Lake Tahoe’s famously clear waters continue to warm, and the surrounding forests face dire threats due to drought, disease and insects, according to the annual Tahoe State of the Lake report by researchers at UC Davis.
The second deepest lake in the United States after Crater Lake, Lake Tahoe has warmed by half a degree Fahrenheit each year for the past four years — 14 times faster than the historic rate, the report said.
Overall, summer weather has been persisting for longer, with earlier spring snowmelts. Last year’s snowmelt began on March 29, 2016 — 19 days earlier than in 1961, the report found. A warming climate may bring changes to Lake Tahoe’s ecosystems and the plants and animals they support.
“It’s making conditions less ideal for the species that are native to the lake that are adapted to the very high UV conditions and cold water temperature conditions,” said Geoffrey Schladow, the report’s author and director of the UC Davis Tahoe Environmental Research Center.
Climate change may also make it harder for native species to defend themselves when an invasive species shows up, he said.
Boat inspection programs have helped keep new invasive species out of the lake, the report said. However, boating on the lake worsens the spread of invasive species already present when creatures such as the Asian clam hitch a ride in the ballast water.
Researchers suspect that Asian clams likely contribute to the growth of thick mats of algae by concentrating the nutrients that these algae need to grow. Recent years have seen more thick mats of algae wash up near Tahoe’s shores.
“They rot and they smell and it’s not what people picture Lake Tahoe looking like,” Schladow said. “If you snorkel down and you try to grab it, they sort of disintegrate in your hands. And essentially, with the right winds, they do get moved onshore.”
This past year was also the fifth in a row without “deep mixing,” when oxygen sinks to the lake’s depths to sustain life there while pulling nutrients back up to the surface, the researchers found. The report said increases in water temperature correspond to years when deep mixing doesn’t occur. Without mixing, the bottom of the lake continues to heat due to the earth’s geothermal heat, raising its temperature by a small amount every year.
Because the warm days of summer heat the surface of the lake, the water near the surface is less dense. In winter, the surface of the lake cools and the cooler, more oxygen-rich water sinks. With fewer days of cool weather, the mixing doesn’t reach deep enough to stir up water at the bottom of the lake.
When algae or fish die, they sink to the bottom of the lake and decompose, releasing the nutrients that they’ve ingested. This causes nutrients to build up there.
“Eventually, maybe next year, maybe three or four years when it does mix to the bottom all these nutrients will suddenly come back up to the surface and the concern is we’re going to feed a very large algal bloom that year,” Schladow said.
“We’re starting to approach some of the highest nitrate levels we’ve ever seen at the bottom of Lake Tahoe,” he said. Nitrate is one of the nutrients that fuel algal growth.
With last year’s increased rain and snow, the lake level rose by more than 20 inches in the 2016 water year, which ran from October 2015 through the end of September 2016. Higher stream flows also washed more nutrients into the lake than they did in 2015.
Despite the changes, long-term trends show that Lake Tahoe’s clarity is continuing to improve, the report said. Clarity is measured by the depth at which a Secchi disk, a 10-inch white circle, disappears from view. In 2016, the average depth was 69.2 feet, the report found. That represented a greater depth of about 4 feet from last year’s measurement, and 5 feet deeper than when clarity was at its worst in 1997.
Winter water clarity improved by nearly 12 feet. However, clarity decreased in the summer when populations of tiny algae grow in the upper 50 feet of the lake. Because of their small size, the algae stay suspended in the water and scatter light.
Tree mortality in Tahoe’s forests has also increased drastically, with the number of dead trees more than doubling from 35,000 in 2015 to 72,000 last year due to the stress of the drought combined with attacks from insects and disease, according to the report. The problem was worst on Tahoe’s north shore, but forests on the east shore were also affected.
Patricia Maloney, a researcher who is part of the Tahoe Environmental Research Center at UC Davis, likens the trees to straws in the ground that compete to suck up the water.
“These bark beetles, they will almost preferentially attack drought-stressed trees,” she said.
Beth Moxley, an arborist and owner of Rockwood Tree Service, has witnessed the damage from the blister rust fungus and pine beetle infestation. She’s worked in the Tahoe area since 1986.
“We’re basically in a crisis,” she said about the large number of dead and dying trees in the Tahoe Basin. “It all started with the drought. The trees become weakened and then they’re susceptible to attack by disease or insect infestation.”
Pines in the area have been severely affected by the blister rust, which has no cure. “I took down a 600-year-old sugar pine the other day,” Moxley said.
“As a tree service here in the Basin, we are overwhelmed. … We feel abandoned by the government agencies,” she said. “We need help.”
The massive tree die-offs also raise the risk of forest fires.
“We’re sitting ducks,” Moxley said. “It’s not a matter of if. It’s when and where.”