Tahoe clarity: Known knowns and movement to understand the lake’s future (Opinion) | TahoeDailyTribune.com

Tahoe clarity: Known knowns and movement to understand the lake’s future (Opinion)

Sudeep Chandra, Alan Heyvaert and Ramon Naranjo / Guest column

The stunning, blue color of Lake Tahoe has long captured our attention. These brilliant hues are a result of the lake’s renowned water clarity. Scientists use clarity as a key measurement to evaluate the lake’s health. 

Clarity data is collected by lowering a 10-inch diameter white Secchi disk into offshore waters and measuring the depth at which it disappears from sight. This data has been collected since the 1960s, making Tahoe one of the longest studied lakes in the world when it comes to water quality and clarity monitoring. This long-term dataset has provided scientists with an opportunity to evaluate changes or patterns in Lake Tahoe’s clarity. Although data indicate the lake has lost about a third of its historic clarity depth since monitoring began, that long-term pattern of decline has started to change. 

Since 2020, the *Tahoe Science Advisory Council has conducted an annual data synthesis and analysis evaluation of the lake data which contributes to an improved understanding of lake clarity and associated conditions. This assessment shows water clarity is linked to ecological characteristics of the lake, including suspended materials (such as fine particles of sediment and algae) that affect the “blueness” of the water, as well as conditions suitable to the native fish community and to other animals and plants living at the bottom of the lake. 

Tahoe Basin resource management agencies are working with the council’s scientists to support continued measurements of clarity and an evaluation of factors that might influence this key ecological trait. Of particular importance for these scientific evaluations are the size and composition of fine particles suspended in Lake Tahoe’s waters, the amount and types of algal species present, and nutrients that can influence the growth of algae.  

The 2022 report, issued last month by the Science Council, concluded that very fine suspended particles, small Cyclotella diatoms (a type of algae), and other algae account for 68% of changes in clarity measurements taken since 2009. When council scientists focus their analysis of clarity on seasonal conditions, some very interesting patterns emerge. The summer average clarity (June–September) continues to decline at a rate of 0.62 feet per year, while the winter seasonal clarity (December–March) has plateaued. Overall, the annual average for lake clarity has also stopped decreasing over the last 20 years.

While analysis of the long-term clarity trajectories suggests that annual average clarity has stabilized, it is not increasing. This stabilization in clarity is progress, however, and good news for Lake Tahoe, if it persists. Additional science-based support is needed to develop informed management strategies that will restore the deep blue waters of the lake. The creation of predictive lake models, inclusion of interdisciplinary science viewpoints, experiments, and new scientific advances can help lead our efforts to restore the lake.

The report notes that since 2017, very fine particles of sediment, soil and algae continue to remain suspended in the water column, keeping clarity levels low. This was despite a series of dry years where reduced runoff from rain and snowmelt transported fewer particles to the lake, which typically leads to an improvement in clarity. 

The Science Council’s findings highlight the need for continued assessment of factors influencing lake clarity and recommends further research to understand why these particles are persisting in the water column, and whether this phenomenon is connected to: particles being introduced from the Basin’s streams, creeks and urban areas; dynamics in the nearshore habitat that link the lake to its watershed; or feedbacks from organisms which affect the clarity. 

Additionally, and importantly, greater understanding of disturbances such as wildfires, climate-driven droughts, large runoff events from atmospheric rivers, increasing temperatures, tourism impacts, and invasive species will be increasingly important to inform policies that help create a more resilient and sustainable Lake Tahoe. 

As a Science Advisory Council, and as an increasingly common practice within the scientific community, we strive to share our recent findings with the public. You can find them, and other Science Council reports, at tahoesciencecouncil.org. We hope you will read the report, contact us with questions, and do what you can to protect and preserve Lake Tahoe. 

Dr. Sudeep Chandra, Tahoe Science Advisory Council Cochair and Professor at the University of Nevada Reno

Dr. Alan Heyvaert, Lead of the Tahoe Clarity Data Synthesis and Analysis Subcommittee and Research Professor at the Desert Research Institute 

Dr. Ramon Naranjo, Tahoe Science Advisory Council Cochair and Research Hydrologist at the U.S. Geological Survey

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