How do we keep Tahoe blue amid climate change? Get rid of the shrimp, scientists say

The famed clarity of Lake Tahoe faces an array of threats related to climate change, which researchers say is already muddying the waters.

But the best bet to keeping Tahoe looking blue and clear? Eliminating billions of tiny shrimp that some lake-goers might not even know exist.

Scientists have launched a pilot project to “climate-proof” Lake Tahoe’s clarity by ridding it of non-native Mysis shrimp that were placed there intentionally decades ago, according to new findings released Thursday by the UC Davis Tahoe Environmental Research Center.

As researchers explain on TERC’s website, Mysis diluviana (or, simply, Mysis shrimp) were introduced to the lake waters in the early 1960s by California and Nevada’s departments of fish and game. The goal was to use the Mysis shrimp as food for lake trout to substantiate their population.

Unfortunately, scientists at the time misunderstood the behavior and diet of those tiny shrimp, which grow to about 22 millimeters (0.86 inches).

“Mysid shrimp are sensitive to light and given Lake Tahoe’s famously clear waters, they chose to spend their days on the dark lake bottom,” a research center webpage explains. This behavior defeated the purpose of their introduction, as trout and most other fish feeding in the lake are “sight feeders.”

Decades later in 2019, the results of earlier scientists’ mistake are becoming apparent. The Mysis shrimp have spent the last half-century feeding on the lake’s native zooplankton, which are an important part of Tahoe’s food ecosystem and play a big role in keeping the water blue and clear.

“UC Davis TERC researchers found that when Mysis shrimp mysteriously disappeared from Emerald Bay in 2011, native zooplankton rebounded almost immediately,” the university said in a news release summarizing findings from its annual “State of the Lake” report, also published Thursday. “... The reverse effect occurred when the Mysis returned.”

The research center says it is halfway through a two-year pilot project dedicated to finding “an effective means of removing enough Mysis shrimp to improve lake clarity indefinitely.”

Environmental scientists earlier this year announced that Lake Tahoe’s clarity “dramatically” improved in 2018, after 2017 marked the worst clarity researchers had ever observed at the lake. Lake Tahoe’s clarity was at its peak in 1968 – the first year TERC started measuring – and gradually dipped since then.

Lake Tahoe clarity depends largely on algae levels, which also correlate positively with stormwater pollution and warm weather. Pollution reaches the lake from nearby roads and urban areas by way of surrounding tributaries, the research center explained in a May news release. The university’s research report showed sediment washing into Lake Tahoe from its major tributaries during spring 2018 measurements only totaled 10 to 25 percent of sediment levels observed the previous spring, which could explain the uptick.

TERC measures lake clarity with a device called a Secchi disk – an object that looks like a small dinner plate, lowered from a boat into set points in the lake. Scientists simply measure clarity by marking the depth in feet at which the Secchi disk remains visible.

Lake clarity started near 100 feet in the late 1960s and gradually fell to 60.4 feet by 2017, before rebounding to 70.9 feet in 2018.

Researchers still don’t know why the shrimp fled from the bay in 2011, or why they’ve returned since then. But the impact on their departure from that part of the lake was enormous.

When the shrimp departed Emerald Bay, Thursday’s news release said, lake clarity improved by a staggering 40 feet.

“Even with climate change, we’re finding that if you get rid of the shrimp, clarity improves,” Geoffrey Schladow, director of UC Davis TERC and an engineering professor at UC Davis, said in a statement. “Their removal allows for the return of native zooplankton, which have the ability to consume both tiny algae and fine clay particles that have reduced clarity in the past. That is huge.”

Weather conditions were optimal last year to restore some clarity. But in correlation with the overall trend of declining clarity, researchers noted in the recent news release that the lake’s water temperature and air temperature have been warming ever since 1968.

Snow to rain

While scientists work on the plan to restore and maintain clear lake water, Thursday’s news release includes daunting projections regarding the impact of climate change on the Tahoe basin. The center’s researchers say the basin’s air temperatures are expected to rise by 9 degrees, on average, by 2100.

That temperature increase would be substantial enough to change much of the region from a snow-based climate to a rain-based climate, which scientists warn could alter stream flows and result in “changes to fish spawning, a loss of water storage and elevated wildfire risk.”

An even more serious climate change risk for Lake Tahoe itself, according to the research center, is presented by a reduction in a physical process called mixing. As the water becomes warmer, the lake does not fully “mix” its nutrients properly, which results in pollutant buildup and algae growth. It’s a cyclical problem, because these two factors further warm up lake temperatures and make it even harder for mixing to occur deep in the water, Thursday’s report says.

The annual State of the Lake report is accompanied by a public presentation set for 6 p.m. Thursday at UC Davis’ Tahoe Science Center in Incline Village, Nevada. This event precedes the annual Lake Tahoe Summit, held Aug. 20, when federal, state and local leaders will gather to discuss research and potential solutions.

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Michael McGough anchors The Sacramento Bee’s breaking news reporting team, covering public safety and other local stories. A Sacramento native and lifelong capital resident, he interned at The Bee while attending Sacramento State, where he earned a degree in journalism.