Report explores ‘amazing’ gain in Tahoe clarity

RENO (AP) — Lake Tahoe experienced an “amazing” rebound in clarity in 2012 to mark a second straight year of improvement, although scientists say studies of long-term trends show climate change continues to take a toll on the waters.

The findings were part of the annual “Tahoe: State of the Lake” report released Wednesday by the University of California, Davis.

“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,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “The real challenge is to be able to sustain the improvements when nature is working against us.”

Scientists have been monitoring the lake’s clarity since 1968, when a white Secchi disk the size of a dinner plate could be seen as far as 102.4 feet underwater.

In 2012, that figure was 75.3 feet — a 6.4-foot improvement from a year earlier. That’s within three feet of the short-term, 78-foot clarity goal researchers have set.

Scientists chalked the good news up to drier conditions that washed fewer pollutants into the lake. It was a relatively dry year for Lake Tahoe, with precipitation at 71 percent of the long-term average.

Other contributing factors include fewer Cyclotella algae and the absence of deep mixing, in which cooler surface waters sink downward and send algae-feeding nutrients to the surface.

While they’re celebrating the clearer waters, study authors pointed to negative consequences of climate change that are apparent over the span of decades. Tahoe’s annual average surface temperature in 2012 was the highest ever recorded, at 52.8 degrees.

Snow also has decreased as a fraction of total precipitation, falling from 51 percent of the total in 1910 to 36 percent in 2012. And a continuing trend of fewer days with below-freezing temperatures caused snowmelt to peak on May 4, earlier than it used to.

Researchers said new tools are beginning to yield valuable insight about the forces behind those changes. An underwater glider that cruised the lake for 11 days in May helped document “internal waves” that can spread pollutants, and a state-of-the-art offshore monitor provides water-quality data in 30-second intervals.

“Some of the new technologies that are being used at Tahoe, combined with participation from talented collaborators from around the world, are not only providing us with new knowledge of the inner workings of our lake, but also teaching us how to sustain freshwater ecosystems globally,” Schladow said.