INCLINE VILLAGE, Nev. — Lake Tahoe’s clarity improved for the second consecutive year in 2012, but trends suggest climate change is continuing to impact the famed Sierra lake.
The average Secchi depth for the lake was 75.3 feet in 2012, an improvement of 6.4 feet from 2011, according to the UC Davis Tahoe Environmental Research Center’s 2013 State of the Lake report, released Wednesday. This value is within 3 feet of a short-term clarity target of 78 feet.
Secchi depths are gathered by lowering a 10-inch, white disc into the lake and recording when the plate is no longer visible to observers.
In 2012, the lake’s average surface water temperature of 52.8 degrees was the warmest on record, according to the report. The average temperature in 1968 — when such recordings began — was 50.3 degrees.
“It’s not as if next year climate change is going to end,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “It’s not ... this is something that is here to stay.”
Deep water temperature has increased approximately 1 degree since 1970. The increase hasn’t been steady, according to the report, but punctuated by occasional drops in temperature that coincide with times when the lake mixes all the way to the bottom.
In 2012, the lake stability index, which measures how resistant to mixing the lake is, also was at a record high. Lake Tahoe mixed only 820 feet last year, likely contributing to the warmer surface temperature and the improved clarity.
Deep mixing brings nutrients to the surface, promoting algae growth. It also moves oxygen to deep waters, promoting aquatic life.
Another reason for improved clarity was due to 2012 being a dry year for Lake Tahoe. Precipitation was 71 percent of the long-term average, according to the report, resulting in fewer pollutants running into the lake.
“We’re encouraged that deep-water clarity is increasing, which shows that restoration projects funded by the Lake Tahoe Restoration Act over the last decade may be working,” said Darcie Goodman Collins, executive director for the League to Save Lake Tahoe. “The report’s findings on the effects of climate change show that continued funding for restoration is more important than ever.”
The lake’s level experienced a net loss in 2012, another consequence of climate change, according to the report. Lake level rose by only 1.3 feet during the spring snowmelt, compared with 3.9 feet in 2011. During summer and fall 2012, lake level fell by 2.3 feet.
During a presentation Wednesday evening at Sierra Nevada College, Schladow proposed that a five-year drought would reduce Tahoe’s level to about 6,215 feet — its natural rim is 6,223 feet — resulting in shoreline impacts.
“We always think about it (climate change) in terms of we’re going to lose our winter economy, we’re going to lose our snow,” he told the 70 people in attendance. “Our summer economy is not going to be so good either. There is not a single dock, a single launch ramp that will have access to the water.”
A new water-quality monitoring station off the West Shore that provides data from lake surface to the bottom every 30 seconds is expected to provide a better understanding of climate change impacts on Lake Tahoe, according to the report.
“In this last year we saw how nature, combined with the results of the many projects that have been completed in the basin, produced an amazing increase in clarity,” Schladow said in a statement. “The real challenge is to be able to sustain the improvements when nature is working against us.”
While the report points to improving clarity, not everyone is encouraged, including Ann Nichols, executive director of the North Tahoe Preservation Alliance.
“I think it’s very misleading to say that the lake is continuing to improve,” she said Thursday. “It’s just a two-year snapshot.”
“It’s not as if next year climate change is going to end. It’s not ... this is something that is here to stay.”
UC Davis Tahoe Environmental Research Center