Lake Tahoe's iconic blueness is most strongly related to algae, not clarity, according to research released from the UC Davis Tahoe Environmental Research Center, or TERC. In the "Tahoe: State of the Lake Report 2015," researchers found the lower the algal concentration, the bluer the lake.
The report also includes updates related to clarity, climate change, drought and new research at Lake Tahoe.
Clear and Blue
The assumption that lake clarity is tied to blueness has driven advocacy and management efforts in the Lake Tahoe Basin for decades. But the report's findings show that at times of the year when clarity increases, blueness decreases, and vice versa. This is due to the seasonal interplay of sediment, nutrients and algal production as the lake mixes.
Clarity is controlled by sediment. Blueness is controlled by algal concentration, which in turn is driven by the level of nutrients available to the algae.
"This is good news," said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center and a civil engineering professor. "It shows that we better understand how Lake Tahoe works, and it reinforces the importance of controlling nutrient inputs to the lake, whether from the forest, the surrounding lawns or even from the air. It's particularly encouraging that blueness has been increasing over the last three years."
Low precipitation helped keep runoff from both nutrients and sediment low in 2014.
Shohei Watanabe, a postdoctoral researcher at UC Davis TERC, led the blueness study in collaboration with NASA-Jet Propulsion Laboratory and Laval University. He produced a Blueness Index, quantifying Lake Tahoe's color for the first time by using data from a NASA-JPL research buoy at the lake and hyperspectral radiometers that measure the amount of light leaving the lake at each waveband -- in other words, its color.
Watanabe combined the Blueness Index with TERC measurements of Secchi depth -- the depth at which a white disk remains visible when lowered into the water. He was surprised to see that blueness and clarity did not correspond. In fact, they varied in opposite directions.
"This does not mean that clarity should be dismissed," said Watanabe. "Rather, it shows that algae concentrations and nutrient input should be managed more closely to truly keep Tahoe blue and clear."
The JPL buoy used in the study is one of four established by NASA with support from TERC to calibrate and validate measurements taken by satellites flying overhead.
"This particular buoy has instruments beneath the water looking up, and an instrument on the buoy looking down" said JPL's Simon Hook, who collaborated with Watanabe.
"The combination of instruments in and above the water are used in this study to look at how light is being scattered and attenuated. That tells you something about both the color and the clarity of the lake."
Other highlights from the State of the Lake Report include:
The State of the Lake report informs nonscientists about the most important factors affecting lake health and helps influence decisions about ecosystem restoration and management within the Lake Tahoe Basin. It can be found online at: http://terc.ucdavis.edu/stateofthelake/
Materials provided by University of California - Davis. Note: Content may be edited for style and length.
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