Lake Tahoe could get clearer over next few years, report says

Lake Tahoe, the world’s largest lake, could become clearer over the next few years due to shifting plankton populations, scientists say in a new report.

Researchers at UC Davis released their annual Lake Tahoe “State of the Lake” report last week, detailing a number of significant changes in the water, including the magnitude of declines in shrimp Mlysis, a zooplankton, which can increase the clarity of the lake.

The team of scientists predict that the decline in the population of the Mlysis shrimp will have a cascading effect on the lake. The zooplankton, about the size of a fingernail, is known to be a native predator of Lake Daphnia, a small crustacean that spends most of its time eating algae, the report said.

The sharp drop in the number of Mlysis shrimp could allow Daphnia to reassert itself, increasing the clarity of Tahoe water, the researchers say. Daphnia was also eaten by Kokanee salmon, and if Daphnia’s population remained high, the salmon could have grown enormously in size, the scientists wrote.

While improved clarity is not a sure bet, the scientists say they are basing their predictions on a similar situation occurring in Lake Tahoe’s Emerald Bay about a decade ago. advance, resulting in a “significant” increase in clarity.

“We expected better clarity, but only temporarily,” said Geoffrey Schladow, a professor of Water Resources and Environmental Engineering at UC Davis.

Schladow said the Mlysis shrimp species will likely return in a few years, but there are ways for humans to eradicate the population.

Lake Tahoe shore with underwater boats, trees and large rocks along the shore.

The shores of Lake Tahoe near Sand Harbor, Nev.

(Max Whittaker / For The Times)

The university has been continuously monitoring Lake Tahoe since 1968. The researchers measure the lake’s clarity by lowering a 10-inch-wide white disc, known as the Secchi disk, into the water and marking the depth at which the disc is located. can be seen.

The average depth at which the disc can be seen in 2021 is 61 feet. Federal and state regulators have set the Secchi depth of 97 feet 4 inches as the lake’s “clarity restoration goal.”

In addition to the appearance of the water, clarity is important as it makes it harder for invasive species to breed in the lake. It also allows Tahoe residents to drink unfiltered water, which Schladow says could change if clarity gets worse.

Despite signaling better lake clarity, 2021 also sees large growth of floating algae near the shore, which the report calls “an increasing ecological threat to the lake.”

The blooms also occur when the most people – residents and visitors – interact with the lake, the report said. “Floating algae abundance also increased 300 percent over the past year, reaching an all-time high annual value in 2021.”

Scientists also tracked the lake’s response to a number of devastating wildfires in the area last year, including the Dixie, Tamarack and Caldor fires.

The UC Davis researchers noted that phytoplankton in the lake emerged much closer to the surface than usual, possibly due to lower amounts of sunlight due to wildfires. High levels of phytoplankton at the lake’s surface make it more difficult to filter water at the top, scientists say.

In addition to the wildfires, drought in the area caused the lake’s surface to decline throughout 2021, with the lake briefly dropping below its “natural belt” over an 11-day period.

“It is almost certain that the lake will fall below its natural belt by the summer of 2022 and stop the river’s flow to the Truckee River,” the scientists wrote.

Schladow said that without the flow into the Truckee River from Lake Tahoe, river rafting would be limited. It can also negatively affect fish that are already endangered. Lake Tahoe could get clearer over next few years, report says

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