Lake Tahoe reaches clarity not seen since the 1980s

The blue of Lake Tahoe is deeper and clearer than it's been in a long time, and it's not just the contrast with the bright white of the snow-plastered Sierra Nevada.

The recently-released Lake Tahoe Clarity Report for 2022 finds in the last five months of last year, it was possible to see more than 70 feet below the surface. That's a 10-foot improvement over the year before.  

"It appears that this is due to a resurgence of microscopic animals known as zooplankton," said Dr. Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. "These zooplankton have the ability to literally clean the water, to filter out the things that cause clarity to be diminished."

These zooplankton, specifically those known as Daphnia and Bosmina, eat algae and sediment and help keep the water clear. But their population has been severely impacted since the introduction of invasive mysis shrimp in the 1960s.

At the time, state officials in California and Nevada hoped the shrimp would serve as food for the local fish. Instead, the zooplankton became a favorite food of the mysis shrimp.

But the shrimp population unexpectedly crashed in late 2021, the report says. Over the past year the zooplankton have rebounded, helping lake clarity. 

Schladow says the effect is likely to be temporary. Runoff from melting snow will bring dirt and debris into the lake, and the mysis shrimp population is expected to recover, which will negatively impact the zooplankton filtering the water.

"The really cold temperatures we had this winter resulted in the lake sort of fully mixing, turning over top to bottom, which gave us a boost in clarity," he said. "But that won't last. As the snowmelt starts to happen, a lot of contaminants will be washed into the lake, fine particles that will in all likelihood reduce the clarity."

The states of California and Nevada have a goal of returning Lake Tahoe to a clarity depth of 97.4 feet.