‘Tahoe will not be blue.’ Why the lake’s warming trends are worrisome

Climate change is gradually warming Lake Tahoe, clouding its clarity and threatening its fabled “blueness,” scientists at UC Davis warned Thursday.

In its annual “State of the Lake” report, the university’s Tahoe Environmental Research Center said surface water temperatures in July 2017 spiked to an average 68.4 degrees. That was the highest since researchers began taking Tahoe’s temperature in 1968, and 6 degrees higher than the year before.

Geoffrey Schladow, director of the Tahoe center, said the 2017 temperature readings may have been a fluke. But there’s little doubt the lake’s waters are getting heated by global warming, he said.

“There is a long-term trend toward the lake getting warmer,” he said in an interview. “The lake surface is getting warmer, it’s getting warmer earlier in the year, it’s getting cooler later in the year.”

Schladow said warming temperatures could hurt water clarity and the overall health of the lake.

If the lake is behaving properly, its waters churn every three or four years: The light, warm water at the surface mixes with the cold, dense water at the bottom. But climate change has shortened the winter season — the time when the churning phenomenon takes place.

Schladow said the lake hasn’t churned completely for seven years.

That could spell bad news for Tahoe. For one thing, it could starve the bottom of the lake of oxygen, creating dead zones that would be harmful to aquatic life. “Anything that’s living down there that requires oxygen to breathe, can’t,” Schladow said.

Just as importantly, the lack of churning means phosphorus and nitrogen will build up. Schladow said eventually the phosphorus and nitrogen will be released into the upper waters.

When that happens, algae will bloom and clarity and Tahoe’s “blueness” will suffer.

“Tahoe will not be blue that year, and possibly for several more years,” Schladow said. “It might be a tipping point? How long that lasts, we don’t know.”

Last year Tahoe’s clarity fell to an all-time low, but that was a freakish event, scientists said. It was caused by record winter rainfalls flushing into the lake a heavy collection of sediments that had built up during five years of drought.

This year UC Davis’ research shows clarity “is back in its normal range,” the report said. Schladow, however, said algae blooms would be far more troubling than sediment.

“Their effects may persist for multiple seasons,” he said.

He said the problems tend to feed on themselves: If the water doesn’t get churned, the surface gets warmer. As it gets warmer, the lake can’t churn like it should.

“There is this feedback going on,” he said.

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