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As Tahoe warms, oxygen and clarity could decrease

There is a small wave inbound on a sandy shore of Lake Tahoe while large rocks are peaking out of the emerald water. A hill covered in trees is visible in the background. A snowcapped mountainscape is visible in the far background, too.
Will Stone
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In the lead up to next week’s Tahoe Summit, researchers are taking stock of how warmer temperatures are impacting the lake’s iconic clear waters. Earlier this week, we looked at concerns about water quality near the shore. But, as it turns out, climate change may also affect oxygen levels in the lake.

 

 

Over the past century, the air temperatures at Tahoe have increased about 4 to 5 degrees. Current predictions anticipate that will continue in the coming decades. This not only means less snow running off into the lake, but, even more worrying, less dissolved oxygen.

“When you have warm very light water floating on top of this cold water at the bottom it’s difficult for that to mix. It’s almost like oil floating on water.”

That’s Geoff Schladow, who leads the U.C. Davis Tahoe Environmental Research Center, which has just released its annual “State of the Lake” report. Because Tahoe is so deep, it depends on the periodic mixing of water on the surface and bottom of the lake in order to distribute oxygen. That hasn’t happened, though, in recent years and Schladow says there are two radically different possibilities for how that could impact the lake’s clarity.

“One is that this lack of mixing may actually help clarity and so, on account of climate change, we will actually see tremendous improvements in clarity in the coming decades. On the other hand, there’s a risk that these low oxygen conditions will allow nutrients to come out of the sediment and clarity will decrease tremendously.”

Schladow says that’s the big research questions going forward: how quickly will oxygen levels decrease? What will be the ultimate effects? For now, clarity has stabilized in the middle of the lake, and, without much runoff from storms, water quality has actually improved.  But, this July, the water temperature at the surface was just over 65 degrees—the warmest in 5 years.

Will Stone is a former reporter at KUNR Public Radio.
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