Global warming may be affecting Lake Tahoe clarity

SOUTH LAKE TAHOE, Calif. - Global warming appears to be affecting normal cycles at Lake Tahoe and could be contributing to a loss of the azure lake's famed clarity, researchers say.

As temperatures in the northern Sierra Nevada increased over the last 30 years, so did the temperature of Lake Tahoe, according to a study led by Robert Coats, a scientist affiliated with the University of California's Davis Tahoe Research Group.

The warming trend - an annual increase of .027 of a degree for a total of about .8 of a degree during the period - already has affected the lake's habit of mixing its dirtier waters from near the surface with the cleaner, deeper waters, the study said.

Exactly how global warming will affect Lake Tahoe and its clarity is an issue yet to be tackled in additional studies in the coming years. Scientists who gathered in South Lake Tahoe last week said they intend to include the temperature changes in future clarity models.

"A warming lake is a more stable lake," Coats told the Tahoe Daily Tribune. He said if the lake mixes less, the fine sediment that contributes to clarity loss is more apt to remain near the surface for longer periods of time.

Ultimately, if global warming continues, a warmer lake could accelerate a condition that would release phosphorous trapped in sediment at the bottom of the lake, Coats said. That phosphorous would fuel a large amount of algae growth.

Coats recently submitted his study, "The Warming of Lake Tahoe," to the scientific journal "Climatic Change" for review. Scientists Joaquim Perez-Losada, Robert Richards, Geoff Schladow and Charles Goldman, who founded the Tahoe Research Group, assisted with the study.

"Any warming is going to stimulate algal growth," Goldman said. "And it may actually have a small impact on fine particles contributing to clarity loss."

Lake Tahoe's clarity decreased last year, slowing a trend of steady improvement that had seen the cobalt mountain waters reach their clearest level in a decade.

The clarity declined last year about 9 percent from the previous year's average. Scientists said that was within normal ranges and should not be viewed as a reversal of significant gains made in recent years to protect the lake.

The decline likely was caused by relatively high precipitation from thunderstorms in 2003, which led to increased runoff of soil and pollutants into the lake, the experts said.

Visible at depths of 102 feet as recently as 1968, a white plate called a "Secchi disk" could be seen at an average depth of 71 feet last year, the new figures show.

In 2002, it was visible at depths of 78 feet, the clearest in 10 years.