Keeping Lake Tahoe blue doesn’t mean keeping it clear

The Amazon River and its tributaries, long watery snakes wind their way through the Amazon rainforest, are a dull, murky, muddy brown. Other waterways show up as green, primarily thanks to photosynthesizing algae. But perhaps the most prized waterways are colored a deep, rich, vibrant blue. Lake Tahoe, the second deepest lake in the US and sixth largest (following the five Great Lakes), is one of the most iconic blue water lakes.

For a long time it’s been thought that the lake’s prized blueness, the focus of local “Keep Tahoe Blue!” campaigns, was related to water clarity. That makes intuitive sense. But the latest State of the Lake, a yearly publication from UC Davis’s Tahoe Environmental Research Center (TERC), reports something different. For the first time ever, researchers there have managed to quantify the lake’s blueness by developing a measurement they call the “blueness index.”

To measure the lake’s blueness, TERC postdoctoral researcher Shohei Watanabe collaborated with researchers from Canada’s Laval University and NASA-JPL to measure the wavelengths of visible light leaving the water. By continuously monitoring those wavelengths, he was able to create a color record over time for the lake.

But it turns out that during parts of the year when clarity increases, blueness actually decreases. Clear water does not a blue lake make.

The lake’s clarity is governed by the presence (or absence) of fine particulate matter that seeps into the lake from the surrounding land. When there are fewer particles floating around, the lake becomes clearer – but not bluer. Blueness, instead, is related to the presence of algae. When there are more algae, the lake appears less blue to the human eye.

Each year the lake’s algal concentration becomes lowest in the summer, because that’s the time when the nutrients inside the lake are most depleted. Summer is also the time when particle concentration is highest, which is why periods of increased blueness actually wind up being associated with periods of decreased water clarity!

Particulate matter washes into the lake most via stormwater runoff from urban areas. Even though those urban areas only comprise 10 percent of nearby land cover, they account for two thirds of runoff. Over time, those particles clump together, and that causes them to become more likely to drift to the bottom of the lake. By the time winter rolls around, the lake starts to look clearer. It’s not that the particles have disappeared, it’s that they’ve sunk.

Meanwhile, the main nutrients that feed the blueness-hating algae are nitrogen and phosphorus. While the nitrogen mainly falls into the lake from the atmosphere, phosphorus gets swept into the lake from the watersheds that flow into it. While nearly half of that phosphorus comes from non-urban landscapes, almost twenty percent of it originates in cities. And all that algae that rely on those nutrients adds up. There are some 30 million trillion algal cells in the lake which, if stacked on top of a football field, would create a 9 foot deep pool of algae weighing some 15,500 tons.

The lake’s blueness has increased every year for the last three years, but could that actually be a bad sign? Blueness may be prized by locals and valued by the 4.5 million tourists that come through the area each year, but the blueness-decreasing algae feeds the entire Tahoe ecosystem. It’s the base of the food web, critical for the lake’s overall health. Nutrient input has been low because rainfall and snowpack – and therefore runoff – have also been low, thanks to California’s multi-year drought. And fewer nutrients means less algae. (Or, in some cases, a shift towards smaller algae species.)

Just what all of this means for the overall health of the Tahoe ecosystem, especially in a period of intense drought and climate change, is still an open question. But at least one thing is certain: the lake’s blueness is just one facet among many that will determine the future of the 39 trillion gallons that comprise Lake Tahoe. – Jason G. Goldman | 29 July 2015

Source: UC Davis Tahoe Environmental Research Center (2015). Tahoe: State of the Lake Report. (Link)

Header image: Lake Tahoe as seen from Emerald Bay, via shutterstock.com

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