Annual UC Davis study finds Lake Tahoe is losing clarity because of algae, shrimp

The University of California, Davis has continually monitored the conditions of Lake Tahoe since 1968. And even a global pandemic didn’t stop the center from its annual findings.

On Thursday, the UC Davis Tahoe Environmental Research Center published its annual Tahoe: State of the Lake Report. The study found the lake’s clarity decreased slightly in 2019 to an average depth of 62.7 feet, an 8.2-foot decrease from 2018. The report cited multiple factors for the decrease, including an increase of mixing of water, algal blooms and above-average stream loads.

The report is intended to inform nonscientists on what affects lake health. It also summarizes 2019 data as part of the center’s decades-long measurement programs.

This year, a central finding was something clearer than Tahoe’s muddying waters: billions of tiny, invasive shrimp currently living in the lake are clouding the future of the lake.

The shrimp — called mysis — were intentionally brought to the lake decades ago to serve as trout feeders. And the center has known for several years the shrimp are a central reason for Tahoe’s decreasing clarity. The report comes on the heels of the center’s June report, which showed that Lake Tahoe’s water clarity significantly worsened in 2019 despite improvements in 2018.

This year further drilled down this conclusion. The report says the shrimp are responsible for not just the removal of cleaners of fine particles from the lake, but for the removal of large algae. These two actions combined have allowed a dominance of the algae cyotella, which severely impacts the lake’s clarity. In fact, the lowest clarity of 2019 was 36.1 feet, which coincided with a cyotella bloom.

How to increase Tahoe’s clarity? Turn shrimp into dog treats

The experience from nearby Emerald Bay, which experienced an increase in clarity when shrimp were removed, suggests the impact is reversible. And with a new plan begun this spring, the shrimp may not remain for long.

In response to the findings, TERC is working together with students from the UC Davis Graduate School of Management and the UC Davis School of Veterinary Medicine to find a way to clear the lake of as many shrimp as possible. They attempted to do it in an unusual way: by turning shrimp into dog treats.

TERC is working with the groups to harvest the shrimp, which are rich in omega-3 fatty acids, to make dog treats as part of efforts to remove them in a cost-sustainable manner, enhance lake clarity and restore native zoo plankton. Plans are underway to create an independent nonprofit where revenues from treat sales cover the cost of removing the lake’s shrimp.

Other Findings

Long-term trends showed the evidence of climate change at Lake Tahoe, where air temperatures are rising and less precipitation is falling as snow. Though 2019 was a colder year than usual, with the coldest February on record since 1956. July’s on-average surface water temperature was about 4 degrees cooler in 2019 than in 2017, landing at 68 degrees. But despite the cooler year, the long-term trends are increasing the length of warmer months, which will also impact clarity.

The report also found the lake is experiencing above-average precipitation and amounts of nitrogen and phosphorus from the Upper Truckee River, though they were below record loads from 2017. Algae attached on rocks were especially heavy, and Nevada’s Zephyr Cove had its second-highest value on record.

In response, TERC is exploring remote techniques to track both rock-attached and free-floating algae, which the public can assist with through the Citizen Science Tahoe App.

The report also included tips for paddleboarders who may be suddenly pushed into water as chilly as 42 degrees due to strong wind events, all using physics to inform their advice.

The report also described how researchers and staff revamped their research during state stay-at-home directives to slow the spread of COVID-19.