By Lake Tahoe News

Lake Tahoe is getting warmer and that is not a good a thing for its clarity.

In 2015 the pristine waters got a little less so in summer and winter, according to data released April 19 by the UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency. The average clarity last year was 73.1 feet; 4.8 feet less than 2014.

“It is commonly believed that drought years produce clearer water conditions, but the reality is more complicated than that,” Geoff Schladow, TERC director, said in a statement.

With more runoff this year compared to the last few springs, the summer 2016 reading could bring even more bad news.

Lake clarity is measured with a Secchi disk, which looks like a white dinner plate. The measurement is based on how far someone can clearly see it from the deck of a boat, with multiple readings taken throughout the lake.

Researchers say the clarity loss is not a long-term trend, but instead can be attributed to year-to-year variability based on the conditions in the lake.

“In 2015, the runoff water into the lake was warmer than the previous year on account of the low snow-to-rain ratio,” Schladow said. “As a consequence, fine particles flowed in closer to the surface, where they impacted clarity, rather than plunging to the deeper parts of the lake.”

Summer clarity averaged 73 feet, a 3.7-foot decline over the preceding year. Winter clarity last year declined by 7.6 feet to 71.6 feet.

During most years, clarity is worse in summer than winter. Schladow said 2015 was an exception because the lack of deep mixing during the winter resulted in little of the pristine, deep water being brought up to the lake surface to dilute the clarity-reducing contaminants. Even so, researchers said long-term decline in summer conditions is still a major concern.