State of Lake Tahoe: Particle and algae levels are soaring — wildfires may be to blame

In 2021, Lake Tahoe’s algae and particle levels soared to unprecedented levels. Long-established phytoplankton behavior changed, and zooplankton populations collapsed.

Scientists think wildfires may be partially to blame.

On Thursday, UC Davis’s Tahoe Environmental Research Center released its annual State of the Lake report, which detailed phenomena observed in Lake Tahoe in 2021. Scientists observed unprecedented changes in aquatic organism populations, phytoplankton behavior and in algal growth.

“Any one of these changes would be a big deal in a single year,” center director Geoffrey Schladow said in a Thursday news release. “All three occurring at once is particularly alarming and a huge opportunity to learn lessons that can be used to inform future management.”

Last summer, the Caldor Fire ripped across the Sierras, prompting evacuations in South Lake Tahoe and charring over 220,000 acres of forest near the lake. Another fire to the north, the Loyalton Fire, also brought smoke to the basin as it charred 47,029 acres. Experts initially celebrated Lake Tahoe’s continued clarity in the aftermath of the fires, but in Thursday’s report, they attributed growing algae and particle levels in part to the wildfires’ effects.

Floating algae increased by 300% in 2021, the report said, reaching record-high levels and exacerbating the threat of algal blooms that could invade popular swimming areas. For the first time on record, the dominant alga in the lake was Leptolyngbya, a genus of cyanobacteria that thrives in smoky conditions.

Particle levels in the lake also hit a record high in 2021, with scientists finding that particles from wildfires “may be a source” of the increase.

But although particle levels are at their highest, Lake Tahoe still remains clearer than it was in 2017, its murkiest year. According to the center’s Lake Tahoe clarity report released this month, 2021’s clarity was the second-lowest on record — 61 feet.

Wildfire smoke may also have caused Lake Tahoe’s phytoplankton to change long-established behaviors. As the summer progressed, phytoplankton moved closer to the lake’s surface, which scientists say “may be due to reduced sunlight and UV radiation during wildfire smoke events.”

Meanwhile, the population of zooplankton plummeted in 2021, along with the population of their predators, Mysis shrimp. Last time the Mysis population shrunk — between 2011 and 2014 — lake clarity increased significantly. Scientists said they will be looking to see if the same phenomenon occurs in the coming years.

“The real question is whether the absence of Mysis from Lake Tahoe will help restore clarity and, if so, is the deliberate removal of Mysis in the future warranted,” the report says.

The report also nods to the significance of climate change on the ecosystem of Lake Tahoe — 2021 was the third-driest year on record in the region, the report says, and that means the lake will “almost certain(ly)” fall below its natural rim and stop flowing into the Truckee River by the end of the summer.

But last year’s State of the Lake report — released in August 2021, and evaluating lake behavior in 2020 — warned that climate change could also cause lake levels to rise and flood surrounding towns. As temperatures rise, climate scientists fear that the Sierra snowpack will deplete, instead turning to water that could flood the lake.

The State of the Lake report is released each year in collaboration with scientific agencies across Northern California and Nevada. It’s available at tahoe.ucdavis.edu/stateofthelake.

This story was originally published July 28, 2022 at 4:27 PM.