Lake Tahoe’s clarity got worse in 2019, UC Davis scientists say in annual report

Lake Tahoe’s water clarity significantly worsened in 2019 despite last year’s improvements, according to the latest study from environmental scientists at UC Davis’ Tahoe Environmental Research Center.

To measure water clarity, scientists lower a small white Secchi disk into the lake and note the depth at which the disk remains visible. In 2018, clarity improved by a remarkable 10 feet, according to researchers. However, a year later, 28 Secchi readings showed an average value of 62.7 feet — a decrease of nearly 8 feet in clarity. The average is close to 2017’s value of 60 feet, the lowest ever measured.

While scientists usually observe one dominant factor that determines water clarity during a given year, a UC Davis officials said the research team isolated several factors responsible for the lake’s 2019 conditions. In addition to precipitation levels and sediment, “the lake mixed all the way to the bottom for the first time in eight years,” officials said in a news release Wednesday.

At first, this brought clear water to the surface and resulted in a clarity value of 112 feet, the highest of the year; however, that water also carried nutrients, leading to algae growth down the line. In fact, algae cells were to blame for the year’s lowest clarity, 36 feet in May.

Climate change played a notable role, as well, officials said. Researchers said that surface warming kept fine particles afloat, making the lake’s famously blue waters murky during the summer. Such a phenomenon aligns with a trend that scientists have observed over the course of two decades: Clarity declines in summer — prime visiting season — but improves in winter. Scientists have yet to fully figure out why.

Over 80 organizations — from governmental agencies to research institutions — continue to work together to restore the lake’s environmental health. Their main focus is on ways to “mitigate climate impacts,” said Geoffrey Schladow, director of the research center. He pointed to the removal of invasive Mysis shrimp, in a project started last year, as a promising means of doing so.

Joanne Marchetta, executive director of the Tahoe Regional Planning Agency, emphasized the science community’s commitment to studying and treating the ecosystem of Lake Tahoe, calling it “our highest priority.”

This story was originally published June 10, 2020 at 6:08 PM.