last year.

Somewhat different patterns are observed in winter and summer. Observations of lake clarity for 2024 show relatively poor winter (December 2023 through March 2024) clarity in an otherwise stable or improving winter trend and relatively poor clarity during the summer (June 2024 through September 2024), which seems to be part of a trend of degrading summer clarity.

In 2023, the high winter clarity (28.0 m) was partly the result of complete lake mixing (turnover) observed in early March 2023. Deepwater mixing brings very clear water from the bottom of the lake to the top, greatly increasing clarity for a limited period. As deepwater mixing did not occur in 2024, the water did not get the increase in clarity that has been associated with past full mixing events.

The seasonal decline in summer clarity in 2024 was consistent with the number of particles observed in the water. Particles that scatter light can include both inorganic (e.g., sediment) and organic materials, such as dead terrestrial matter washing in from the watershed, as well as phytoplankton that may grow in response to favorable growing conditions (e.g., nutrients and higher temperatures). However, summer water clarity remained low after concentrations of observed particles dropped, suggesting unobserved factors or more complex dynamics affecting summer clarity.

With trends now appearing somewhat stable overall on an annual average – highly variable and generally not improving – future research should focus on examining the nature of the particulates that affect water clarity. The relative importance of sediment and detritus entering from the watershed and airshed, and phytoplankton growth in response to changing environmental conditions can imply different processes at play in water clarity.