Lake Tahoe could become clearer in coming years with small changes, report says

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Lake Tahoe, the world’s clearest large lake, could become even clearer in the coming years due to changes in plankton populations, scientists say in a new report.

UC Davis researchers released their annual Lake Tahoe “State of the Lake” report last week, detailing several key changes in the water, including declining levels of Mysis shrimp, a type of zooplankton, that are affecting the lake’s clarity. could increase.

The team of scientists predicted that the decline in the Mysis shrimp population will have a cascade effect on the lake. About the size of a fingernail, the zooplankton species are known to feast on the lake’s native Daphnia, a small crustacean that spends much of its time devouring algae that would otherwise dilute the lake’s clarity. , the report said.

The plummeting numbers of Mysis shrimp could allow the Daphnia to reattach itself, increasing the clarity of Tahoe’s water, researchers said. The Daphnia is also eaten by Kokanee salmon, and if the Daphnia population remains high, the salmon could grow massively in size, scientists wrote.

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While the improved clarity isn’t certain, scientists said they’re basing their predictions on a similar situation that happened in Lake Tahoe’s Emerald Bay about a decade ago, resulting in a “dramatic” increase in clarity.

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

Schladow said the Mysis shrimp will likely return in a few years, but there are ways the population can be culled by humans.

The shoreline of Lake Tahoe near Sand Harbor, Nev.

(Max Whittaker / For The Times)

The university has continuously monitored Lake Tahoe since 1968. The researchers measured the lake’s clarity by lowering a 10-inch-wide white disk, called a Secchi disk, into the water and marking the depth at which the disk remains visible.

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The average depth at which the disc could be seen in 2021 was 61 feet. Federal and state regulators have set a Secchi depth of 97 feet 4-inches as the “target for restoring lake clarity.”

In addition to how the water looks, clarity is important because it makes it more difficult for invasive species to reproduce in the lake. It also allows Tahoe residents to drink unfiltered water, which Schladow says could change as the clarity gets worse.

Despite the harbinger of better clarity in the lake, 2021 also saw massive growth in nearshore floating algae, which the report called “a growing ecological threat to the lake.”

“The flowers also occur where the greatest number of people – residents and visitors – interact with the lake,” the report states. “The abundance of floating algae also increased by 300 percent last year and reached an all-year high in 2021.”

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

The UC Davis researchers noted that phytoplankton in the lake were getting much closer to the surface than normal, which could be due to the lower levels of sunlight resulting from wildfires. The high levels of phytoplankton at the lake’s surface make it more difficult to filter the top water, scientists say.

On top of the wildfires, dry conditions in the area caused the lake’s surface to decline in 2021, with the lake briefly falling below its “natural edge” for an 11-day period.

“It is almost certain that the lake will fall below its natural edge in the summer of 2022 and stop the river’s outflow into the Truckee River,” the scientists wrote.

Without an outflow to the Truckee River from Lake Tahoe, river rafting would be limited, Schladow said. It can also negatively affect already endangered fish.