Snowpack, lake clarity among major threats to Lake Tahoe's ecological future

INCLINE VILLAGE, Nev. — Following the annual State of the Lake Report, Dr. Geoffrey Schladow took center stage in Incline Village recently to discuss the findings from this year's study.

Among the topics discussed during the roughly hour-long presentation on lake data from 2018 were invasive species, temperature and precipitation, deep water mixing, forest health, and whether Lake Tahoe's famed clarity can be safeguarded against climate change.

“We all love Tahoe,” said Schladow, director of the University of California, Davis Tahoe Environmental Research Center. “Tahoe's beautiful. It's like no other place on earth, but it is changing. Go to the South Shore, the beaches there have a lot of clam shells in them, you're not just walking on white sand anymore, you're walking on clam shells. Metaphyton, these are like these long filamentous, stringy algae, they're increasingly washing up on the beaches of the South Shore, and when they wash up there they decompose.

"And there are sand flies and it's not nice. It's happening more and more every year.”

FROM SNOW TO RAIN

Meteorologically, 2018 was a very uneventful year, according to the report, with air temperatures and precipitation similar to long-term trend lines. The report also noted that the percentage of snow in the total precipitation was 31.5%, which is almost identical to 2017, but down compared to 100 years ago when it was closer to 50%.

During the past 107 years, daily air temperatures measured in Tahoe City have increased. The average daily maximum temperature has risen by 2.25 degrees Fahrenheit, and the average daily minimum temperature has risen by 4.43 degrees. According to the report, the number of days when air temperatures averaged below freezing have declined by about 30 days since 1911, though year-to-year variability is high.

Part of the annual report included projections of what the lake and basin will look like by the end of the century.

The study, which used the Representative Concentration Pathway 8.5 model (a scenario where not much is done in the future about climate change), predicted yearly average maximum temperatures would rise by 9 degrees Fahrenheit in the basin by the end of the century.

Furthermore, rising temperatures are forecast to change the Tahoe area from a snow-based hydrology to a rain-based hydrology by the final third of the century, moving the time of peak streamflows from June to January.

“When you suddenly don't have a snowpack storing water, you're getting rain reaching the stream much sooner, much higher flows, potential for more erosion, potential for bridges to be washed out, and we as engineers can deal with that,” said Schladow. “Think, however, if you're a fish and your time of year for spawning is when those flows are happening … used to happen in June, now it happens in January. I wish I could tell you what that means.”

While Schladow painted a grim picture for the future of the area as the climate warms, he indicated there is hope to protect the lake's clarity against warming temperatures.

“We cannot keep the lake cooler,” he said. “We cannot stop snow from turning into rain, but maybe we can maintain clarity in the face of all these things.”

MYSIS INVASION

From 1963-1965, the California and Nevada Departments of Fish and Game introduced non-native Mysis shrimp into Tahoe, believing the population would provide plentiful food supply for lake trout.

Once the shrimp became established, however, they did not successfully supplement the food supply for sport fishes, and instead caused unexpected negative impacts on the lake, damaging clarity levels, and nearly eradicating the native zooplankton, Daphnia and Bosmina, which act as the lake's natural cleaners.

Then in 2011, Tahoe Environmental Research Center teams observed that all of the Mysis shrimp in Emerald Bay had mysteriously disappeared. Within two years, Daphnia and Bosmina reappeared in large numbers and water clarity in the bay almost doubled. In a five-year study, funded by private donors, researchers confirmed a link between the shrimp and negative impacts on clarity.

When shrimp aren't present, the Daphnia and Bosmina are able improve the lake's clarity by consuming fine particles and Cyclotella, an algae, in the water column, turning them in fecal pellets, which sink to the bottom of the lake.

Due to its tiny size, Cyclotella, a natural phytoplankton in the lake, acts just like the fine particles that are at the core cause of clarity decline. Since 2006, researchers have observed an increase in the phytoplankton. This, according to Schladow, is due to lake's lack of mixing, meaning larger phytoplankton have settled out, leaving the Cyclotella with no competition for resources.

“(Cyclotella's) been here all along, but there's just a lot more of it now than there was in the '80s and '90s,” said Schladow.

In the following years, the Mysis returned to Emerald Bay, and the clarity has gone back to where it was prior to 2011.

The research center is currently halfway into a two-year pilot project, trawling for the shrimp at night in order to find an effective means of removing enough Mysis to improve lake clarity.

The annual average Secchi depth in 2018 was measured at 70.9 feet, which is a 10.5-foot increase over 2017, and according to researchers, is being attributed to the return of more normal conditions following a five-year drought and the heavy snow year in 2017.

Summer clarity has been declining in the long term at Tahoe, according to the report, and is largely offsetting gains made in winter clarity. Annual winter depth measurements from 1968 to present indicate winter clarity is showing an overall improvement. In 2018, however, clarity decreased by 5.2 feet, which was largely due to the carryover of conditions from 2017's extremely low clarity. The winter average of 73.5 feet in 2018 was still well above the worst winter, in 1997, when the average was 65.6 feet.

Summer clarity in 2018 averaged 61.7 feet, an increase of 8.2 feet compared to 2017. The cause of improvement, according to researchers, was due to more normal summer conditions.

WATER TEMPERATURES

For 2018, the average surface temperature of the water was 53.2 degrees Fahrenheit, which is the second highest since measurements began in 1968. The maximum daily summer surface water temperature of 77.5 degrees, recorded on Aug. 6, 2018, was one of the highest observed.

The water temperature at a depth of 1,320 feet, according to researchers, is indicative of conditions in the deeper waters of the lake. Since 1970, deep water temperatures have increased by 1.19 degrees. The increase has been punctuated by several occasional dips in temperatures, which coincide with the lake mixing completely to the bottom, allowing a large amount of heat to escape.

DEEP MIXING

Research indicated that for the seventh straight year, Tahoe didn't mix all the way to the bottom.

This deep mixing, which typically occurs between February and March, brings nutrients to the surface, where they promote algal growth, and moves oxygen downward, promoting aquatic life throughout the water column.

“As the surface of the lake gets warmer, it gets lighter — less dense, and when it's less dense and the wind blows over it, it's harder for it to get mixed in with the rest of the water,” said Schladow. “It's like oil floating on water.”

MOTHS ON THE ATTACK

Tahoe's forests recently experienced a drought, and now a new threat has emerged to the health of the forest's Aspen trees.

The white satin moth has been defoliating (removing leaves) stands of Aspen in parts of the basin, including trees old enough to bear the marks left by Basque sheepherders from the early 1900s.

The moth, which was first detected in the area in 2011, is now defoliating numerous aspen stands, and has recently been detected in the West Shore.

The annual Tahoe: State of the Lake Report is produced by the University of California, Davis Tahoe Environmental Research Center and presents data from 2018 regarding lake clarity, temperature, snowpack, invasive species, algae, nutrient loads, and more in the context of the long-term record.

Justin Scacco is a reporter for the Sierra Sun. Contact him at jscacco@sierrasun.com.