Secret behind Lake Tahoe’s blueness revealed


The secret behind Lake Tahoe’s awesome blueness is not to do with the clarity of the lake, but is rather strongly related to algae, it has been revealed.

According to a research published by researchers at UC Davis Tahoe Environmental Research Center, or TERC, the blueness of the lake is directly related to lower level of algal concentration. This means the lake will appear more blue when there is lower algal concentration and if the algal concentration increases, the lake will appear less blue.

In their “Tahoe: State of the Lake Report 2015,” researchers have also included updates related to clarity, climate change, drought and new research at Lake Tahoe.

The motive behind the study was that it has long been assumed that lake’s clarity was the reason behind the blueness of the lake. However, the latest research found that at certain 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.

Researchers reveal that the clarity of the lake is controlled by sediment, while the blueness is controlled by algal concentration, which in turn is driven by the level of nutrients available to the algae.

Researchers say that the revelation of the secret is a good thing for the lake as they now understand how Lake Tahoe works. Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center and a civil engineering professor, said that their findings highlights and reinforces the need to control the nutrient inputs to the lake, whether from the forest, the surrounding lawns or even from the air.

Low precipitation helped keep runoff from both nutrients and sediment low in 2014.

Shohei Watanabe, a postdoctoral researcher at UC Davis TERC, who led the blueness study in collaboration with NASA-Jet Propulsion Laboratory and Laval University 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 colour.

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.”