Lake’s clarity worsened in 2019
Scientists spotlight invasive shrimp in Tahoe, among other factors
RENO— Lake Tahoe’s fluctuating clarity got worse last year during an especially cold and wet winter as sedimentation, algae growth and an invasive shrimp continued to pose restoration challenges for the famed clear water of the mountain lake straddling the California-nevada line.
The clarity decline came a year after the lake had showed improvement from its worst level in a half-century as climate change continues to drive long-term trends, with rising air temperatures and less precipitation falling as snow, according to the UC Davis Tahoe Environmental Research Center.
A white, dinner plate-sized disc used to measure clarity was visible at an average depth of 62.7 feet in 2019. That is down from 70.9 feet measured in 2018.
While the average annual clarity isbetterthanitwasinprevious decades, it’s still short of the restoration target of 97.4 feet set by state and federal regulators, the research center said in its annual State of the Lake Report.
Scientists hope efforts to combat threats to the lake’s clarity posed by development and climate change will return Lake Tahoe to its historical
clear depth of 100 feet.
The lake’s cobalt waters exceeded that at one point in 2019, reaching a maximum depth of 112 feet on Feb. 19. Last year’s worst reading of 36.1 feet was recorded on May 8, coinciding with an algae bloom.
The scientists said the data
reinforces the need to continue to expand efforts to remove the invasive shrimp. Introduced to the lake in the 1960s, Mysis shrimp are driving out native zooplankton that keep the water clear by consuming algae and other small particles.
Last year’s precipitation was a foot more than the average of the past
110 years. Average air temperature in February 2019 was several degrees lower than the long-term average —
and it was the coldest February since 1956.
The average surface water temperature of 68 degrees in 2019 was down from 72 degrees in 2017.
But the surface water temperature in Lake Tahoe has been on the rise since at least 1968, when regular measurements began.
Warmer temperatures have reduced the percentage of annual precipitation that comes in the form of snow from
about 50 percent 100 years ago to around 30 percent today, which means warmer water flows into the lake.
The lake’s clarity was at its best when levels were first recorded in 1968, with an average depth of 102.4 feet. The disc typically was visible at depths of 85 feet or deeper through the 1970s.
The lake’s clarity levels registered their worst performance during a severe drought in the late 1990s.