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Climate change may be muddying Lake Tahoe waters

By Mike Taugher (San Jose Mercury News) - August 15, 2011

Lake Tahoe’s legendary clarity fell to its second-worst reading ever last year, just inches better than the record low measured 14 years ago when a $1.5 billion program to fix the lake’s environment was launched.

The average depth at which a 10-inch plate could be seen from a research boat was just 64.4 feet last year, a decline of 3.7 feet from the previous year. That compares with 102.4 feet when researchers began measuring the decline of Lake Tahoe’s clarity in 1968.

Researchers were not alarmed, however, saying the annual readings bob up and down and that they continue to believe that the rate of decline is slowing.

More vexing are indications that pollution-control projects in the Lake Tahoe area might actually be working, but that larger forces are overwhelming them, researchers said.

“It looks like climate change may be behind what we’re seeing this year or the last couple of years,” said Geoffrey Schladow, director of UC Davis’ Tahoe Environmental Research Center.

The measurements taken in winter are showing improvement, which Schladow said could be evidence that projects meant to reduce the flow of fine sediments into the lake are working.

But the lake is getting cloudier in the summer.

In the latest “Tahoe: State of the Lake” report released Friday, researchers at Schladow’s center suggested that could be because of climate change.

An earlier study showed that as the upper waters at Tahoe have warmed, the difference in temperature between layers of water make the lake less likely to mix. Larger, heavier algae sink deeper into the lake and cannot ride back to the surface as they would when the lake mixes.

As a result, the bigger algae are disappearing and a tiny species of algae is exploding in numbers during the summer, Schladow said.

The huge numbers of smaller algae do more to cloud the lake than fewer big algae.

“While we’re still maintaining the decadelong trend of slowing the decline in clarity, the new forces of climate change and the unusual concentrations of algae have us concerned,” said Joanne S. Marchetta, executive director of the Tahoe Regional Planning Agency, in a news release.

Her agency administers the Lake Tahoe Environmental Improvement Program, which was launched in 1997 as a $900 million commitment of federal, state, local and private investment to address some of Lake Tahoe’s most serious environmental problems, including diminishing clarity.

That program grew to more than $1.5 billion, but funding for its future is becoming uncertain.

Schladow said one of the most important things the program did was to define fine sediment, and not nutrients, as the primary culprit causing the decline in clarity. That sediment, mostly finely ground bits of granite, comes ´╗┐largely from urban areas that cover just 10 percent to 15 percent of the basin’s land.