CARSON CITY, Nev. (AP) — An annual report released Thursday detailing the status of Lake Tahoe's health and clarity has researchers rethinking some long-held assumptions and focusing on trends that may be linked to climate change.

"This last year has defied conventional wisdom in many ways," said Geoffrey Schladow, director of Tahoe Environmental Research Center, part of the University of California, Davis.

"The forces that drive the lake are starting to change," he said. "Climate change is one of those forces. The role it's playing is evolving."

Schladow said researchers were surprised that after the monster winter of 2010-2011, Lake Tahoe's clarity improved last year by 4.5 feet to an annual average of 68.9 feet. Harsh winters that bring heavy runoff from snowmelt are generally believed to decrease clarity.

"In the past, very wet years have led to decreases in lake clarity, whereas we are now seeing the opposite," he said.

Researchers also said that the 2007 Angora Fire on Tahoe's west shore that burned more than 3,000 acres and destroyed hundreds of homes has had little effect on the lake's water clarity, another surprise.

"There's always been that fear of wildfire here and what that would do for restoration efforts," Schladow said. But the latest research suggested that "the system is pretty resilient to the effects of fires and it will self-heal in a matter of a few years."

"A bigger fire would have more effects," he said, but noted that the data is encouraging.

The 2012 State of the Lake report, released in advance of this year's annual Tahoe Summit being held Monday at Edgewood, also calculated for the first time the lake's stability — the energy needed for mixing layers of lake water.

Researchers found that during the past 43 years, the length of time Tahoe waters are stratified during the summer into layers of different temperatures increased by almost 20 days and is likely the result of climate change, the report said.

"During the summer, Lake Tahoe and every lake gets warmer on top, while the bottom stays quite cold," Schladow explained. But the ecosystem also requires deep water mixing in the winter — when the warm upper levels cool and sink — to replenish oxygen at the deeper levels.

"We've been saying for a few years now that one of the consequences of climate change was that this mixing may take place less vigorously or not as frequently," Schladow said.

That could change the chemistry at the bottom of the lake, causing the release of nutrients and spur algae growth that clouds Tahoe's clarity.

Scientists until now have been taking measurements of Tahoe's deep water oxygen levels once a month. But Schladow said new equipment is being installed to record those levels every 30 minutes to give researchers more detailed data.