Pummeled by drought and climate change, beloved Lake Tahoe in hot water

Snow on the mountain peaks at the Squaw Valley Ski resort near Truckee, Calif., as seen on Tuesday Nov 4, 2014. With the recent snowfall in the Sierra Mountain near Lake Tahoe over the weekend ski resort operators are cautiously optimistic. less Snow on the mountain peaks at the Squaw Valley Ski resort near Truckee, Calif., as seen on Tuesday Nov 4, 2014. With the recent snowfall in the Sierra Mountain near Lake Tahoe over the weekend ski resort ... more Photo: Michael Macor, The Chronicle Buy photo Photo: Michael Macor, The Chronicle Image 1 of / 4 Caption Close Pummeled by drought and climate change, beloved Lake Tahoe in hot water 1 / 4 Back to Gallery

Lake Tahoe and the community around it are increasingly battered by climate change and drought, with the lake’s temperature climbing 10 times faster than the historic average in the past four years and algae threatening the Sierra Nevada gem’s famous emerald and blue clarity.

Intense seasonal changes in 2016 — hallmarks of climate change — killed huge swaths of forest around the lake and nourished invasive species, according to the annual Tahoe State of the Lake Report released Thursday by the UC Davis Tahoe Environmental Research Center.

The beloved vacation spot, researchers said, now sees summer conditions for 26 more days than it did in 1968, boosting the danger of devastating wildfires, while the spring snowmelt has moved up 19 days since 1961. The report was based on conditions measured throughout last year.

“People come to Tahoe for the trees and because the beaches are clean,” said report author Geoffrey Schladow, the director of the UC Davis center. “The lake is nestled in the mountains and surrounded by a beautiful forest. If it is increasingly dominated by dead and dying trees and stringy algae washing up on the beach, it will change the way people experience the environment here.”

Noting efforts to preserve the lake, which have been embraced by lawmakers in Sacramento and Washington, he added, “But I am optimistic because, at Lake Tahoe, most people are pretty much on the same page of where they want the lake to go.”

The report called climate change an overarching factor in many of the area’s challenges. Insects, disease and stress — combined with the transition from a historic drought to unprecedented rainfall last year — felled tens of thousands of trees.

Invasive Asian clams, spread around the lake by boaters who take on and then dump water during wakeboarding excursions — a heavier craft creates a bigger wake — repopulated on the northern end. The mollusks then attracted huge mats of algae, which wash up on beaches and decompose.

Average water clarity, one of the most noticeable symptoms of a warming planet, degraded to 69.2 feet in the nation’s second-deepest lake, a 3.9-foot decrease from 2015. Researchers track clarity by submerging a white dinner-plate-size tool known as a Secchi disk and measuring how far down they can view it.

Mark Twain likened boating on Lake Tahoe to floating on air, and when they first started measuring clarity in 1968, researchers could see the Secchi disk 102.4 feet below the surface.

But that’s not the case anymore. The problem, UC Davis experts say, is that fine particles from urban runoff are fueling the growth of algae, which in turn absorbs light, increases temperatures and reduces clarity. Algae thrive in warmer water.

Last summer, over just a few months, clarity dropped by 16.7 feet to 56.4 feet as tiny algae amassed in the lake’s upper reaches, clouding the view. Deep mixing, an important blending of the lake water, remained shallow for the fifth year in a row.

The mixing of waters is critical to add oxygen and distribute nitrogen that typically accumulates at the lake’s depths. Without it, nutrients are choked on its sandy floor.

The findings didn’t come as a surprise, said Darcie Goodman Collins, executive director of the League to Save Lake Tahoe, an advocacy group better known as Keep Tahoe Blue. The lake is facing a worsening of issues it has grappled with for years, she said.

“The report continues to show the impact of climate change that we already knew was an issue,” Goodman Collins said. “We are seeing warmer temperatures and longer periods of drought followed by intensive storms. That is characteristic of 2016.

“The marriage of science and advocacy is more important now,” she said. “They give us strong and important protections for the lake we all love. It is data and reports like this that inform our decisions.”

In the last four years, the average temperature of the lake at all depths has increased by an average of 0.26 of a degree a year to 43.3 degrees Fahrenheit — a rate 10 times greater than the long-term warming rate, the report found.

Surface temperatures did fall by 2.5 degrees last July — but it was an anomaly caused by strong summer winds, according to the research center, which started tracking water temperature records in 1970.

“Everybody is worried about climate change,” said Tom Lotshaw, a spokesman for the Tahoe Regional Planning Agency, which works to preserve and restore Lake Tahoe. “Our focus is really on trying to restore and build the resiliency of our natural environment and its ecosystems.”

For forest managers, that means chopping down dead trees, clearing brush and doing controlled burns. For boating regulators, it means continuing mandatory watercraft inspections, which ensure all vessels are clean and dry — with no tiny hitchhikers — before hitting the waves.

It’s a start, Lotshaw said.

“Hopefully, we keep coming up with projects and programs to address all of the issues facing the lake,” he said. “We have a lot to do.”

Lizzie Johnson is a San Francisco Chronicle staff writer. Email: LJohnson@sfchronicle.com Twitter: @LizzieJohnsonnn