Lake Tahoe clearer, but report mostly bleak
The news from Lake Tahoe and its overstressed health is mixed this year: Its legendary blue depths are a tiny bit clearer, but the polluting effects of the Angora Fire that burned 3,100 acres and destroyed 254 homes a year ago are still unknown.
As for global warming, its effects on the entire Tahoe Basin are measurable: Nights are warmer, the lake's temperature has been slowly rising for many years even though it dipped a bit last year, and last winter saw less snow and more rain than usual.
All this comes from a 60-page report on Tahoe's condition released Tuesday by UC Davis scientists who have monitored the lake's weather, water and life for more than 40 years.
In the second annual report from the Tahoe Environmental Research Center, Director Geoffrey Schladow says in his introduction that the patterns of the lake's condition are "the results of natural forces and human actions that operate over time scales ranging from days to decades. Lake Tahoe is a complex ecosystem, and it behaves in ways we don't always expect."
Here are some highlights:
The Angora Fire that started with an illegal campfire in June 2007 dumped tons of nitrogen and phosphorus into the lake - far more than the amounts in a normal summer, but so far, the fire has had a "negligible impact" on lake clarity and algal biomass," says the report. On the other hand, there was "no significant runoff" from the Upper Truckee River during the rest of 2007.
Still, long-term algae growth in the lake - a major contributor to its loss of clarity and much of it caused by human-generated pollution - has steadily increased since scientists first started measuring it annually in 1968. Last year, algae concentrations on rocks around the shoreline were above average, and the scientists found that the rate of algae growth was the highest on record - five times greater than in 1959.
However, the water's clarity actually improved slightly in 2007 compared to the year before, say the scientists. To measure the lake's clarity, scientists toss 10-inch white plastic "Secchi disks" - named after a 19th century Italian astronomer and priest named Angelo Secchi who first used them in Rome - into the water throughout the year and measure how deep the disks sink before they can no longer be seen.
Tahoe's average "Secchi depth" has become steadily shallower since 1968, although the loss of clarity has slowed a lot in the past 20 years. Last year, the "Secchi depth" was 70.1 feet, or 2.4 feet deeper than the year before, while in 1968, when fewer summer homes ringed the lakeshore, the water was clear to more than 100 feet in depth.
-- The lake's water was clear down to 70.1 feet deep last year, but 40 years ago its legendary clarity extended to 100 feet.
-- Since 1970, the lake's average surface water temperature has warmed from 41.7 degrees Fahrenheit to 42.6 degrees.
-- In 2007, winter temperatures dropped below freezing during 63 days - about 30 days fewer than in 1910, when records began.
-- Compared to 1961, the peak time of snowmelt around the lake is 2 1/2 weeks earlier.
-- With development has come pollution, resulting in a loss of water clarity. Nearly three-quarters of the fine particles polluting Lake Tahoe originate from runoff from towns and built-up communities. Most of the suspended sediments and about 30 percent of the polluting phosphorous come from the Upper Truckee River, Trout Creek, Blackwood Creek and Ward Creek.
The full 2008 report can be found at: links.sfgate.com/ZEMN
For more on the Tahoe Environmental Research Center: links.sfgate.com/ZEMO